Puberty in the Female and Its Disorders

Fetus

Ovary

The ovaries differentiate in the urogenital ridge adjacent to the anlage of the adrenal cortex and the kidney. The granulosa cells are the homologues of the Sertoli cells of the testes. The theca, interstitial, and hilus cells are the homologues of the Leydig cells; hilus cells may even contain crystalloids like Leydig cells. Adrenocortical rests occasionally have been found in the hilus of the ovary. Conversely, ovarian rests have been identified in the adrenal glands.

The primitive germ cells migrate into the ovary from the yolk sac endoderm during the first month of gestation. The testes become histologically discernable by 8 weeks’ gestation. The ovaries develop in the absence of testicular development being switched on by the signaling cascade initiated by the SRY gene on the Y chromosome. Activation of the β-catenin signaling pathway by Wnt-4 and R-spondin1 permit forkhead ( Fox ) L2 transcription factor expression by germ cells to activate ovarian differentiation by sustaining oocyte and granulosa cell development and suppressing Sertoli and Leydig cell differentiation; they also support later aspects of follicle development. Steroidogenic factor-1 ( SF-1 ) WT-1 , LIM-1 , and possibly DAX-1 genes play roles in the formation of the ovaries. Germ cell bone morphogenetic proteins (BMPs) are necessary for primordial germ cell proliferation.

Primitive germ cells undergo mitotic division to become oogonia, a process that is maximum at 8 to 12 weeks. Oogonia then undergo oogenesis, entering the prophase of meiosis to become primary oocytes beginning at 12 to 16 weeks. The number of oocytes reaches a peak at 20 weeks when there are 6.8 million germ cells, of which 80% appear to be viable ( Fig. 16.3 ). When oocytes enter the diplotene stage of meiotic prophase they must be furnished with granulosa cells to form a primordial follicle, or else they undergo atresia.

The ovary remains histologically undifferentiated until primordial follicles appear at about 16 weeks, when the epithelium of the secondary sex cords provides granulosa cells to the oocytes. However, the fetal ovary has the capacity for androgen and estrogen formation and signaling, although at a far lower level than the testes, by 12 weeks. Primordial follicles become primary follicles when the encircling granulosa cell layer becomes cuboidal. Primordial and small primary follicles ( Fig. 16.4 ) are resting follicles, which are the major repository of germ cells. This stock of germ cells is depleted only very slowly during childhood (see Fig. 16.3 ). Residence of primordial follicles in the ovarian cortex restrains their progression partly because of cortex mechanical rigidity. Mechanical effects are mediated by the growth-restrictive Hippopotamus signaling pathway and by vascular permeability via the vascular endothelial growth factor signaling pathway.

Secondary follicles and preantral follicles, characterized, respectively, by organization of a distinct theca cell layer and proliferation of granulosa cell layers, then appear successively. Preantral follicles develop at 24 to 26 weeks. Antral (graafian) follicles appear near term, and those granulosa cells enveloping the oocyte to become the cumulus. Ovarian estrogen production appears to be virtually unresponsive to gonadotropins until early antral follicles develop at near term gestational age. One or two antral follicles of 1 to 2 mm in diameter are present in the ovary at term. At this time, ovarian follicle development is complete, and the complement of ova is greater than at any other time during postnatal life (see Fig. 16.3 ), totaling 2 million, of which half appear atretic.

Both X-chromosomes are active in oocytes, and the oocytes secrete factors, such as growth differentiation factor-9 (GDF9), necessary for the induction of the granulosa cell layer that is necessary for oocyte survival. Oocyte-specific chemokines and transcription factors then coordinately direct the formation of primordial follicles and their subsequent development to primary follicles. GDF9 interaction with growth factors, such as BMP 9 and transforming growth factor beta (TGF-β), is then critical for primary follicle granulosa cell proliferation. Then preantral follicles develop when GDF9, in coordination with other growth factors, induces the theca cell layer from fibroblast-like stem cells. A host of local factors then regulate further follicle growth and development; for example, the forkhead transcription factor FOXL2, expressed specifically in granulosa cells, restrains GDF9 from prematurely activating follicle growth.

Estrogen receptor (ER) expression is critical for development of the granulosa cell layer. Insulin and androgen promote the primordial-primary follicle transition. Only upon reaching the early antral follicle stage does further follicle development become strictly dependent on FSH action.

Follicle number is determined by the balance between survival and atresia of ovarian germ cells. The endowment of ovarian germ cells has been thought to be determined during fetal life since the germ cells of the ovary, unlike those of the testes, seem to be a nonrenewing population. However, female germline stem cells can replicate, which suggests that local environmental factors extrinsic to the oocyte hold it in a state of suspended animation. The endowment of follicles may also be influenced by circulating factors, such as toxins and placental insufficiency. Some clinical evidence suggests that fetal undernutrition slows the rate of atresia. Studies in mice indicate that puberty appears to be a critical developmental window for the regulation of the follicle population because a wave of primordial follicle depletion is triggered by gonadotropin action on the intrinsic apoptotic pathway.

Adolescent

The endocrinological changes of puberty actually begin in late preadolescence before secondary sex characteristics appear, as just reviewed. The underlying basic event is increasing secretion of hypothalamic GnRH. Puberty is the consequence of the hypothalamus releasing GnRH with increasing frequency and amplitude, first only at night, then gradually throughout the day.

Increased GnRH secretion in man was initially deduced when Kastin, Job, Grumbach and their collaborators demonstrated that preadolescent children had GnRH-releasable pituitary stores of LH and FSH ( Figs. 16.6 and 16.9 ). Subsequently, it was reported that in man, the output of an immunoreactive fragment of GnRH begins to rise in late childhood and increases to adult levels during puberty. Studies in the rat suggest that hypothalamic GnRH increases through puberty.

Knobil subsequently showed that puberty can be induced in the immature female rhesus monkey by administering GnRH in hourly pulses that yield blood levels of about 2000 pg/mL. Prolonged administration of GnRH according to this regimen first gradually brings about transient increases in LH and FSH. This then induces cyclic follicular development. The resultant moderate estradiol surge is of such magnitude as to result in menarche because of withdrawal menstrual bleeding in an anovulatory cycle ( Fig. 16.10 ). Continuation of the same GnRH regimen leads to development of normal monthly ovulatory menstrual periods. Physiological pulses of GnRH in man probably attain lower concentrations (200 pg/mL) and occur at slightly wider intervals than in monkeys. Consequently, LH pulses in mature women occur at intervals of approximately 1.5 hour during the follicular phases, slowing during the luteal phase.

Puberty begins in response to increased GnRH secretion. Serum LH first begins to rise disproportionately to FSH; this LH-FSH disparity is particularly evident during sleep, which is reflected in responses to GnRH or GnRH agonist ( Table 16.1 ). Puberty becomes clinically apparent as thelarche when estradiol levels are sustained > 10 pg/mL. It seems likely that a rise in inhibin-B as increasing ovarian follicles develop plays a key negative-feedback role in limiting further increase in FSH levels during puberty. FSH levels become less GnRH-dependent during puberty. The mechanisms for differential regulation of FSH and LH are discussed later in this chapter.

Pubertal gonadotropin cycles seem to develop well before menarche and are capable of inducing cyclic estrogen production. Our working model of the nature of pituitary-ovarian dynamics in early puberty is illustrated in Fig. 16.11 .

Puberty progresses as LH rises. Whereas serum FSH levels rise about 2.5-fold over the course of puberty, LH levels rise 25-fold or more. The initial change in LH secretion at the beginning of puberty is a nightly increase in LH secretion that begins within 20 minutes of the onset of sleep. Subsequently, LH increases more with the onset of sleep, stays up longer, and falls less during waking hours. As the child approaches menarche, the daytime LH levels continue to increase until the diurnal rhythm is typically lost. FSH levels follow a similar pattern, although the FSH changes are less striking. The gonadotropin diurnal rhythm during puberty seems entirely related to sleep, unlike the cortisol circadian rhythm. There is a delay of about 12 h between the peak LH level during sleep and the estradiol zenith, such that estradiol levels are maximal between late morning and early afternoon. The gonadotropin and estradiol rhythms in an early pubertal girl are shown in Fig. 16.12 .

Augmentation of the bioactivity of serum LH occurs during pubertal progression. Plasma LH bioactivity rises nearly fivefold more during the course of puberty than does LH as measured by polyclonal RIA. The change in bioactive LH is mirrored well by the “third-generation” monoclonal antibody-based immunometric (“pediatric”) assays that have very high specificity for bioactive LH epitopes. However, disparities in the ratio of bioactive to immunoreactive LH (B/I) persist with these assays, for reasons related to the molecular microheterogeneity of gonadotropins, which is discussed later. Serum FSH rises during puberty according to immunoassay more so than by bioassay.

Estradiol output increases rapidly in the year approaching menarche. This seems to be the result of a variety of autoamplification phenomena that facilitate puberty, maturation of the dominant follicle, and ovulation. These are summarized in Box 16.1 . These phenomena occur at all levels of the axis. The CNS is stimulated by preovulatory levels of estradiol to increase GnRH pulse amplitude. At the pituitary level, there is the self-priming effect of GnRH, whereby a pulse of GnRH sensitizes the pituitary to have a greater LH response to a subsequent identical GnRH pulse. Critical patterns of estradiol and progesterone secretion enhance the pituitary LH and FSH responsiveness to GnRH. At the gonadal level, the cascade of events is augmented by the FSH induction of aromatase activity and progestin production in granulosa cells, phenomena in which androgens play a synergistic role. Furthermore, FSH stimulates granulosa cell mitosis and induces LH receptors, phenomena in which estradiol may play a synergistic role. Subsequently, LH is able to further enhance the aromatase and progesterone effects. Progesterone itself plays a synergistic role in stimulating granulosa cell progesterone and prostaglandin synthesis in concert with both FSH and LH. In the rat, ovarian GnRH receptor sites also diminish just before ovulation, and at about this time the ovary changes its pattern of metabolism so that the secretion of androstanediol-3β-monosulfate decreases to levels that are no longer inhibitory to LH secretion.

The preovulatory gonadotropin surge occurs when all these cascading processes culminate in activation of the positive feedback mechanism, a unique feature of the female neuroendocrine system. Positive feedback refers to the neuroendocrine system acquiring the ability to secrete a midcycle surge of LH in response to the increasing estrogen secretion by a preovulatory follicle, that is, when the ovary signals via increasing estrogen secretion that it is prepared for ovulation.

Menarche does not necessarily indicate full maturation of the neuroendocrine-ovarian axis. As the studies of Knobil illustrate (see Fig. 16.10 ), menarche can be caused by estrogen-withdrawal bleeding—and it is about half of the time—but ovulatory cycles may follow in short order. General characteristics of the mature ovary are shown in Fig. 16.4 .

The morphology of the normal adolescent ovary has long been considered polycystic, and histological examination typically has shown thecal luteinization. In the perimenarcheal period, the combination of a high number of follicles and mature gonadotropin stimulation leads to a greater number of 2 to 9 mm antral follicles within a year after menarche than at any other stage (see Fig. 16.3 ). This often leads to a “multifollicular” ultrasonographic appearance that overlaps adult criteria for polycystic ovary morphology in one-third to one-half of normal adolescents (see section on polycystic ovary syndrome).

Adult

The follicular phase of each menstrual cycle recapitulates puberty in many respects. Gonadotropin and sex hormone levels are low during the premenstrual phase of the mature cycle ( Fig. 16.13 A). Gonadotropin concentrations then increase at the time of menstruation, FSH predominating in the early follicular phase, whereas nocturnal LH pulsation is slow ( Fig. 16.13 B). LH pulsation increases to a circhoral pattern around a stable baseline, and estradiol production slowly begins as antral follicles develop ( Fig. 16.13 C). Estradiol levels gradually increase and serum FSH levels fall reciprocally ( Fig. 16.13 D). Upon formation of a dominant follicle, serum estradiol concentrations increase geometrically. This selectively begins to amplify the pituitary's LH response to GnRH as estradiol reaches about 90 pg/mL for over 3 days ( Fig. 16.13 E).

When the serum estradiol rises to over 200 to 300 pg/mL for 36 hours, the positive feedback mechanism is activated and the midcycle gonadotropin surge commences ( Fig. 16.13 F). Estradiol then appears to induce PR expression in the hypothalamus and pituitary. An increase in progesterone to 100 ng/dL facilitates the LH surge, shortens the duration of time over which estradiol is required for the surge to 24 hours, and brings about an FSH surge. The mechanism of progesterone action involves inhibition of GnRH cleavage. Androgens may also play a role in facilitating FSH and GnRH release. The LH surge is then primarily responsible for luteinizing the preovulatory ovarian follicle (see Fig. 16.13 F). At this time, LH pulses become larger in amplitude but slower in frequency and their apparent bioactivity increases. Ovulation then results.

As the follicle is disrupted by ovulation, estrogen levels fall ( Fig. 16.13 G). Meanwhile, serum progesterone increases steadily as the corpus luteum begins to form, and comes to be sustained at very high levels for several days, along with a lesser increase in 17-hydroxyprogesterone (17-OHP) and a return of estradiol to late follicular phase levels ( Fig. 16.13 H). In response to the high progesterone level, LH pulses become slow and large. In the absence of increasing human chorionic gonadotropin (hCG) from a conceptus, the corpus luteum's life span is exhausted and its production of progesterone and estradiol wanes. Subsequently, FSH begins to rise out of proportion to LH. Shortly after the sex steroids withdraw from the scene, the endometrium sloughs, giving rise to menstrual flow. Meanwhile, the follicular growth induced earlier by FSH begins to gain momentum and the next cycle begins.

Follicular (Proliferative) Phase Ovary

The hormonal functions of the follicle have dual purposes that must be closely coordinated: to change the milieu of the ovum to prepare for ovulation and to signal the pituitary to send the signal to ovulate, that is, the LH surge. Thus the ovary is the zeitgeber for the cycle; the normal cyclic pattern of ovarian hormone secretion induces the midcycle surge of pituitary gonadotropins, even in the presence of unchanging circhoral pulses of GnRH. Ovarian hormones also augment the amplitude of the GnRH response, which is a “fail-safe” mechanism that “guarantees” a preovulatory gonadotropin surge.

Ovarian follicular development and steroid secretion in relationship to changing gonadotropin levels are illustrated in Fig. 16.14 . FSH and LH play major roles in granulosa and thecal cell differentiation, respectively, whereas a host of local factors modulate gonadotropin action. For example, follicular maturation in response to gonadotropins is enhanced by insulin-like growth factors (IGFs), TGF-β, and fibroblast growth factor, whereas it is inhibited by TGF-α.

Primordial follicle growth and development is gonadotropin independent. Subsequently, granulosa cells of preantral follicles develop FSH receptors, and theca cells, which encircle granulosa cells, develop LH receptors (see Fig. 16.14 A). Activin causes FSH-independent upregulation of FSH receptors in preantral follicles, although it opposes FSH stimulation of antral follicle development. Primordial follicle growth is constitutively repressed by nuclear forkhead transcription factor Foxo3; when Foxo3 is released in response to stimulation of the PTEN-PI3K-Akt pathway, follicular growth progresses to the point where follicles become responsive to FSH.

Antrum formation requires a trace (prepubertal) amount of FSH receptor activation (see Fig. 16.14 B). FSH stimulates androgen receptor expression in primary follicles, and androgens in turn stimulate further expression of FSH receptors and the early stages of follicular growth. Androgen action is also necessary for the development of a full complement of follicles, and androgen excess stimulates excessive follicle number. LH stimulates the appearance in thecal cells of the enzymes necessary for androgen biosynthesis. Evidence that theca cells of small antral follicles form estradiol is meager.

As antral follicles grow over 2.5 mm in diameter, their granulosa cells begin to form estradiol from androgen supplied by theca cells (see Fig. 16.14 C). Androgen production at low levels may synergize with FSH to stimulate aromatase activity within the granulosa cells.

At this stage, follicles are increasingly FSH dependent and consequently uniformly FSH responsive. IGF-1 is required for follicular growth beyond the early antral stage in response to FSH. Antral follicles do not grow over 5 mm in diameter without a pubertal degree of FSH stimulation. By the midfollicular phase, the proliferation of FSH-responsive granulosa cells results in an accelerating rate of estradiol production and preferential conversion of androstenedione to estradiol rather than dihydrotestosterone (DHT) by these cells (see Fig. 16.14 D). Estradiol itself clearly stimulates proliferation of granulosa cells and oocyte survival in rodents. In humans, estradiol appears to promote antral growth independently of LH and is synergistic with FSH in bringing about the development of the dominant follicle.

A dominant follicle is selected at the beginning of the menstrual cycle from a crop of follicles that were recruited 2.5 months prior. Recruitment of a group of follicles is normally promoted by the midcycle FSH surge and regresses with increasing corpus luteum progesterone secretion. Another wave of follicle growth in the late luteal phase is promoted by the rise of FSH as luteal progesterone and estradiol secretion wanes. The selected follicle is the one that is the most sensitive to FSH (lowest "FSH threshold"). FSH is critically important during the follicular phase for optimal development of this dominant follicle. By the midfollicular phase of the cycle this follicle becomes virtually the sole source of estradiol (see Fig. 16.14 E). Typically, there is only one such follicle. Only this follicle continues to grow so as to reach a diameter of 10 mm or more. All other gonadotropin-dependent follicles undergo atresia.

At this stage, the rising estradiol level is suppressing FSH secretion and augmenting pituitary LH responsiveness to GnRH. FSH is more bioactive in the dominant follicle because it is more efficiently concentrated and because local factors increase ovarian responsiveness to FSH. The increased LH causes further proliferation of thecal cells and an increase in their LH receptor content. Androgen production is consequently increased. This synergizes with FSH to both augment aromatase activity and bring about increasing progesterone secretion by the well-estrogenized granulosa cells of these follicles. Progesterone then enhances the synthesis of both itself and estradiol. The increased thecal androstenedione production is diverted much more to estradiol than to DHT biosynthesis. Antral fluid steroid concentrations reflect these changes (see Fig. 16.15 ). Activin acts to prevent premature luteinization of granulosa cells, and activin tone seems to wane as the preovulatory phase approaches.

FSH next induces LH receptors in the granulosa cells, luteinizing them (see Fig. 16.14 F). Androgen and insulin synergize with FSH in this induction of LH receptors. LH then joins FSH in acting on luteinized granulosa cells to augment estradiol and progestin production.

The LH and FSH surge then occurs in response to the positive feedback action of estradiol at both the CNS and pituitary levels, an effect amplified by the rising levels of progesterone. The final steps in follicle maturation ensue rapidly: the LH surge induces granulosa cell PR and prostaglandin synthase while inhibiting cyclin gene transcription, and the FSH surge upregulates vascular endothelial growth factor. In the absence of these critical steps, ovulation and follicular rupture do not occur. Then the follicle promptly becomes desensitized to LH and FSH and ceases to grow. This is followed by an inflammatory-type response. Protease activity, prostaglandin production, and vascular permeability increase; cell junctions loosen; and cumulus cells form a mucopolysaccharide envelope around the oocyte (cumulus expansion).

Oocyte meiotic maturation resumes in response to a specific phosphodiesterase, forming the haploid gamete (secondary oocyte) and the first polar body in response to the LH surge. Ovulation of the cumulus-oocyte complex then occurs. The presence of a favorable follicular steroidal milieu is necessary both for ovulation (a premature LH surge in a subject with an unripe follicle will not result in ovulation) and subsequent developmental competence of the oocyte. Meiosis will go to completion and the second polar body will be extruded only in response to contact with a sperm.

The processes stimulating dominant follicle emergence are delicately balanced by those preventing it. It seems critical that the intraovarian concentration of androgens not become excessive. Androgen excess interferes with follicle viability beyond about the 8-mm stage and synergizes with FSH to cause premature luteinization. These interfere with the emergence of dominant follicles. Follicles arrested in their growth become atretic, and atretic follicles contain relatively high concentrations of androgens (see Fig. 16.15 ). Progesterone also suppresses further differentiation of nondominant follicles by some of the same mechanisms. High concentrations of estrogen play a critical role in inhibiting selection of the dominant follicles in primates. If there is interference with estrogenization, multiple large cystic follicles develop that are impaired in their ability to ovulate and undergo androgen-dependent atresia.

Anti-Müllerian hormone (AMH) and inhibins have emerged as other granulosa cell factors important in the regulation of follicular development. AMH is the major hormonal paracrine inhibitor of primordial follicle progression. It is produced by the granulosa cells of small growing follicles. As follicles grow, intrafollicular AMH levels rise sufficiently to inhibit recruitment of primordial follicles to the primary follicle stage; it also inhibits P450c17 activity, GnRH release, and FSH stimulation of aromatase activity. Because estradiol inhibits AMH production, there exists an intrafollicular short negative feedback loop confining AMH expression to follicles up to about 8 mm in diameter. Thus AMH appears to act as a follicular gatekeeper, ensuring that each small antral follicle produces little estradiol before selection of the dominant follicle, which allows a direct ovarian-pituitary dialogue regulating the development of the follicle selected to undergo ovulation.

Inhibin-B is the predominant form of inhibin. It arises from granulosa cells in small follicles before aromatase is expressed and is regulated by FSH in a sluggish negative feedback loop. It upregulates thecal steroidogenesis, as discussed later. Inhibin-A is a product of the preovulatory follicle (and corpus luteum) that responds to both LH and FSH.

Atresia is the fate of all except the few hundred follicles chosen for ovulation during an individual's life span. Most follicles beyond the primordial stage become atretic. Atresia occurs by the process of programmed cell death. This apoptotic process has diverse determinants, including cell death inducer and repressor genes. FSH support becomes increasingly necessary for survival as the follicle matures, and it is normally only the follicle that has the lowest FSH threshold that escapes atresia.

Factors Controlling the Onset of Puberty

Pubertal onset is under the control of a complex regulatory network that is able to dynamically respond to numerous endogenous and environmental signals. GnRH neurons play a critical hierarchical role in the direct and indirect integration of these central and peripheral signals. Reproductive development is coupled with metabolic cues that may disrupt the maturational process. The mechanisms by which neuroendocrine and genetic factors control pubertal development remain unknown. Epidemiological studies indicate that nutrition, ethnicity, and genetic factors, are normally important in the pubertal process. Environmental chemicals and chronic inflammatory disease can disrupt the process.

Evidence that there are genetic factors involved in the timing of puberty comes from multiple studies. It has been estimated that between 50% and 80% of the variation in the timing of puberty is genetically determined. Several large genome-wide association studies (GWAS) of age at menarche, examined pubertal timing in healthy females to identify the genes responsible. These studies demonstrated that there is significant genetic heterogeneity in pubertal timing in the general population that is likely to involve hundreds of common variants. The gene Lin-28 homolog B ( LIN28B ) was the first locus associated with age of menarche. LIN28B is the human ortholog of the Caenorhabditis elegans gene that controls developmental timing through micro ribonucleic acid (microRNA). Mutations in LIN28B have not been identified in humans with disorders of puberty. The 1000 Genomes Project studied genotype data in about 370,000 women and identified 389 independent signals ( P < 5 × 10 − 8) for age at menarche, with effect sizes perallele ranging from 1 week to 5 months. These signals explain only about 7.4% of the population variance in age at menarche. Genes implicated in GnRH signaling, pituitary development, hormonal regulation, fatty acid biosynthesis, and energy homeostasis have been implicated. Although mutations in these genes have been shown to cause physiological interruptions in development, their role in the initiation of puberty remains unknown. Specifically, single nucleotide polymorphisms (SNPs) in the GnRH and GnRH receptor genes have not been associated with variations in the timing of puberty in the general population.

The key in the initiation of puberty is the activation of the hypothalamic GnRH pulse generator. The molecular events that control the pulse generator include a complex interplay between both inhibitory and stimulatory factors. The mechanism of central activation of puberty first appears to be a consequence of a removal of a restraint mechanism, with a rise in gonadotropin secretion (initially during sleep). This restraint in the GnRH pulse generator is independent of the presence of gonads and more intense in males. A targeted gene approach in mice has confirmed that ERα (also termed ESR1) in Kiss1 neurons mediates feedback suppression of both Kiss1 expression and gonadotropin secretions during the prepubertal period.

However, the high levels of testosterone to which the male fetus was exposed during the period of sexual differentiation may be responsible for the more prolonged suppression of GnRH release in males than females. A role for decreased estrogen feedback sensitivity by the hypothalamic pulse generator near the time of puberty has also been shown.

Recent evidence points to an important role for the kisspeptin 1 receptor (KISS1R), a G-protein–coupled receptor (previously known as GPR54), and its ligand, kisspeptin, an excitatory neuropeptide, as a signal for pubertal GnRH release. Expression of both proteins has been found to increase before pubertal onset in association with the increase in GnRH pulse generator activity in the hypothalamus. Kisspeptin binding to its receptor on GnRH neurons stimulates GnRH secretion. Mice with knockout of Kiss1r were found to be infertile despite having normal GnRH neurons. Leptin and androgen synergistically upregulate this system, and estrogen antagonizes it. Mutations in KISS1R result in hypogonadotropic hypogonadism. However, mutations in KISS1R have not been found in boys with pubertal delay, nor have polymorphic sequences been associated with delay of pubertal development. Elegant studies in primates have demonstrated an increase in kisspeptin during pubertal development with a corresponding increase in KISS1R associated with an increase in LH. The maximum level of expression of kisspeptin and KISS1R in the hypothalamus in both males and females occurs at puberty. For each Tanner stage, girls tend to have higher kisspeptin levels than boys, potentially explaining their earlier onset of puberty.

Chronic administration of kisspeptin to immature female rats induces precocious activation of the central axis. In addition, chronic treatment with kisspeptin restores pubertal development in a rat model of undernutrition. Kisspeptin may thus not only influence the priming of puberty, but also the integration of nutritional and energy status. Although it is clear that kisspeptin activation of GnRH neurons occurs at puberty and that GnRH is increasingly sensitive to kisspeptin activation during development, other pathways contribute to GnRH activation since the hypogonadism associated with deficiency of KISS1 or KISS1R is not complete.

Neurokinin B (NKB) signaling seems to be critical for the initiation of puberty. Some kisspeptin neurons, KNDy neurons, coexpress NKB, dynorphin A, and their receptors (TAC3R and KOR), the primary function of which seems to be synchronizing kisspeptin neuron pulsatility. Receptors for NKB are also located on GnRH neurons, where they seem to modulate GnRH release or transport. Loss-of-function mutations in TAC3 and its receptor TACR3 in patients with normosmic GnRH deficiency and pubertal failure have identified a role for this neuropeptide in the control of GnRH secretion. Although kisspeptin directly regulates GnRH expression and secretion, NKB agonists failed to stimulate GnRH release in rodents. It appears most likely that a collaborative mechanism that includes both kisspeptin and NKB signaling to GnRH neurons is necessary for reproductive function in females. To investigate the interactions of kisspeptin and NKB in humans, the effects of the coadministration of kisspeptin-54, NKB, and an opioid receptor antagonist, naltrexone, on LH pulsatility were studied. Subjects receiving kisspeptin and naltrexone increased LH and LH pulsatility, whereas NKB alone did not affect gonadotropins. NKB and kisspeptin given together had significantly lower increases in gonadotropins compared with kisspeptin alone. These results suggest significant interactions between the KNDy neuropeptides on GnRH pulse generation in humans. Further, Tacr3 knockout mice are infertile, although they appear to have reversible central hypogonadism. Interestingly, a mutation in TAC3R was found in one patient with constitutional delay of growth and pubertal development (CDGP) in a study of 50 patients, whereas none have been reported in TAC3.

Disrupting mutations in makorin ring finger protein 3 (MKRN3), a paternally expressed, imprinted gene located in the Prader-Willi syndrome locus, are associated with central precocious puberty. This indicates the presence of a GnRH release-inhibiting pathway centered in the arcuate nucleus.

Initiation of puberty involves coordinated changes in transsynaptic and glial-neuronal communication. Mediating pubertal restraint are the major inhibitory systems: gamma-aminobutyric acid (GABA)ergic, some opioidergic contribution, and gonadotrophin-inhibiting hormone (GnIH), an RFamide-related peptide (RFRP). The major excitatory systems involve glutamate and kisspeptin signaling, with glial cells facilitating GnRH secretion in diverse ways ( Fig. 16.16 ). It appears that GABA receptor signaling develops in advance of glutamate signaling. Increased signaling via glutamate receptors of several types (ionotropic and metabotropic) appears to be the major proximate change in neurotransmission involved in puberty onset. At puberty, however, seemingly as a consequence of glutamate receptor signaling, GABA-A receptor signaling on GnRH neurons increases GnRH secretion. Glial cells facilitate the process through elaboration of TGFs (especially TGF-β1), IGF-1, neuregulins, prostaglandin E ₂ , and the elaboration of enzymes that control the concentration of glutamate (glutamic dehydrogenase, which catalyzes the synthesis of glutamate, and glutamine synthase, which converts glutamate to glutamine).

The basis of the change in neurotransmitter balance is becoming clearer. A second tier of control seems to be modulation of these processes by increased hypothalamic expression at puberty of tumor-suppressor genes that act to integrate glial-neuronal interactions. A yet higher echelon of candidate hypothalamic genes have been identified that are transcriptional regulators of the second-tier genes. These genes include Oct-2 , a regulator of the POU-domain homeobox genes, enhanced at puberty 1 ( EAP1 ), knock-out of which delays puberty and decreases fertility of mice, thyroid transcription factor I ( TTF1 ), yin yang 1 ( YY1 ), and CUX1 . Genes contiguous to elastin appear to be involved in the pace of puberty: deletion of chromosome 7q11.23 in Williams syndrome typically leads to an early normal onset but rapid pace of puberty with an abbreviated pubertal growth spurt. Substantial redundancy of these networks and the signaling neurochemicals exists since the onset of puberty is dependent on the expression of many genes, likely arranged in a coordinated network. The gene products may function as activators or repressors of targets important for pubertal onset and progression. Sex steroids have been implicated as important modulators in pubertal onset.

MicroRNAs, specifically the miR-200/429 family and miR-155, have been shown to be important in the epigenetic regulation of puberty by regulating GnRH gene transcription. miR-7a2 is critical for normal pituitary development and deficiency results in gonadotropin deficiency.

Thus the onset of puberty is controlled by an opposing increase in excitatory and a corresponding decrease in inhibitory signaling from neural networks targeting the GnRH neuron. Lesioning studies indicate that inhibitory tracts mainly seem to be routed through the posterior hypothalamus and stimulatory ones through the anterior hypothalamic preoptic area. These studies have been complemented by studies in genetically engineered mouse models. In one such model, the anteroventral periventricular nucleus (AVPV) population of neurons was shown to be the site of estrogen positive feedback in the control of pubertal progression, and kisspeptin cells in the arcuate nucleus of the hypothalamus were shown to be critical for estradiol negative feedback. Indeed, it appears that KNDy neurons integrate negative feedback of sex steroids to regulate GnRH secretion. Postmortem hypothalamic tissues were collected by The Netherlands Brain Bank, and sections were stained for kisspeptin by immunohistochemistry to determine the number of kisspeptin-immunoreactive neurons within the infundibular nucleus. This study showed that the number of kisspeptin neurons is greater in the infant/prepubertal and elderly periods compared with the adult period. In MTF transsexuals, but not homosexual men, female-typical kisspeptin expression was observed. The authors suggest that infundibular kisspeptin neurons are sensitive to circulating sex steroid hormones and that the sex reversal observed in MTF transsexuals might in part reflect an atypical brain sexual differentiation. Neonatal androgenization, which ablates the ability to generate a midcycle LH surge, was shown to selectively inhibit development of the AVPV population of kisspeptin neurons.

An overview of the systems involved in regulating the initiation of puberty is shown in Fig. 16.17 . Pubertal maturation and skeletal maturation seem to have common determinants. Abundant clinical evidence indicates that sex steroid hormones are among these determinants. Thus genes involved in sex steroid hormone metabolism and action are candidate regulators of the onset of puberty. There is limited and inconsistent data on the role of endocrine-disrupting chemicals on the timing of puberty, although some animal and epidemiological evidence supports the potential for some compounds to accelerate the time of pubertal onset and for others to delay the timing. Experience with diethylstilbestrol indicates that fetal exposures can have epigenetic effects. The growth hormone (GH)-IGF system is another determinant. GH facilitates the onset and tempo of puberty. Experimental studies suggest that this occurs through GH or IGF actions at all levels of the neuroendocrine-ovarian axis. Girls generally enter puberty when they achieve a pubertal bone age. Pubertal stage normally correlates better with the bone age (r = 0.82) than with the chronological age (r = 0.72, RLR unpublished data), particularly as menarche approaches. Skeletal age correlates better with menarche than chronological age, height, or weight, and its variance at menarche is half that of chronological age. The bone age at the onset of breast development averages about 10.75 years, and that at menarche averages about 13.0 years. Disorders that accelerate bone maturation, such as congenital adrenal hyperplasia (CAH) or hyperthyroidism, tend to advance the age of onset of true puberty. Disorders that retard skeletal maturation, such as GH deficiency, hypothyroidism, or anemia, tend to delay the onset of puberty. On the other hand, some data suggest that factors linked to intrauterine growth retardation, although not necessarily the growth retardation itself, predispose to sexual precocity.

Optimal nutrition is clearly necessary for initiation and maintenance of normal menstrual cycles. The hypothesis that body fat is the weight-related trigger for pubertal development originated with the discovery by Frisch and coworkers that weight correlated with initiation of the pubertal growth spurt, peak growth velocity, and menarche better than chronological age or height. Midchildhood may be a critical period for weight to influence the onset of puberty. Suboptimal nutrition related to socioeconomic factors is an important factor in the later onset of puberty in underdeveloped than in developed countries. Conversely, obesity appears to be an important factor in advancing the onset of puberty in the United States. Some of the obesity effect may be mediated by IGF-1 and adrenal androgen.

Leptin appears to be an important link between nutrition and the attainment and maintenance of reproductive competence. Leptin deficiency causes obesity and gonadotropin deficiency. Paradoxically, prolonged leptin excess can downregulate the leptin receptor and GnRH release. Leptin is secreted by white adipose cells, acting on the hypothalamus to reduce appetite and stimulate gonadotropin secretion. A critical threshold level appears to signal that nutritional stores are sufficient for mature function of the GnRH pulse-generator and, thus to be permissive for puberty. Blood leptin levels rise throughout childhood and puberty to reach higher levels in girls than boys and are positively related to adiposity and negatively related to testosterone levels. Leptin binding protein, a truncated form of the leptin receptor, falls as puberty begins, which suggests that circulating leptin becomes more bioavailable. Whether leptin has a direct role in the pubertal activation of the GnRH pulse generator is unknown. In models of leptin insufficiency, the administration of kisspeptin induced LH secretion. Conversely, leptin’s effect on puberty did not require signaling in kisspeptin neurons in other mouse models.

Other factors also link nutrition and gonadotropic function. Part of the leptin effect is mediated by inhibition of hypothalamic neuropeptide Y (NPY) formation. NPY is a potent appetite-stimulating member of the pancreatic polypeptide family that directly inhibits GnRH release during food deprivation. However, in the preovulatory state, it stimulates GnRH release, an effect mediated by a different neural network acting on a different NPY receptor subtype on the GnRH neuron. NPY is also inhibited by the anorexogenic peptide YY (PYY), a gut hormone secreted in response to food and inhibited by GH; the pubertal fall in PYY has been postulated to permit the coordinated pubertal rise in appetite and gonadotropins. Insulin may also signal nutritional status to KNDy neurons, since deletion of the insulin receptor in KNDy neurons in genetically modified mice resulted in pubertal delay and reduced serum LH levels in both sexes. Interestingly, adult fertility was not affected.

GWAS studies of pubertal timing implicated several genes associated with body weight other than leptin and leptin receptor and include fat mass and obesity-associated protein (FTO) , SEC16 homolog B (SEC16B) , transmembrane protein 18 (TMEM18) , and neuronal growth regulator 1 (NEGR1). Rare heterozygous variants in FTO have been identified in pedigrees with CDGP associated with low body mass index (BMI) and growth and pubertal delay. Mice made heterozygous for the FTO gene knockout displayed delayed puberty, but did not manifest low body mass. Other mediators linking nutrition and puberty include melanocortin (MC)3/4 receptors, signaling from alpha-melanocyte–stimulating hormone (MSH) to increase Kiss1 expression and mediate the permissive effects of leptin on puberty, and ghrelin and mutations in the ghrelin receptor growth hormone secretagogue receptor (GHSR). A small cohort of 31 CDGP patients was analyzed for mutations in GHSR , and 5 patients were found to have point mutations in this gene.

Other cues that provide information on nutritional status to the central reproductive axis may include glucose, ghrelin, and insulin. The effect of these factors on LH pulsatility may be mediated directly at the level of the gonadotroph or indirectly by changes in GnRH secretion. There is little evidence for the role of pineal secretions in human reproduction that is found in lower animals.

The essential element for the onset of puberty is an increase in pulsatile hypothalamic GnRH secretion that is regulated by a complex interplay of excitatory and inhibitory signals that have yet to be fully understood or elucidated. During childhood the activity of the GnRH pulse generating system is restrained, an awakening of the pulse generator occurs gradually during late childhood, and the tempo of GnRH neuronal activation increases during puberty. The underlying mechanisms for all these changes are unclear. The pubertal diminution in tone of the CNS centers that inhibit hypothalamic GnRH secretion during childhood has traditionally been considered to result from decreasing sensitivity of a “gonadostat” to negative feedback by sex steroids. However, this now seems an overly simplistic concept for a mechanism that seems to involve a change in the balance of neural inhibitory and stimulatory signals that impinge upon the GnRH neuron.

Many studies have been performed to help understand the initiating developmental events or the “trigger” for pubertal onset. In fact, it is becoming increasingly clear that there is no single “trigger” for puberty, but a gradual increase in GnRH pulsatility associated with a complex interplay of factors and hypothalamic developmental programs. Thus the apparent “sensitivity of the gonadostat” seems increasingly likely to reflect the degree of activity of the GnRH neuron. That is, when GnRH secretory activity is attenuated, the pulse generator is easily inhibited; when the GnRH neuron is active, the pulse generator is relatively insensitive to negative feedback.

The integration of hypothalamic signaling systems along with the developmental changes in the control of GnRH neuronal function seem to converge to trigger the onset of puberty. In the rat, structural remodeling of the GnRH neuron was demonstrated during pubertal progression by an increase in the density of dendritic and somal spines; the percentage of total neurons with spines being lowest at birth and increased gradually postnatally until puberty. The spiny processes of neurons are the location of excitatory synapses important in neuronal plasticity. The greatest percentage of complex neurons is in the peripubertal period, with the percentage decreasing after completion of puberty. These developmental changes are correlated with an increase in excitatory synaptic input to the GnRH neuron triggering the onset of puberty in mice. Which excitatory synaptic input (e.g., glutamatergic, kisspeptinergic, or yet unknown neurochemical signals) plays a role in the pubertal increase in GnRH secretion is unknown. Whether primate or human GnRH neurons undergo synaptic excitatory remodeling during development is also unknown.

Since its discovery, numerous studies have demonstrated the expression of the kisspeptin-signaling system in several peripheral sites implicating it in biological processes, such as the regulation of ovarian function, embryo implantation, placentation, angiogenesis, and insulin secretion. However, whether kisspeptin is secreted from sites of peripheral expression and the impact on the reproductive axis are currently unclear.

Regulation of Gonadotropin Secretion

An essential feature of the mature HPG axis is the long-loop, negative-feedback control of gonadotropin secretion by gonadal secretory products, as depicted in Fig. 16.17 . The generally tonic nature of gonadotropin secretion is punctuated by two prominent types of periodicity: two- to threefold pulsations of LH above trough levels at 1.5-hour intervals and, in the sexually mature female, by a transient, midcycle, preovulatory gonadotropin surge. The latter is characterized by a greater than 10-fold, rapid rise of LH and a lesser rise of FSH. This surge is brought about by positive feedback when a critical level of estradiol, facilitated by a modest rise in progesterone, is achieved for a critical period of time, as discussed in relation to Fig. 16.13 .

Estradiol, in concert with inhibin, reciprocally regulates FSH in a sensitive, log-dose, negative-feedback loop. Progesterone in high (luteal phase) concentrations is a major negative regulator of GnRH-LH pulse frequency. Androgens have a biphasic long-loop feedback relationship with gonadotropins: at modest elevations they stimulate gonadotropin release and at very high levels they inhibit it.

Estradiol exerts triphasic, and progesterone biphasic, effects on gonadotropin secretion. As estradiol rises after the midpoint of the follicular phase it selectively reduces the FSH response to GnRH, and when it reaches preovulatory levels it transiently exerts positive feedback effects on LH and, to a lesser extent, FSH. At sustained high levels estradiol suppresses both gonadotropins. As progesterone reaches a preovulatory level, it enhances the estradiol positive feedback effect, but at the higher levels that ensue during the luteal phase, it suppresses LH pulse frequency while enhancing LH pulse amplitude.

The GnRH neurons primarily responsible for maintenance of the reproductive cycle are those of the arcuate (infundibular) nucleus ( Fig. 16.18 ). GnRH neurons are inherently pulsatile. Synchrony is promoted by fluxes of ionic calcium into these cells and autocrine GnRH inhibitory feedback. GnRH secretion is modulated by the variety of neurotransmitters and growth factors involved in initiating puberty. Synchrony of the network of GnRH neurons that accounts for pulsatility is conferred when the hypothalamic concentration of GABA periodically falls from levels inhibitory to GABAA receptors in the presence of an excitatory neurotransmitter. EAP1, a hypothalamic protein previously shown to be important for pubertal onset, has also been implicated in the control of menstrual cyclicity in primates.

Sex steroid signals are in part conveyed to GnRH neurons indirectly. Regulation of GnRH secretion by estrogen involves in part induction of PRs in the hypothalamus. GnRH neuronal cell lines have been studied in which estradiol directly stimulates and inhibits GnRH gene expression under different experimental conditions. Although progesterone exerts its main inhibitory effect on GnRH secretion, it has effects at higher CNS levels and at the pituitary level. Prolactin suppresses both hypothalamic and gonadotropin GnRH receptor expression.

Other clinically relevant factors affecting GnRH release are sleep, endorphins (endogenous opioids), and interleukins. In sexually mature women, sleep inhibits GnRH pulse frequency and this effect seems to be amplified by female hormones. Endorphins are important physiological regulators of GnRH release after puberty has begun. Hypothalamic β-endorphin suppresses oophorectomy-initiated GnRH secretion, and opiate antagonists reverse this effect, as well as the sleep effect. The inhibitory effect of stress on gonadotropin release appears to be mediated by β-endorphin released from proopiomelanocortin in response to corticotropin releasing hormone (CRH). Interleukins also inhibit gonadotropin release. Serotonin seems to modulate LH pulsatility and facilitate the LH surge.

GnRH receptors on the gonadotroph are maintained in an optimally active state only when GnRH is delivered in pulses approximately 1 to 2 hours apart in man. Pulses substantially less frequent result in a hypogonadotropic state. Paradoxically, continuous administration of an initially stimulatory dose of GnRH results in downregulation of gonadotropin production, after an initial burst of gonadotropin release. This is the physiological basis for the success of long-acting gonadotropin agonists in suppressing puberty in children with true central precocious puberty. However, while gonadotropins are downregulated, free alpha-subunit production is elevated and responsive to GnRH.

Hypothalamic GnRH receptor function is modulated by autocrine and paracrine factors, including GnRH itself and kisspeptin. Pituitary GnRH receptors appear to be directly and indirectly downregulated by GnRH, gonadotropins, and inhibins, as well as sex steroids. LH and FSH themselves inhibit GnRH release (short-loop feedback) and inhibit their own release (autocrine feedback).

How is differential regulation of gonadotrope LH and FSH release accomplished in response to a single GnRH pulse? The frequency of the GnRH pulse is one determinant. An increased frequency of this signal stimulates LHβ-subunit gene expression, whereas slowing this signal stimulates FSH β-subunit and suppresses follistatin gene expression, altering the FSH/LH ratio. Pituitary adenylate cyclase activating polypeptide amplifies LH responses to GnRH while blocking its effect on FSH.

The sex hormone milieu is also clearly a major differential modulator of gonadotrope LH and FSH release. FSH is more sensitive than LH to inhibition by estrogen; this effect of modest levels of estradiol is of rapid onset and sustained. LH is the more sensitive to the stimulatory effects of higher estradiol levels; this effect is of later onset and short-lived. Similar relationships pertain in aromatase null mice. ER null mice have revealed ER-alpha as the predominant receptor isoform that conveys negative feedback regulation to the gonadotroph. Progesterone exerts both negative and positive feedback effects at the pituitary level, and these effects are antagonized by androgen. The progesterone metabolite 3α-hydroxyprogesterone suppresses FSH release.

Androgens have complex effects on gonadotropin dynamics. Normal androgen action facilitates the midcycle gonadotropin surge in response to positive feedback. Elevated testosterone increases baseline LH pulse amplitude and frequency while inhibiting the capacity to mount the gonadotropin surge. These actions appear to result from antagonizing progesterone action.

Inhibins of gonadal origin seem to be the major nonsteroidal-specific negative feedback regulator of pituitary FSH synthesis and secretion. Inhibins inhibit FSH release at the pituitary level, but they may act at a higher level as well. Serum levels of both inhibins rise upon FSH stimulation. Inhibin-B, produced by small antral follicles in response to FSH, is virtually the only inhibin moiety in blood during puberty. Its blood levels rise during the early follicular phase and then fall thereafter except for a small postovulatory peak, generally paralleling the changes in serum FSH; the latter peak may function to attenuate the FSH surge. Serum inhibin-A, a marker of the preovulatory follicle and corpus luteum, begins to rise in the late follicular phase and thereafter parallels levels of progesterone; its fall late in the luteal phase appears to contribute to the early follicular phase rise in the FSH level.

The structurally related activins seem to be important as regulators of both pituitary and ovary function. Activin is formed by gonadotropes themselves and its primary role is to stimulate FSH release. It also upregulates the activin binding protein follistatin, which arises within folliculostellate cells of the anterior pituitary. Follistatin, by competitively inhibiting binding of activin to its receptor, specifically inhibits activin stimulation of FSH secretion.

Infant and Child

Neuroendocrine Unit

The hypothalamic-pituitary-gonadal (HPG) axis is transiently active during the neonatal period. This is sometimes termed the minipuberty of the newborn ; unlike true puberty, the clinical manifestations are only nascent and do not progress. The regulation of neonatal gonadotropin secretion, like that during puberty, is incompletely known.

Serum FSH and LH are low in cord blood and remain low until estrogen concentrations fall from inhibitory levels upon disruption of the fetoplacental unit at birth. Then the LH and FSH levels of neonates promptly begin to rise in pulsatile fashion to early pubertal levels in the first week of life (see Fig. 16.5 ).

Serum LH and FSH levels rise higher in female than in male premature infants, reaching into the postmenopausal range. This sexual dimorphism seems to be related to lack of negative feedback because of lagging ovarian follicular development: antral follicle development begins near term gestational age. There is parallel hyperprolactinemia without sexual dimorphism.

At their peak between term and 4 months of age, serum gonadotropins and LH/FSH ratios are lower in girls than in boys, apparently because girls lack androgen-programmed accentuation of GnRH pulsatility. Responses to GnRH and GnRH agonist are similar to those of early puberty (see Fig. 16.6 ). In congenital agonadism, gonadotropins reach postmenopausal levels during the neonatal period.

After about 4 months postterm, gonadotropin and prolactin levels begin to gradually fall into the prepubertal range (see Fig. 16.5 ). FSH is higher in girls than in boys, a tendency that tends to persist into early childhood. This appears in part related to negative feedback by the higher activin-A and lower inhibin-B serum levels of girls than boys. GnRH secretion also appears to be greater in girls than in boys at this time.

The decline in gonadotropins may in part be related to the maturation of neural tracts that conduct inhibitory signals from the CNS and/or to an increase in hypothalamic sex steroid receptors. Hypothalamic ERs increase in a pattern reciprocal to the fall in serum gonadotropins in the rat (see Fig. 16.7 ), as do hypothalamic DHT receptors. Increasing sensitivity of the hypothalamus to sex steroid hormone negative feedback could account for the inhibitory effect of the small amounts of circulating estradiol and testosterone.

A nadir in both serum gonadotropins occurs by about 6 years of age (see Figs. 16.2 and 16.5 ). At this age, the LH and FSH response to GnRH is minimal, apparently from lack of GnRH stimulation. Furthermore, at this stage, agonadism is seldom reflected in a rise in serum gonadotropins or gonadotropin reserve.

However, gonadotropin production is not completely suppressed in midchildhood. Gonadotropins have been detected in the urine of young prepubertal children, at the limits of sensitivity of classic bioassays: LH excretion averaged 3% and FSH 15% of the adult amounts. Specific monoclonal antibody-based assays have revealed that LH falls to less than 0.2 U/L during the day whereas FSH remains detectable and that the gonadotropins produced at this stage are secreted in micropulses that approximately double in association with sleep. The gonadotropins also appear to be bioactive judging from their sensitivity to estradiol negative feedback in the primate and the active formation of antral follicles during childhood, which indicates gonadotropin stimulation, as discussed in the following section on the adult.

Between 7 and 10 years of age, even prepubertal girls experience subtle but significant increases in gonadotropin levels. This change corresponds with rising secretion of GnRH. These data indicate that the hormonal secretory pattern of the prepubertal 10-year-old child is different from that of the 7-year-old and indicate that the hormonal changes signaling the development of puberty are found late in the first decade of life, antedating by some time the development of secondary sex characteristics.

Ovary

The ovary of the infant and child is not quiescent. Initiation of growth and development of resting follicles occurs throughout childhood. The neonatal ovary typically contains an antral follicle with thecal luteinization, and the number of antral follicles approximately doubles over that in infancy by 7 years and quadruples by 9 years (see Fig. 16.3 ). All these antral follicles normally undergo atresia in childhood, and this augments the amount of stroma. As a result, by midchildhood, the ovaries of normal girls have up to five antral follicles 4 to 9 mm in diameter, and ovarian volume increases up to approximately 3.5 cc. Ovarian follicular development begins to accelerate just before the onset of clinical signs of puberty.

During the first few months of life, early pubertal blood levels of ovarian hormones are found as part of the transient activation of the HPG axis that occurs in the newborn. Serum estradiol and inhibin-B levels parallel those of FSH. In the neonatal period they begin rising to early pubertal levels, remain there for the first few months of life, and fall to low levels during childhood (see Fig. 16.8 ). Specifics about the hormonal changes are discussed later (see Normal Hormonal and Sexual Developmental Stages).

Regulation of Ovarian Secretion

Ovarian secretion results from the combined actions of LH and FSH, as discussed earlier with regards to Figs. 16.13 and 16.14 . The early follicular phase follicle functions according to the two-cell, two-gonadotropin model illustrated in Fig. 16.19 . In response to LH, androstenedione, the most abundant steroid formed in the ovary, is secreted by the theca-interstitial-stromal (thecal) cell compartment. In response to FSH regulation, aromatase then forms estrogen from precursor androstenedione in granulosa cells. FSH also stimulates granulosa cells to secrete inhibins. As by-products of the secretion of both ovarian estradiol and adrenal cortisol, androgens do not normally contribute to negative feedback regulation of gonadotropins. However, they have a biphasic effect on gonadotropin secretion: modest elevations increase GnRH pulse frequency by interfering with progesterone negative feedback and very high levels directly inhibit gonadotropin secretion.

The regulation of the intraovarian androgen concentration is critical to ovarian function. Androgens are important for ovarian function. Androgens are obligate substrates for estradiol biosynthesis. Androgens also increase recruitment of primordial follicles into the growing follicle pool and then act in conjunction with gonadotropins on granulosa cells to stimulate preantral follicle development into small antral follicles, which enhances FSH upregulation of aromatase activity. Androgens also synergize with FSH to luteinize follicles by inducing LH receptors. However, in excess androgens impair selection of the dominant follicle of women; this appears likely to result from premature luteinization of follicles, thus committing the follicle to atresia. Therefore androgen synthesis must be kept to the minimum necessary to optimize follicular development. This means that the synthesis of ovarian androgens must be coordinated with the needs of the follicle. This is achieved by intraovarian intracrine, autocrine, and paracrine modulation of LH action (see Fig. 16.19 ).

LH stimulates theca cell development and steroidogenesis and is necessary for the expression of gonadal steroidogenic enzymes and sex hormone secretion. However, once adult LH levels are achieved, further LH increase normally has little further effect on androgen levels because excess LH causes homologous desensitization of theca cells. Desensitization involves downregulation of LH receptor expression and steroidogenesis. Because steroidogenic downregulation is primarily exerted on 17,20-lyase activity, which converts 17-hydroxycorticoids to 17-KS, 17-OHP levels rise in response to increased LH levels, but the rise in androgens is limited.

A model of the intraovarian interaction among the major regulators of steroidogenesis is shown in Fig. 16.19 . Stimulation of androgen secretion by LH appears to be augmented by specific intraovarian FSH-dependent factors, such as inhibins and IGFs. These processes seem to normally be counterbalanced by other FSH-dependent processes that downregulate androgen formation as LH stimulation increases. Androgens and estrogens themselves seem to mediate at least a portion of this desensitization to LH, with estrogens being critical through an ERα-dependent mechanism.

Insulin and IGFs are important coregulators of ovarian function. Insulin upregulates theca cell LH receptor sites and action and to a lesser extent estrogen biosynthesis in response to FSH. The entire IGF system is represented in the ovary: IGF-1 augments FSH receptor expression and action. and appears to mediate GH promotion of granulosa cell steroidogenesis. Insulin is equipotent with IGF-1 in stimulating thecal androgen biosynthesis, and, although insulin can act through hybrids of the insulin and IGF-1 receptors and at very high levels interacts with the IGF-1 receptor, it appears to primarily act through its own receptor.

Androgen-expressing steroidogenic cells express a previously unrecognized protein variant, DENND (differentially expressed in normal and neoplastic development) 1A.V2, that facilitates steroidogenesis: it upregulates basal and cAMP-stimulated cytochrome P450c17 and side chain cleavage activities. The mechanism by which DENND1A.V2 regulates steroidogenesis is currently unknown. DENND1A is a member of the connecdenn family of proteins, which are involved in protein trafficking, endocytotic processes, and receptor recycling. Thus it is tempting to speculate that it acts by upregulating LH receptor signaling.

Many other peptides modulate ovarian cell growth or function in response to gonadotropins. Inhibin stimulates ovarian androgen production, whereas androgens reciprocally stimulate ovarian inhibin production. Activin opposes the inhibin effect. A variety of other ovarian peptides are also capable of modulating thecal androgen synthesis. Stimulators include catecholamines, for which an intraovarian system exists, prostaglandin, and angiotensin. Inhibitors include leptin, CRH, epidermal growth factor (EGF), tumor necrosis factor, TGF-β, and GDF9. Leptin antagonizes IGF-1 effects. TGF-β is particularly interesting because it suppresses androgen biosynthesis and stimulates aromatase activity; it also stimulates meiotic maturation of the oocyte. Other peptides acting on granulosa cells include cytokines, which have diverse effects, and AMH, which inhibits aromatase. GnRH is also capable of modulating thecal steroidogenesis. A GnRH-like protein has been described in the ovary that may act through ovarian GnRH receptors to suppress steroidogenesis in the human ovary. It inhibits FSH induction of progesterone secretion, aromatase activity, and LH receptors in granulosa cells, downregulates LH receptors, and inhibits the hCG stimulation of progesterone secretion by luteal cells.

Prolactin has complex effects on steroidogenesis. In low concentrations, it enhances ovarian estradiol and progesterone secretion by increasing LH receptors. On the other hand, high levels of prolactin inhibit ovarian estradiol and progesterone biosynthesis. Prolactin also stimulates adrenal androgen production.

Peptide Hormones

Peptide hormones act after binding to specific receptors located in the plasma membranes of target cells. GnRH receptors and gonadotropin receptors are members of the 7-transmembrane receptor family. These receptors are necessary for the actions of their cognate hormones. Receptors expressed in nonclassical sites are not necessarily functionally mature. Mature receptors signal after coupling to a guanine nucleotide (G-protein) subunit ( Fig. 16.21 ). Gs signaling activates adenylate cyclase and acts via phosphodiesterase-regulated cAMP to activate protein kinase A. Gq signaling activates phospholipase C, which acts via protein kinase C and Ca ^{2 +} ; Ca ^{2 +} may also be mobilized by other factors that influence ion channels. Phosphorylation of various cytoplasmic and nuclear proteins ultimately mediates the action of the peptide hormones, and secondarily involves the RAS and EGF signaling cascades in the case of gonadotropins. The diversity among target cells in their responses to the action of protein kinases in part relates to diversity and type of kinase, intracellular compartmentalization, substrate availability, and other differences in gene expression that are specific to each type of target cell.

GnRH is a decapeptide [pyro]Glu-His-Trp-Ser-Tyr-Gly-Leu-Arg-Pro-Gly-NH ₂ . One gene encodes the single precursor protein for both GnRH and prolactin release-inhibiting factor. GnRH not only effects prompt release of preformed gonadotropins (the “readily releasable pool”), but also stimulates the synthesis of gonadotropins (the “reserve pool”). Repeated administration of GnRH augments the pituitary responsiveness to subsequent GnRH pulses (“self-priming”). This has been ascribed partly to upregulation of GnRH receptors. GnRH has an important paradoxical effect. As discussed earlier, it acutely stimulates gonadotropin secretion, but, upon protracted, continuous administration it downregulates pituitary gonadotropin secretion. The significance of the expression of GnRH and its receptor in nonhypothalamic reproductive tissues is unclear.

An evolutionarily conserved form of GnRH (GnRH-II) acts primarily through the type 2 GnRH receptor; GnRH-II and the type 2 GnRH receptor are products of unique genes, rather than being modified products of the GnRH or type 1 GnRH receptor genes. The cell bodies of the GnRH-II neurons lie predominantly in the midbrain and only a minority project to the hypothalamic-pituitary area. GnRH-II function in humans is unknown; there is speculation that it is a neurotransmitter involved in sexual behavior. A recent study using functional neuroimaging demonstrated that kisspeptin administration enhanced limbic brain activity in response to sexual and couple-bonding stimuli and attenuated negative mood. Whether kisspeptin may become a therapeutic agent for patients with reproductive function dysfunction is currently unclear.

LH and FSH are synthesized in a single type of cell, and both are sometimes identified within the same cell. A vestigial population of hCG-secreting pituitary cells has been described. LH, FSH, and hCG are glycoprotein hormones that consist of two chains. After synthesis of these hormones on the ribosomes, the carbohydrate moieties, which constitute about 16% of the weight, are added in the rough endoplasmic reticulum and Golgi apparatus. The α chains of LH, FSH, hCG, and thyroid-stimulating hormone (TSH) are identical in amino acid sequence (92 amino acids). Although the β chain of each hormone is different in both primary amino acid sequence and length, these β chains nevertheless share 30% to 80% amino acid homology. Biological activity is conferred when an α and β chain are glycosylated and assemble within the cell. The α/β dimer is stabilized by a β-subunit derived “seat belt” that wraps around the α-subunit. Neither the isolated α nor the β glycosylated protein subunit exhibits biological activity unless noncovalently bound to one another.

The gonadotropins exhibit considerable molecular heterogeneity. The major basis for this is variation in the relative degree of glycosyl sialylation or sulfonation, steps which occur reciprocally in the pituitary gland. These differences affect in vitro and in vivo bioactivity. Polymorphisms in amino acid sequence of the LH-β and hCG-β gene also may affect the expression or bioactivity of LH or hCG. Reproductive status affects isoform distribution, with sialylated forms predominating in the hypogonadal state. Androgens increase and estrogens decrease in vitro LH biopotency by altering LH sialylation. Thus the pituitary gland contains multiple isoforms of LH and FSH that vary in bioactivity. Consequently, different pituitary LH and FSH standards, as well as serum, contain variable proportions of immunoreactive material of varying bioactivity.

One corollary of the molecular heterogeneity is that the antibodies generated from these gonadotropin moieties detect heterogeneous epitopes that are not necessarily bioactive; indeed some may even act as gonadotropin antagonists. These factors combine to cause the gonadotropin B/I to vary in a wide variety of circumstances. The purest of standards, even recombinant ones, interact very differently in the diverse immunoassay systems. Likewise, the putative level of LH or FSH in a serum sample differs substantially among immunoassays. Furthermore, bioactivity assessments vary with the bioassay model system. Monoclonal antibody–based immunometric assays yield results that correlate with, but are not necessarily equivalent to, those by bioassay. The “third-generation” immunometric assays have the advantage of being more sensitive and specific for low levels of gonadotropins in serum than polyclonal antiserum-based RIA, but B/I discrepancies remain.

The major determinant of in vivo gonadotropin bioactivity is the serum half-life. Terminal sialic acid residues retard clearance by the liver, the primary site of metabolism, whereas sulfonated ones facilitate clearance. About 10% to 15% of gonadotropins are excreted in urine according to RIA ; only about one-third of this is in a biologically active form.

LH is cleared more rapidly from the blood than FSH or hCG. LH disappears from blood in an exponential pattern: RIA indicates that the half-life of the first component is about 20 minutes and that of the second component is about 4 hours. The bioactive LH half-life is about 25% to 50% shorter. These respective components for immunoreactive FSH are 4 and 70 hours; those for hCG are 11 and 23 hours. Hormone production rates in follicular phase women, which approximate midpubertal values, are given in Table 16.2 .

Prolactin has structural and functional similarities to GH and placental lactogen. Prolactin has a considerable degree of structural heterogeneity; this results from genetic and posttranslational events within pituitary cells, as well as modifications, such as glycosylation in the periphery. Lactotrope growth and prolactin secretion are stimulated by estrogens. Prolactin release from the anterior pituitary is primarily under the control of hypothalamic inhibition, probably primarily mediated by dopamine. A prolactin release-inhibiting factor has been described within the same precursor protein as GnRH, thus providing a potential mechanism for reciprocal control of these two peptides. Prolactin secretion also is inhibited by thyroxine and is directly responsive to thyrotropin-releasing hormone (TRH). Estrogen and suckling are stimulatory. These signals may be positively mediated by α-MSH.

Inhibins and activins are members of the TGF-β superfamily and signal accordingly. Inhibin was discovered as the result of the search for the nonsteroidal gonadal hormone capable of specifically suppressing FSH. Activin was serendipitously discovered as the FSH-stimulating activity in the side-fractions in these studies. These hormones are formed by the differential disulfide-linked dimerization of two of three subunits (α, β _A , and β _B ), each encoded by a distinct gene. The combination of an α - and β-subunit yields the inhibins, inhibin-A (αβ _A ) and inhibin-B (αβ _B ). Activins are dimers of β-subunits, β _A β _A , β _B β _B , and β _A β _B (activin-A, B, and AB). Inhibin antagonizes all known actions of activin. The genes for all three subunits are differentially expressed in a wide variety of tissues. Furthermore, these factors, particularly activin, have proven to exert effects not only on gonadotropes, but within other pituitary cells, the gonads, and in nonsexual target tissues.

Steroid Hormones

The ovary and adrenocortical zona reticularis share the core of the steroid biosynthesis pathway ( Fig. 16.22 ). Gonadal cholesterol seems to be derived mostly from the cholesterol esters of low-density lipoprotein in man. Most steroidogenic steps are mediated by cytochrome P450 family members. These are the terminal enzymes in electron transfer chains, which include P450 oxidoreductase (POR) as the clinically relevant electron donor for all in the endoplasmic reticulum. The initial step in the biosynthesis of all steroid hormones is the conversion of cholesterol to pregnenolone. This is a two-stage process. The rapidity of the process depends upon the transport of cholesterol from the outer to the inner mitochondrial membrane by the steroidogenic acute regulatory protein (StAR). The conversion itself is carried out by the cholesterol side chain cleavage activity (scc) of cytochrome P450scc. The next steps are either the 3β-HSD step or 17 α-hydroxylation. 3β-HSD converts △ ⁵ -3β-hydroxysteroids to steroids with the △ ⁴ -3-keto configuration (e.g., pregnenolone to progesterone, 17-hydroxypregnenolone to 17-OHP, and DHEA to androstenedione). This step is obligatory for the synthesis of all potent steroid hormones. The type 2 3β-HSD isozyme accounts for the vast majority of the 3β activity in the human ovary and adrenal; the type 1 isozyme accounts for 3β-HSD activity in liver and skin. Pregnenolone alternatively undergoes a two-step conversion to the 17-KS DHEA along the △ ⁵ -steroid pathway: this conversion is accomplished via cytochrome P450c17. P450c17 is a single enzyme with 17α-hydroxylase and 17, 20-lyase activities, the latter being less efficient and critically dependent on electron transfer from cytochrome b . Progesterone undergoes a parallel transformation to androstenedione in the Δ ⁴ -steroid path: 17α-hydroxylation of progesterone by P450c17 forms 17-hydroxyprogesterone, but in humans P450c17 does not efficiently utilize 17-hydroxyprogesterone as a substrate for 17,20-lyase activity, so P450c17 seems to form little if any androstenedione. There is some evidence for the existence of a P450c17-independent Δ ⁴ -pathway to androstenedione, but most seems to be formed from DHEA by the action of 3β-HSD. Sulfotransferase 2A1 is uniquely expressed in the adrenal zona reticularis and requires the cofactor 3’-phosphoadenosine-5’-phosphosulfate synthase type 2. Other sulfotransferases (e.g., for formation of estrone sulfate) and steroid sulfatase (for the reverse reaction) are widely expressed.

17β-Hydroxysteroid dehydrogenase (17β-HSD) and aromatase activities are required for the formation of potent sex steroids. In the ovary, androstenedione is the major precursor for sex steroids. The conversion of 17-KS to 17β-hydroxysteroids by 17β-HSDs is essential for the formation of both androgen and estrogen: testosterone is formed in the ovary by 17β-HSD type 5 (also termed aldoketoreductase , AKR,1C3), whereas estradiol formation requires 17β-HSD type 1. Aromatase activity, effected by P450arom, is essential for estradiol formation. Alternate promoters are used by the P450arom gene in the gonads, placenta, and adipose tissue, which yields alternatively spliced forms of aromatase. The organization and regulation of steroidogenesis in the developing follicle is depicted in Fig. 16.19 .

The ovary normally accounts for about 25% of testosterone secretion in the mature female (0.06 mg daily), but it secretes about 30 times as much androstenedione (1.6 mg daily). These amounts are similar to those secreted by the adrenal. However, the ovary secretes less than 1/10 as much DHEA as the adrenal.

The “production rate” of a hormone equals its secretion rate plus (in the case of hormones formed outside of endocrine glands) the rate of formation of the hormone by peripheral conversion of secreted precursors. The “blood production rate” is calculated as metabolic clearance rate X serum concentration; in the steady state the amount of hormone irreversibly leaving the plasma compartment equals the amount entering it. Because of extensive steroid interconversions, the quantity of these hormones excreted in urine is not necessarily indicative of the amount reaching target tissues. For example, so large a fraction of urinary testosterone glucuronide is formed directly from androstenedione by compartmentalized metabolism within the liver that the range of urinary excretion of testosterone in women overlaps that in men ( Fig. 16.23 ). Estrone sulfate, like DHEAS in the androgen pathway, forms a circulating reservoir of inactive estrogen that can be returned to the active pool by hepatic sulfatase activity. The blood production rates of representative steroid hormones are given in Table 16.2 and are shown for estrogens in Fig. 16.24 . During the luteal phase of the menstrual cycle, estradiol production doubles and progesterone production rises 16-fold or more.

Sex hormones also have environmental origins. Structurally distinct biological estrogens include equine estrogens and plant-derived phytoestrogens. Synthetic estrogens include pharmacological compounds, such as ethinyl estradiol, diethylstilbestrol, selective ER modulators (SERMs), and some industrial chemicals, such as organochlorines (p,p”-dichlorodiphenyltrichloroethane [DDT] and others) and plasticizers (such as bisphenol A and phthalates). Endocrine-disrupting chemicals (EDCs) interfere with any aspect of hormone action, with mechanisms including mimicking or blocking hormone signaling through its receptor, and modulating the synthesis, release, transport, metabolism, binding or elimination of natural hormones. These compounds therefore may impact development of the reproductive tract and function of the reproductive axis. Animal studies have indicated that EDCs can impair ovarian development, inhibit ovarian follicle growth, increase follicular atresia, and disrupt steroid hormone levels.

Peripheral conversion of secreted prehormones by nonendocrine organs accounts for a major portion of sex hormone production. The ovary and the adrenal cortex are sources of prehormones, as well as secreted hormones. About 50% of serum testosterone (0.1 mg daily) normally is formed indirectly by peripheral conversion. Although 85% of normal estrogen production in women arises by secretion in midcycle, 50% of estrogen production can arise from extraglandular sources during the low-estrogen phases of the menstrual cycle. Peripheral formation of active steroids occurs in a wide number of sites, including liver, fat, and target organs. For example, the liver has high levels of 3α- 3β-, and 17β-hydroxysteroid dehydrogenase and 5α-reductase activities (see Fig. 16.22 ).

Peripheral androgen metabolism is not tightly regulated by the neuroendocrine system. It seems determined to some extent by the perinatal androgenic milieu, the effect of which is possibly mediated by GH. Postnatally, it is influenced by the sex hormone binding globulin (SHBG) level and the state of nutrition. Adipose tissue becomes a major site of conversion of androstenedione to both estrone and testosterone in the obese. Cytochrome P450 mixed function oxidases, the most important of which is CYP3A4, affect steroid efficacy by forming hydroxylated steroid metabolites of varying potency. They are subject to induction or inhibition by numerous drugs. Phytoestrogens increase estradiol bioavailability by inhibiting hepatic sulfotransferase.

Plasma steroids appear to reach their sites of action and metabolism by simple diffusion from the vascular compartment. The bioactive portion of serum testosterone seems to be the free testosterone and a portion of the albumin-bound testosterone that differs among tissues according to the diffusion characteristics of the vascular bed. About 98% of serum testosterone and estradiol are bound to albumin and SHBG. The SHBG concentration determines the fraction of serum testosterone and other ligands (e.g., estradiol, DHT) that are free or bound to albumin. It is also a major determinant of ligand egress from plasma ( Fig. 16.25 ). Some sex steroid effects may be mediated by SHBG binding to membrane receptors and activation of adenylate cyclase. A number of physiological and pathological states affect the SHBG level. It is increased by estrogen and thyroid hormone excess; it is decreased by androgen, insulin-resistant obesity, glucocorticoid, GH, and inflammatory cytokines.

Target cell metabolism influences the cell's response to the steroid hormones that reach it ( Fig. 16.26 ). The intracellular conversion of testosterone to DHT by one of the two isozymes of 5α-reductase is important for many but not all effects of testosterone, dependent upon the tissue-specific pattern of steroid metabolism. An important mode of testosterone action is via estradiol, notably within the brain. Although transformation is not fundamental to the mode of action of estradiol, estradiol effectiveness is influenced by target cell metabolism: the induction of 17β-hydroxysteroid oxidation in target tissues by progesterone, resulting in conversion of estradiol to the less potent estrogen estrone, counterbalances estrogenization. There is also evidence that novel steroid metabolites exert tissue-specific effects.

Within target cells, all steroid hormones regulate the genome similarly, starting with binding to high-affinity intracellular receptors ( Fig. 16.27 ). The steroid hormone receptors belong to the superfamily of nuclear hormone receptors. The estrogen, progesterone, and androgen receptors are, thus, homologous. Classic sex hormone effects are exerted by the interaction of steroid with receptor, not by either alone. Steroid binding triggers the dissociation of inhibitory chaperone heat shock proteins from the receptor. The active receptor-ligand complex then undergoes noncovalent dimerization and binding to its specific hormone response element on the gene. The deoxyribonucleic acid (DNA) bound steroid-receptor complex acts as a transcriptional regulator of the target gene promoter. Sensitivity to steroids is also modulated by molecular chaperone proteins that influence receptor configuration, intracellular trafficking, and receptor turnover, all which are determinants of steroid action.

The binding properties of steroids to their cognate receptors are the initial determinants of classical steroid action. Ligand-based selectivity is one element of this interaction. Estradiol is a more potent estrogen than estrone and estriol partly because it binds best to the steroid binding domain of the ER. DHT is an inherently more potent androgen than testosterone mainly because of its higher association rate constant and its lower dissociation rate constant.

The antiestrogens tamoxifen and clomiphene and the antiandrogens cyproterone acetate and spironolactone competitively inhibit the active ligands from binding to their specific receptor sites by weakly and transiently occupying receptor sites. These differences result from potent agonists snugly fitting into the binding pocket, which induces a receptor conformation different than that of antagonist-bound receptor. One such change is the C-terminal tail of the receptor flipping over to close the “door” when a potent agonist enters; this simultaneously provides a different outer surface for interaction with coregulator proteins.

Thus ligand-based selectivity arises not only because of tighter ligand binding, but because alternative ligands produce both intermediate and unique conformational changes in the receptor, which in turn induce altered receptor interactions with coregulator proteins that result in a spectrum of activities. Thus steroids do not simply switch receptors on; they induce selective functions that depend on the nature of the coregulators that are recruited to the complex. In part, this selectivity arises because different domains of these receptors mediate these different functions. For example, the activation function (AF)-1 domain of the ER mediates interactions with mitogen-activated protein (MAP) kinase and TGF-β3, whereas the AF-2 domain mediates interactions with coregulator proteins. Coactivators, in turn, regulate alternative splicing, gene activation and repression (in some cases via their dual enzymatic functions), ubiquitin-proteosome-mediated turnover of the receptor-coregulator complex, and also determine cell-specific, site-based actions, as discussed later.

Receptor-based selectivity is a second element in steroid action. There are now known to be two isoforms of each of the sex steroid receptors. The α and β forms of the ER, although homologous, are coded by separate genes. A and B forms of the progesterone and androgen receptors exist. These forms of the PR arise by transcription from alternate promoters within the same gene, whereas those of the androgen receptor arise from posttranslational modification of a single messenger (m)RNA. These isoforms have variously been shown to manifest a differential tissue expression pattern and respond differentially to antagonists. Interactions of a steroid with different forms of receptors can regulate some target genes differentially. One role of ERβ is apparently to modulate ERα activity: ERα and ERβ can have opposite actions at AP-1 and SP-1 sites, and studies of transcriptional activity in bone and breast tissue of mice indicate a restraining effect of ERβ on responses to estradiol. Thus different target tissues exposed to the same hormone may respond selectively because of a distinct repertoire of receptor isoform expression. Some examples are notable. Although both forms of ER are expressed in most target tissues, the classic form of the ER, ERα, plays the key role in regulation of LH and estrogen actions on the uterus, breasts, sex-specific behavior, and bone. In the mouse ovary, knockout experiments show that ERα is expressed in thecal cells where it prevents androgen excess in response to LH. In contrast, ERβ is expressed only in granulosa cells, where its inhibition of androgen receptor expression is critical to prevent premature follicular atresia. Both are necessary for oocyte survival and the ability of preovulatory follicles to rupture. Furthermore, loss of both causes transdifferentiation of granulosa cells to Sertoli-like cells and massive oocyte death. Liganded PR _A is essential for ovulation and is the more effective antagonist of ER action. In addition, sequence variation in hormone response elements contributes to differential gene regulation.

Effector site-based selectivity is a third variable in classical sex hormone action. In other words, the potency and character of a response to a ligand-receptor complex are not simply inherent properties of the complex. Rather, they depend on the array of effector molecules present in the site of action. Thus the array of genes expressed locally and the relative expression level of coregulators (coactivators and corepressors) are extremely important in the determination of appropriate and graded responses to a ligand by a target cell. Heterodimerization of the ER with other nuclear receptors can modulate its action. Androgens appear to exert some of their genomic effects by directly complexing with transcription factors other than the androgen receptor. Both estrogen and androgen appear to exert antiapoptotic effects in osteoblasts and osteocytes by activating a ligand-dependent, but nongenomic, kinase-mediated signaling pathway.

Nuclear receptor coactivators are critical in sensing cell-specific environmental signals and to coordinate signals emanating from membrane receptors with nuclear receptor action. Surface receptors send signals through kinase pathways that result in specific serine/threonine phosphorylation patterns of coactivators. These phosphorylation patterns serve as a code for the coactivator to preferentially bind and activate distinct sets of downstream transcription factors (see Fig. 16.27 ). Overexpression of steroid receptor coactivator-3 (SRC-3) is as important in the pathogenesis of some breast cancers as is ER positivity.

The effects of a given ligand-ER complex often differ from those of the estradiol (E2)-ER complex among cell types. This is the basis for the development of SERMs: these compounds exert effects in a tissue-specific manner depending on the cell context. The chemical structure of a SERM—or any ER ligand, for that matter—determines the configuration of the ER, resulting in a spectrum of activities from agonist to antagonist, depending on which coregulators are available for recruitment in the target cell. Raloxifene is an estradiol agonist in bone and epiphyseal cartilage but is antiestrogenic in uterus and breast; tamoxifen is estrogenic in uterus but antiestrogenic in breast and bone. Both appear to retain neural and endothelial estrogenic activity.

Nonclassical mechanisms play a role in sex steroid action. The nonclassical mechanisms are of two general types: (1) genotropic estrogen response element (ERE)-independent signaling, in which liganded ER acts as a coregulator of other transcription factors that act through their specific DNA response elements and (2) nongenotropic signaling, in which E2 binding to membrane-associated receptors, including ERα, rapidly stimulates phosphorylation pathways.

The nongenomic effects via membrane signaling occur rapidly (within minutes) and can mediate cell proliferation, apoptosis, and migration in cell-specific ways. Nongenomic E2 actions account for most of the LH-inhibitory and energy balance effects of E2. These effects can be mediated by binding to nuclear ER in plasma membrane domains provided by scaffolding proteins, such as caveolin. On such platforms, the E2-ER complex acts like a membrane receptor, coupling with G-proteins and activating cytoplasmic pathways involving SRC and MAP kinase. Androgens appear to act similarly. Nongenomic actions of nuclear PR have also been reported. Some nongenomic effects seem to involve the activation of novel G-protein–coupled transmembrane receptors for E2 and progesterone that interact either with steroids or their metabolites.

Genomic ER signaling may also be ligand independent. For example, cell membrane signaling by growth factors or other peptides stimulate ER phosphorylation. EGF activates phosphorylation of the ER and simulates diverse estrogen effects. Activation of unliganded ERα seems to be involved in repressing expression of the androgenic 17β-HSD testicular isoform in the ovary.

Steroids that act by binding to membrane-bound receptors in the brain are termed neuroactive . Neuroactive steroids synthesized in the brain are termed neurosteroids . The best documented of these effects are on neurotransmitters, which control ion channels. Allopregnanolone (3α-hydroxy-5α-tetrahydroprogesterone) and 3α-androstanediol are GABA _A receptor agonists and so have sedative and antiepileptic properties. Pregnenolone sulfate and DHEAS have the opposite effect, the former also stimulating the glutamate receptors. Receptors for 5-hydroxytryptamine have been implicated in mediating some of the effects of sex steroids and certain of their metabolites. Some estrogen effects in brain are membrane-mediated.

The tissue-specific posttranscriptional events involved in sex steroid signaling are poorly understood. Estradiol and progesterone modulate the actions of each other through effects on their specific receptors: increased estrogens in the preovulatory phase of the cycle upregulate target organ receptors for both estradiol and progesterone; luteal phase levels of progesterone then suppress the production of both receptors. Estrogen prevents bone loss by blocking the production of proinflammatory cytokines. Androgen action has been reported to be mediated by prostaglandins in genitalia, and testosterone stimulates the IGF-1 system in epiphyseal cartilage.

Genital Tract

The Müllerian system of the embryo gives rise to the uterus, cervix, upper vagina, and fallopian tubes in the absence of AMH secretion by fetal testes during the first trimester of gestation. Genital swelling develops to engulf the base of the penis-like clitoris between 11 and 20 weeks’ gestation in parallel with the development of the ovarian follicular system. ERs are expressed in the labia minora, prepuce, and glans in females, but not in the homologous structures of males. An association between antiestrogen and genital ambiguity has been reported. Diethylstilbestrol induces dysplasia of the genital tracts. These data suggest that estrogen may play a direct role in female genital tract differentiation. However, knock-out of ERs has no obvious effect on genital tract differentiation.

The infantile uterus and cervix enlarge under the influence of estrogen during puberty. The endometrium and cervical glands then undergo cyclical changes in concert with cyclic ovarian function. In response to rising estrogen during the follicular phase of the cycle, the endometrial epithelium and stroma proliferate. The uterine glands increase in number and lengthen. Endometrial hyperplasia is prevented by progestin and androgen excess. In response to progesterone secretion after ovulation, the endometrium increases in thickness: stromal edema occurs, and the uterine glands enlarge, become sacculated, and secrete a glycogen-rich mucoid fluid. The coiled arteries lengthen further during this time and become increasingly spiral. These changes are critical to permit implantation. High-dose progestin is an effective postcoital contraceptive because it prevents implantation when taken within 3 days of unprotected intercourse.

Endocervical gland secretions lubricate the vaginal vault. The endocervical mucus is scanty and relatively thin during the low-estrogen phase of the cycle. The increase in mucus flow with advancing follicular development seems to require tissue-specific stimulation of the cystic fibrosis transmembrane regulator by estrogen. Cervical mucus becomes more viscous and elastic as estrogens rise in the later follicular phase of the cycle—the extent to which it can be stretched into a long spindle, spinnbarkeit, is a function of the estrogen level.

The mucosa of the vagina and the urogenital tract is comprised of hormone-responsive stratified squamous epithelium ( Fig. 16.28 ). The basal layer is the regenerative area. In the absence of estrogen, there is only a parabasal layer of cells over this, and the vagina is thin, with a tendency to alkalinity, which predisposes it to local infection (nonspecific vaginitis). In response to estrogen, epithelial proliferation occurs, with formation of successive intermediate and superficial layers. With this maturation, the cytoplasm of each cell first expands, leading to formation of small intermediate cells. With further estrogenization, the nuclei become pyknotic and large intermediate cells form. Greater estrogenization brings about their transformation to cornified squamous superficial cells: the cytoplasm changes from basophilic to acidophilic with the accumulation of glycogen. Resistance to infection of the fully developed vaginal mucosa results from its thickness and from its acid pH, which occurs from the fermentation of the glycogen of the superficial cells. In response to luteal phase progesterone, degenerative changes appear in vaginal mucosal cells: superficial cells decrease, the cytoplasm assumes a “crinkled” appearance, cells degenerate, and bacterial proliferation increases.

Vaginal smears show the characteristic cyclic changes in the cell types comprising the vaginal epithelium (see Fig. 16.28 ). In the prepubertal years, parabasal cells predominate, and characteristically 10% or less are small intermediate cells. A pattern consisting entirely of intermediate cells is typical of early puberty. The early follicular phase of the menstrual cycle is characterized by the predominance of large intermediate cells with few, if any, superficial cells. Peak maturation is reached at midcycle, at which time 35% to 85% of the cells seen on vaginal smear are superficial; the remainder are large intermediate cells. This cornification develops over a 1-week period in response to estradiol levels of about 70 pg/mL and persists 1 to 2 weeks after estrogen withdrawal (see Fig. 16.11 ).

Progesterone antagonizes estrogen effects on the vaginal epithelium and cervix. Inhibition of cervical ripening by progestins is used to prevent recurrent spontaneous preterm delivery.

Many normal variations have been recognized in the appearance of the hymen. The transverse diameter increases with age.

Mammary Glands

Multiple rudimentary branching mammary ducts are found beneath the nipple in infancy; they grow and branch very slowly during the prepubertal years. Estrogen stimulates the nipples to grow, mammary terminal duct branching to progress to the stage at which ductules are formed, and fatty stromal growth to increase until it constitutes about 85% of the mass of the breast. GH (via IGF-1) and glucocorticoids play a permissive role. These hormones interact with breast stroma and local growth factors to stimulate the development of breast epithelium. Lobulation appears around menarche, when multiple blind saccular buds form by branching of the terminal ducts. These effects are caused by the presence of progesterone. The breast stroma swells cyclically during each luteal phase. Full alveolar development normally only occurs during pregnancy under the influence of additional progesterone and prolactin. Prolactin does not play a role in breast growth without priming by female hormones.

Estrogen and progesterone also play a role in breast cancer susceptibility. Earlier than average age at menarche is a modest risk factor for breast cancer, regardless of BRCA status; notably, however, breast cancer risk has not been shown to be increased in precocious puberty, The BRCA1 gene normally restrains mammary growth, at least in part, by inhibiting expression of ERα and PRs, and cancer-related mutations reverse these processes.

Pilosebaceous Unit

The pilosebaceous unit (PSU), with but few exceptions, consists of both a piliary and a sebaceous component. Androgens are a prerequisite for the growth and development of PSUs in their characteristic pattern. Androgens exert their effects both on the dermal papilla, which regulates the hair growth cycle, and on PSU epithelium. Before puberty, the androgen-dependent PSU consists of a prepubertal vellus follicle in which the hair and sebaceous gland components are virtually invisible to the naked eye ( Fig. 16.29 ). Under the influence of androgens, in the sexual hair areas, the PSU switches to producing a medullated terminal hair follicle that expresses a unique type of keratin that is androgen responsive. The difference in the apparent density of sexual hair between men and women is caused by differences in the density of terminal hairs that develop in response to androgen. In the balding-prone area of scalp in individuals genetically predisposed to male-pattern alopecia, androgens weakly attenuate the hair growth cycle, so that the PSU gradually generates only vellus follicles. In acne-prone areas, androgen causes the prepubertal vellus follicle to develop into a sebaceous follicle, in which the sebaceous epithelium develops and the hair remains vellus. Adrenarchal levels of androgens suffice to successively initiate sebaceous gland development and the growth of pubic hair. Progressively, greater amounts of androgen are in general required to stimulate terminal hair development along a pubic to cranial gradient. All these effects of androgen are to some extent reversible by antiandrogens.

Estrogens modestly stimulate hair growth, probably by inhibiting the catagen (resting) phase of the hair cycle; this may well be caused by induction of androgen receptors by estrogen. Estrogens also directly inhibit sebum secretion. GH synergizes with androgen action on the PSU, in part via IGF-1 signaling. Retinoic acid receptor agonists antagonize the effects of androgen on the sebaceous gland by inhibiting sebocyte proliferation and differentiation. Insulin, prolactin, glucocorticoids, thyroxine, and catecholamines also play roles in PSU growth, development, and function.

Bone

Increased secretion of sex hormones clearly initiates the pubertal growth spurt. About half of this effect of sex hormones is caused by their stimulation of the GH-IGF axis. The remainder of the effects of sex steroids on skeletal growth is direct.

Differences between the actions of sex hormones contribute to women’s bones being shorter and narrower than men’s. The basis for these differences are diverse and involve interactions with IGF-1 and effects on cortical, cancellous, and periosteal bone formation.

Estrogen and androgen both stimulate epiphyseal growth. Estradiol is the critical hormone that brings about epiphyseal closure. Estrogen also is particularly effective in reducing bone turnover. To some extent these effects may be prenatally programmed. Bone accrual during puberty is a major determinant of adult fracture risk. Menarche after 15 years carries a 1.5-fold increase in fracture risk, and the risk rises with age of menarche.

Adipose Tissue

Women have a greater percentage of body fat than men. During puberty, they develop both more and larger fat cells than men in the lower body, which favors a lower body (gluteofemoral) fat distribution, in contrast to men’s upper-body (visceral) fat accumulation. The critical periods for establishment of the adipocyte population are fetal life and adolescence, after which lipid accumulation occurs primarily by cell hypertrophy. Serum levels of leptin rise throughout puberty to reach higher levels in females than males, whereas levels of the antilipolytic adipocytokine adiponectin remain stable in females but fall in males in response to androgen.

Insulin signaling is of major importance to the size and function of adipose tissue—stimulating adipogenesis (development of preadipocytes into adipocytes) and lipogenesis, while inhibiting lipolysis. Beta-adrenergic catecholamines stimulate lipolysis, countering inhibition by insulin. Visceral white adipose tissue (VWAT) lipolysis is less sensitive to insulin and more sensitive to catecholamines than subcutaneous SCWAT.

Androgens cause a masculine physique at puberty primarily by inhibiting adipogenesis reciprocally to stimulating myogenesis. They act by inhibiting adipogenic differentiation of human mesenchymal pluripotent stem cells reciprocally to their stimulation of the myogenic lineage, in a dose-dependent fashion. Local androgen generation by adipose tissue as it differentiates in response to insulin likely serves to limit insulin-generated adipogenesis. In adipocytes, androgen has been reported to inhibit lipogenesis. Androgen also inhibits catechol-stimulated lipolysis in female SCWAT, opposing the effect of insulin, but not in omental WAT.

Estrogen has been reported to suppress lipogenesis through inhibition of adipocyte lipoprotein lipase activity, according to most in vitro studies. Estradiol attenuates the lipolytic response to catecholamines, specifically in SCWAT adipocytes, but promotes lipolysis by stimulating hormone-sensitive lipase. The development of obesity in postmenopausal women and oophorectomized animal models has led to the concept that estrogen deficiency causes obesity, but the role of FSH elevation in mediating this obesity by inhibiting the formation of brown adipose tissue confounds the interpretation of all such studies. Progesterone generally counters estrogen and androgen effects on white and brown adipose tissue in experimental models.

Female sex steroid effects on serum lipids are modest. Physiological (transdermal) estradiol replacement slightly raises high-density lipoprotein cholesterol (HDL-C) and lowers very low-density lipoprotein (VLDL) triglycerides. The lower LDL-C during the normal luteal phase seems primarily because of its consumption by the corpus luteum as steroidogenic substrate. Oral estrogen replacement therapy raises HDL-C more than does transdermal replacement, but differs from it in raising VLDL-triglycerides and decreasing LDL-C, whereas androgenic progestins lower HDL-C.

Progesterone deficiency is responsible for the increased postprandial chylomicron triglyceride concentrations that occur as the result of low peripheral lipoprotein lipase activity when the pituitary-gonadal axis is acutely suppressed. In humans, use of the progesterone analogue megestrol acetate is approved to stimulate appetite and weight gain; use of progestins is associated with insulin resistance.

Muscle

Testosterone administration increases muscle mass and decreases fat mass reciprocally. Androgen does so by promoting the commitment of mesenchymal pluripotent stem cells to the myogenic lineage while inhibiting adipogenesis. Testosterone effects are exerted via androgen-receptor–mediated and nonclassical pathways. Human skeletal muscle formation of DHT is mediated primarily by type 3 5-alpha-reductase. Androgens then exert dose-related stimulation of muscle cell hypertrophy, as well as hyperplasia along with associated tissues, such as motor neurons. Hyperandrogenic women have increased muscle mass and strength, which seems to give them an advantage in athletic competition. Consequently, there is active debate about regulation of women’s androgen levels in elite athletic competition.

Central Nervous System

Concordance of gender identity (self-identification as male or female) and gender orientation (sexual preference) with gender assignment on the basis of genital anatomy is the norm, which is consistent with an important role of androgen in programming these aspects of neuropsychosocial development. However, nonhormonal genetic and epigenetic factors influence sexually dimorphic aspects of human development.

Before sex hormone differences are detectable, several genes are differentially expressed in the brains of male and female mice. Sex chromosomes directly program sexually dimorphic neuronal differentiation and behaviors, such as aggression, parenting, and social interaction. The maternally inherited X chromosome is preferentially expressed in glutamatergic neurons of the cerebral cortex, and sex-specific imprinting of autosomal genes of the hypothalamus is common and appears to be the default state in females.

Testosterone exposure during both the period of transient activation of the HPG axis in the fetal-perinatal period and again during puberty plays a role in organizing neural gene expression and development in a sexually dimorphic manner according to extensive studies in animal models, which are consistent with observations in humans. The critical period for this hormonal programming on behavioral patterns closes after puberty. This has consequences both for sex-typical neuroendocrine function and sex-typical and nonsexual behavior that is activated by the pubertal hormonal milieu.

The critical period for hormonal sensitivity of sexually dimorphic areas of the brain occurs during the early newborn period of rodents, which is thought to be comparable to the early second trimester of humans. In rats, the preoptic nucleus of the hypothalamus is larger in males, and treatment of newborn females with testosterone (or estradiol) permanently increases neuronal development to duplicate this effect and causes subsequent masculinized sexual behavior and anovulation. The anovulation results from masculinization of the LH secretory pattern (that is, both increased LH secretion and suppressed capacity to mount LH surges in response to estrogen priming) that appear to be the consequence of permanent suppression of hypothalamic PR expression.

Testosterone administration to experimental animals stimulates the growth of sexually dimorphic brain areas to adult male size. Maintenance of differences in adult nuclear size and androgen receptor expression of sexually dimorphic areas of the brain is dependent upon the ambient androgen level. Peripubertal testosterone and female hormone administration have different effects on behavior.

Several human brain structures are sexually dimorphic, some becoming so at puberty. Characteristic cortical and subcortical sex differences are discernable at puberty by structural magnetic resonance imaging (MRI), although most human brains have a mosaic of “male” and “female” features. Fetal amniotic testosterone reportedly correlates positively with many of the regional male sexual dimorphisms in the gray matter, including the amygdala, as with diverse differences in gender behavior. Functional MRI has shown endogenous testosterone levels to correlate with, and exogenous testosterone administration to females to activate, amygdala and parahippocampal regions and other brain areas in response to social-affective stimuli. Studies in transsexuals have shown that virilizing doses of testosterone affects the size of specific cortical and subcortical areas of the brain, and antiandrogen/estrogen treatment robustly inhibited hippocampal size.

Some testosterone effects are androgen specific. However, many testosterone behavioral effects appear to be mediated by intraneuronal aromatization of testosterone to estradiol in a manner that is regulated in site-specific fashion by androgen and estrogen. Thus it has been postulated that low levels of estradiol promote the development of the brain and greater amounts masculinize it. These higher levels of estradiol are generated in the male brain by neuronal aromatization of circulating testosterone. ERα knock-out in female mice reduces sexual behavior and parenting behavior, while increasing aggressiveness.

The mechanism by which estrogens mediate androgenic masculinization of rodent sexual behavior involves prostaglandin E ₂ mediation. Estradiol acts through ERα to induce microglia, the resident immune cells of the brain, to secrete prostaglandin E ₂ , which reduces DNA methyl transferase activity so as to release epigenetic repression of the default female behavioral state; this initiates enhanced neurite dendritic spine formation and masculinized behavior. Release of epigenetic repression appears to be an important postreceptor mechanism of testosterone action in the brain, also affecting embryonic neural stem cells. The involvement of prostaglandin E ₂ in mediating the androgen effect on neural synapsing lends credence to a role for androgen excess contributing to autism spectrum disorders.

This hormonal organization of the brain involves hormone-specific effects on cell proliferation versus survival and synapse formation versus pruning. Androgens have a trophic effect on the dendritic spine cells of sexually dimorphic nuclei of rodents that promotes increased synaptic density. Estrogen alters the pattern of synaptic connections in spatially-specific and precise patterns that appear to fine-tune the sensitivity of certain regions of the brain to excitatory and inhibitory amino acids. Hypothalamic changes in synaptic remodeling have been correlated with the preovulatory surge of GnRH. There is also sexual dimorphism in cerebral progestin receptors, and progesterone attenuates testosterone effects on the brain. Progestin-estrogen administration is neuroprotective in animal studies. These hormones may counteract brain and spinal cord injury in adult women, but not in the sexually immature state.

On average, women tend to perform better than men on tasks that involve object memory, verbal skills, processing speed and accuracy, and fine-motor skills, whereas men tend to excel in visual-spatial memory, while the sexes do not differ in vocabulary or math skills. These differences are quantitatively modest, of the order of 0.4 to 1.0 standard deviation (SD), leading, therefore, to large overlaps in these skills among the sexes. The male advantage in visual-spatial skills is established by 4.5 years of age. Because both boys and girls who are congenitally sex hormone deficient are relatively poor in visual-spatial abilities, and sex hormone treatment at puberty does not ameliorate these deficits, this difference seems to be the result of estrogen-mediated patterning in both sexes. The extent to which this difference is innate or because of sociocultural factors is a subject of considerable debate.

A wide variety of gender dimorphic behaviors are found in young children, but normally they have a different character than in adults. Gender identity is established in midchildhood, probably by 3 years of age. Sexual orientation is established by 10 years of age; it has been postulated that this is dependent upon adrenarche rather than true puberty. Early pubertal amounts of androgen or estrogen have little effect on sexual behaviors, but increase some aspects of aggressive behavior. Only in later puberty is there activation of the sex drive, which has been programmed in earlier development.

Discordant gender identity (transsexualism/transgenderism) and sexual orientation (homosexuality, bisexuality) occur in a small proportion of the population. Their prevalence seems to be increased in disorders of sex development (DSD). Studies of DSD indicate that a male level of androgen acting through the androgen receptor pre- or perinatally is an important determinant of male gender behavior (role) and mildly disruptive to female gender identity. However, DSD is uncommon among homosexuals and transsexuals, whereas heritability estimates approximate 20% to 60%. Neuroimaging studies suggest that these disorders have a biological basis. Homosexuality is associated with loss of sex differences in brain structures and transsexuality with less differentiation of brain areas dealing with body and self-perception. Neuroimaging indicates that male homosexuals have a pattern of nuclear activation in response to pheromone-like chemosignals resembling heterosexual women rather than that of heterosexual men, and homosexual women have an intermediate type of activation.

Androgen and estrogen metabolites in sweat and urine, which contain unusual steroids, such as androst-4,14-diene-3-one, have been found to exert sexually dimorphic activation of the anterior hypothalamus that is independent of their odor. Therefore they appear to act as pheromone-equivalent chemosignals. Human pheromones appear to modulate the timing of ovulation and mood. It is likely that a dedicated population of olfactory receptors that project to GnRH neurons act as pheromone receptors.

Other Targets of Sex Hormone Action

Sex steroid hormones affect a wide variety of tissues in ways that are often unrecognized. An estrogen effect on stabilizing muscle integrity has been noted in muscular dystrophy.

Autoimmune disorders are in general more common in females, particularly after puberty. Estrogen downregulates blood levels of the inflammatory cytokine interleukin-6 and thymic autoimmune regulator ( AIRE ) gene expression. Progesterone has a similar effect and androgen the opposite effect on AIRE. Sex dimorphism in predisposition to autoimmune disorders is partly explicable by sex hormone action on AIRE network genes during the neonatal minipuberty. However, sex genotype influences the autoimmune system independently of sex steroids.

The cardiovascular effects of estrogen include upregulation of estrogen and PRs in vascular tissue and nongenomic effects on endothelial nitric oxide synthase. Estrogen improves the disturbed endothelial dysfunction of young hypogonadal women and is necessary for the cardioprotective effect of exercise. Estradiol replacement therapy, oral or transdermal, lowers blood pressure, although estradiol causes salt and water retention. This contrasts with contraceptives containing the more potent estrogen ethinyl estradiol, which raise blood pressure significantly, unless containing an antimineralocorticoid progestin.

Estrogens and progestins also exert hemostatic effects that are associated with increased resistance to the anticoagulant action of activated protein C. Combined oral contraceptives containing estrogen carry about a fourfold increased risk of venous thromboembolism in first-time users. The risk falls with decreasing dose of estrogen and duration of use and rises about 50% in those containing third-generation (e.g., desogestrel) and antiandrogenic progestins. Nevertheless, the risk is less than that of pregnancy. Progestin-only contraceptives are not associated with any increased risk of venous or arterial thrombosis.

The differences between the sexes in lipid levels are not explained by physiological differences in estrogen levels. Although oral estrogens raise triglycerides, this is caused by a first-pass hepatic effect. Differences in androgen (lowers HDL-cholesterol) and progesterone (lowers triglycerides and HDL-cholesterol) levels only explain part of the difference.