Puberty and Its Disorders in the Male

Development of the Gonadotropin-Releasing Hormone Neuronal Network

In postnatal life, the gonadotropin-releasing hormone (GnRH) neurons are located in the hypothalamus. These neurons produce intermittent discharges of GnRH into the hypophysial-portal circulation to stimulate the pituitary gonadotropes to synthesize and secrete the gonadotropins follicle-stimulating hormone (FSH) and luteinizing hormone (LH).

The development of the HPG axis is exceptional in that GnRH neurons develop in metazoan embryos outside the central nervous system (CNS). Immature GnRH neuronal precursors become detectable in the olfactory placode and in the anlage of the nose (vomeronasal organ) from an early embryologic stage (around the sixth week of gestation in human and embryonic days E10.5–E11 in mice) and then begin their complex journey toward the hypothalamus. The embryonic migration of GnRH neurons to the hypothalamus is key for the development of the neuroendocrine network that allows normal pubertal development.

The pathway along which the GnRH neurons travel includes scaffolds formed by olfactory, vomeronasal, and terminal nerves 3 and 4. Migratory GnRH neurons are known to have receptors for at least 20 signaling molecules. They receive a plethora of guidance and movement-inducing messages during their journey, which appear to be distinct depending on the stage of their migration. These signals may act directly on GnRH neurons or indirectly on olfactory axons, as disruption of the olfactory tract “scaffolds” themselves can disrupt GnRH migration. The molecular signals involved include adhesion molecules (e.g., neuronal cell adhesion molecule [N-CAM]), extracellular matrix molecules (e.g., heparin sulphotransferases), cytokines (e.g., leukemia inhibitory factor, hepatocyte growth factor), those controlling cell-to-cell interactions (membrane receptors [e.g., neuropilin-2]), and transcription factors (e.g., Ebf2,) as well as both chemoattractants and chemorepellents (e.g., Reelin). Gradients of chemokines (e.g., CXCL12) may be particularly important for promoting and guiding the movement of GnRH neurons. This combination of factors has a high degree of redundancy, such that loss of any one factor may not fully disrupt GnRH neuronal migration.

The neurons begin their migration in the nasal compartment in or around the vomeronasal organ, pass the cribriform plate and penetrate the CNS in close proximity to the olfactory bulbs, and finally migrate apposed to a subset of vomeronasal nerves that diverge caudally into the basal areas of the forebrain. At the end of their journey, GnRH neurons diverge from guiding axons to disperse into their final positions in the septohypothalamic region, including the diagonal of Broca, the medial septum, and the preoptic areas of the hypothalamus. Eventually, these neurons reach the hypothalamus where they extend projections to the median eminence to form a network, which completes this aspect of GnRH neuronal development.

The whole process of migration involves no more than a few hundred neurons per hemisphere in mice and several thousand in primates or humans. The absolute number of GnRH neurons required for pubertal development is not known, but there appears to be a degree of redundancy in the system. Rodent studies suggest that around 12% of the GnRH neuron population is sufficient for pulsatile gonadotropin secretion and puberty onset, whereas between 12% and 34% are required for estrous cycling and ovulation in adult female mice. In addition, adult Reeler mice (which were initially identified based on an ataxia phenotype) have significantly fewer GnRH neurons in the hypothalamus and display a phenotype of delayed pubertal maturation and low fertility.

GnRH neurons extend their neurites to the median eminence under the control of factors that remain mostly unknown. Fibroblast growth factor receptor 1 (FGFR1) signaling is known to be important for this process of axon extension, as evidenced by reduced projections to the median eminence in transgenic mice expressing a dominant negative FGFR1 (dnFGFR1) in GnRH neurons.

The final stage of GnRH neuronal development comprises functional activity. By the 15th week of human gestation, the GnRH pulse generator is modulating the function of the fetal gonadotropes. The entire HPG axis is functionally active for the first time during fetal development, and after a brief perinatal pause, it continues to function in infancy (during the so-called minipuberty ), until it enters a relative quiescent state, often referred to as the juvenile pause or prepuberty .

As discussed subsequently, pathological abnormalities of pubertal development have been identified with each aspect of GnRH neuronal development: (1) defects in the synthesis of GnRH, which result mainly from an abnormal migration of GnRH neurons from the olfactory placode toward the hypothalamus during the first trimester of fetal life; (2) defect in the maturation and function of the GnRH neuronal network; (3) loss of function of GnRH itself or its receptor also known as a defect of the bioactivity of GnRH ⁴ ( Fig. 18.1 ).

As a prismatic example , ANOS1 , previously known as KAL1 , encodes anosmin-1, an extracellular matrix protein that regulates axonal pathfinding and cellular adhesion. Anosmin-1 promotes branching of olfactory bulb neurons. Subjects with ANOS1 loss-of-function mutations have arrest of both olfactory bulb neurons and GnRH neurons at the level of the cribriform plate, resulting in abnormal sense of smell and hypogonadotropic hypogonadism (HH). Additional genetic factors involved in GnRH neuronal development and function are described in detail in the “Delayed Puberty, Hypogonadotropic Hypogonadism” section later in this chapter.

Pituitary Development

The pituitary consists of the anterior lobe (or adenohypophysis) and the posterior lobe (or neurohypophysis), as well as the pars intermedia and the infundibulum (or pituitary stalk). The anterior lobe derives from epithelial precursors/oral ectoderm and the posterior lobe derives from neural ectoderm, both under the control of a cascade of transcription factors. The spatial and temporal expression of these factors is critical, and mutations in the genes encoding these factors can lead to HH, as well as combined/multiple pituitary hormone deficiencies (see “Delayed Puberty” section later; reviewed in ).

Organogenesis of the pituitary gland begins during the fourth week of gestation with an upward extension of the oral ectoderm (to form Rathke’s pouch) toward the neural ectoderm. Simultaneously, the neural ectoderm of the ventral diencephalon extends downward, allowing for the connection of these two elements and leading to the eventual formation of the composite, two-lobe structure of the adult pituitary.

Initiation of organogenesis and formation of Rathke’s pouch involves signaling molecules, such as Hesx1, Otx2, Pitx1/2/3, Sox2, FGF8, and Lhx3. Subsequent cell lineage determination depends on molecules, such as Prop1, POU1F1 (Pit1), and Nr5a1(SF1) among others. Because of the complex pattern transcription factor control, different molecules are key to the development of different cell lineages at different times. For example, Lhx3 and Hesx1 affect the development of all lineages as does Pitx1/2/3; Prop1 affects differentiation of gonadotrophs, thyrotrophs, somatotrophs, lactotrophs but perhaps plays a less independent role in development of corticotrophs; POU1F1 regulates differentiation of thyrotrophs, somatotrophs, and lactotrophs; Nr5a1 largely affects gonadotrophs. As noted, in the absence of functional redundancy, mutations in the genes for these regulatory factors can lead to pituitary hormone deficiencies, with the hormones affected largely predicted by the cell types whose development is dependent on the particular transcription factor. However, some mutations, such as in HESX1 or OTX2 , can lead to combined pituitary hormone deficiencies, as expected, but also to isolated growth hormone (GH) deficiency and to septooptic dysplasia.

The pituitary develops early in gestation and, similar to the GnRH neuronal network, becomes functional in utero. The gonadotrophs are the last anterior pituitary cell types to develop, but gonadotropin release is detectable by week 14 in response to GnRH secretion. Gonadotropin secretion peaks around 20 to 22 weeks in utero and subsequently increases and decreases in accordance with the dynamic hypothalamic GnRH secretion, described earlier, and with positive and negative feedback loops, described later.

Testicular Development

A brief summary of prenatal testicular differentiation and development follows; a more detailed description of testicular differentiation and sex development can be found in Chapter 6 . Beginning at approximately 4 to 6 weeks of gestation, the primordial bipotential gonad arises from a condensation of the mesoderm of the urogenital ridge. During this time, the primordial germ cells proliferate and migrate from the hindgut to colonize the developing gonad. In the human male fetus, the testicular compartments, tubular and interstitial components, and specific cell types, Leydig, Sertoli, and germ cells can be visualized by 11 weeks of gestation. During early gestation, placental human chorionic gonadotropin (hCG) governs Leydig cell proliferation and secretion of testosterone and insulin-like factor 3 (INSL3); endogenous LH begins to regulate these activities in midgestation. Because of the role of placental hCG during early gestation, gonadotropin deficiency does not influence male sexual differentiation. However, LH secretion does influence the number of fetal Leydig cells, as demonstrated by a reduced number in anencephalic fetuses and an increased number in 46,XY fetuses who have elevated gonadotropin concentrations secondary to complete androgen insensitivity. Testosterone production secondary to fetal LH secretion is also critical for phallic growth during the second and third trimesters; males with LH deficiency typically have normally formed but small penises at birth. FSH secretion influences Sertoli cell differentiation and anti-Mullerian hormone (AMH) and inhibin B secretion.

Testicular descent occurs in two phases. The transabdominal phase begins at approximately 12 weeks of gestation and is influenced by the Leydig cell product, INSL3, and its cognate receptor, leucine-rich repeat-containing G-protein–coupled receptor 8 (LGR8). The second androgen-dependent phase, descent of the testes through the inguinal canal, is usually accomplished by the end of week 35.

Following a brief pause in activity after birth, the hypothalamic-pituitary-testicular axis is active during the first few months of life with testosterone concentrations peaking at 1 to 2 months of age. By approximately 6 months of age, testosterone concentrations decline to prepubertal levels. During the brief period of increased neonatal HPG activity, sexual hair does not develop and gametogenesis does not occur because of limited androgen receptor (AR) signaling in certain tissues (e.g., Sertoli cells). During infancy and childhood, seminiferous cords are solid and generally filled with immature Sertoli cells. The germ cells are limited to spermatogonia and Leydig cells are rarely visualized. Inhibin B and AMH continue to be secreted during childhood and serve as valuable markers of Sertoli cell function.

Hypothalamic KNDy and Gonadotropin-Releasing Hormone Neurons

GnRH release is coordinated by inhibitory and excitatory neuronal and glial inputs ( Fig. 18.2 ). Retrograde tracing studies in mice have demonstrated that GnRH neurons are controlled by a complex neuronal network of inputs from many regions of the brain, including the brain stem, limbic system, hypothalamic nuclei, motor and sensory circuits, and basal ganglia.

Among the various regulators of GnRH neurons, kisspeptins and neurokinin B are the most essential. The distribution of kisspeptin neurons in the hypothalamus varies with species. Kisspeptin neurons have been anatomically mapped in mice to reside within the hypothalamus caudally, in the mediobasal hypothalamus, which includes the arcuate nucleus (ARC)/infundibular region. In mice, a second population is located in the rostral part of the hypothalamus called the anteroventral periventricular ( AVPV ) region. In rats, monkeys, and humans, this regional distribution of kisspeptin neurons is less clear. The two populations of kisspeptin neurons in mice exhibit marked functional differences, with, for example, the AVPV kisspeptin neurons having a role in mediating the positive-feedback effects of estrogen, leading to the GnRH/LH surge that triggers ovulation, and the ARC kisspeptin neurons having a role in mediating negative-feedback effects of sex steroids on tonic activity of the HPG axis. AVPV kisspeptin neurons exhibit clear sexual dimorphism, with female rodents at puberty having a much greater number of kisspeptin neurons in this area.

Kisspeptin, an excitatory neuropeptide, was identified as a vital permissive factor in puberty onset by the discovery of patients with GnRH deficiency with loss-of-function mutations in the kisspeptin receptor, KISS1R , previously known as GPR54. Subsequently, an activating mutation in KISS1R was identified in a girl with central precocious puberty, and an activating mutation in KISS1 , which encodes kisspeptin itself, has been reported in a child with central precocious puberty. Moreover, mice with knockout of Kiss1r are infertile, despite normal distribution of GnRH neurons and normal hypothalamic GnRH levels, and mice with Kiss1 knockout also have a phenotype consistent with normosmic GnRH deficiency.

Kisspeptins are synthesized by hypothalamic neurons that contact GnRH neurons and their projections. Kisspeptin neurons are located outside the blood-brain barrier and therefore come directly in contact with peripheral sex hormones. Most GnRH neurons express the kisspeptin receptor and kisspeptin neurons express steroid receptors (estrogen receptor alpha, androgen receptor, and progesterone receptor) and are the main relay for the negative and positive feedback of steroid hormones on the gonadotropic axis.

Kisspeptin signals directly to GnRH neurons to control pulsatile GnRH release. Kisspeptin is upregulated in both primates and mice in the peripubertal period, and its administration to pubertal rodents advances the onset of sexual maturation. Interestingly, kisspeptin also appears to be downregulated in functional amenorrhea, suggesting its role as a mediator of environmental factors, such as emotional well-being and nutritional status. In addition, as noted, kisspeptin has been shown to be an important neuroendocrine regulator of ovulation. In mice, kisspeptin neurons located in the AVPV mediate the positive feedback of estradiol on the GnRH neuronal network and therefore the generation of the LH preovulatory pulse. In contrast, estrogens have an inhibitory effect on Kiss1 neurons in the ARC. Thus kisspeptin signaling appears to have both positive and negative feedback loops in the HPG axis. Whilst kisspeptin has been identified as a pivotal upstream regulator of GnRH neurons, whether kisspeptin is the key factor in triggering the onset of puberty remains unclear.

Another excitatory neuropeptide, neurokinin B, has been implicated in the upstream control of GnRH secretion. Identification of this pathway was based on discovery of loss-of-function mutations in TAC3 , encoding neurokinin B, and its receptor TACR3 , in patients with pubertal failure caused by normosmic HH. Kisspeptin neurons located in the ARC synthesize neurokinin B and dynorphin A and have been called KNDy ( K isspeptin, N eurokinin B, Dy norphin) neurons. Both TAC3 and KISS1 expression in the ARC is downregulated by estrogen. Therefore these neurons are considered as the relay of the negative feedback of steroid hormones on the gonadotropic axis. KNDy neurons also express the neurokinin B receptor, NK3R, suggesting that paracrine and autocrine loops control GnRH release. Dynorphin inhibits the release of GnRH, and together these peptides are currently believed to play a fundamental role in regulating the pulsatile GnRH release.

More recently, another RF-amide–related peptide (RFRP1, RFRP3), the mammalian orthologue of the avian peptide gonadotrophin-inhibiting hormone (GnIH) , was discovered as another inhibitory regulator of the gonadotropic axis by directly controlling GnRH neurons. GnIH plays an essential role in the inhibition of the HPG axis in several species. In addition to neuropeptides, several neurotransmitters participate in the control the GnRH network. Gamma aminobutyric acid (GABA) and glutamate control GnRH neuronal excitability in the ARC. In female rats, glutamine synthase is downregulated and glutamate dehydrogenase becomes more abundant in the hypothalamus at puberty, both leading to increased availability of glutamate, which acts through its n-methyl-D-aspartate (NMDA) and kainate receptors. The administration of glutamate agonists to prepubertal primates can stimulate the onset of puberty.

The neural network of GABA is quite complex because some of these neurons will have a direct effect on GnRH neurons, and others will act on interneurons. The inhibitory role of GABAergic neurotransmission in restraining the initiation of puberty has been clearly shown in primates but is more ambiguous in rodents. GABAergic signaling pathways are likely to be important in the stress-induced suppression of LH.

Some recent evidence highlights the importance in mice of micro ribonucleic acid (microRNAs) (particularly the miR-200/429 family and miR-155) in the epigenetic upregulation of GnRH transcription during the murine equivalent of the minipuberty. Moreover, miR-7a2, has been demonstrated to be essential for normal pituitary development and HPG function, with deletion in mice leading to HH and infertility.

Corticotropin-releasing hormone (CRH) signaling has long been known to play an important role in the stress-induced suppression of the GnRH pulse generator in rodents. Intracerebroventricular (ICV) administration of CRH decreases LH pulse frequency in rats, whereas stress-induced suppression of LH pulses by insulin-induced hypoglycemia is blocked by ICV administration of a CRH antagonist. Juxtaposition of CRH immunoreactive fibers to GnRH neurons has been observed in the infundibular region of the hypothalamus in the human.

Finally, the synchronous pulsatile secretion of GnRH is also controlled through neuron-glia signaling pathways. Glial inputs appear to be predominantly facilitatory and they consist of growth factors and small diffusible molecules, including transforming growth factor (TGF)β1, insulin-like growth factor 1 (IGF-1), and neuregulins, that directly or indirectly stimulate GnRH secretion. Glial cells in the median eminence regulate GnRH secretion by production of growth factors, acting via receptors with tyrosine kinase activity. FGF signaling is required for GnRH neurons to reach their final destination in the hypothalamus, as well as for GnRH neuronal differentiation and survival. In addition, GnRH neuron secretory activity is facilitated by IGF-1 and by members of the epidermal growth factor family, such as neuregulin 1β. Plastic rearrangements of glia-GnRH neuron adhesiveness, mediated by soluble molecules, such as N-CAM and synaptic cell adhesion molecule (SynCAM), coordinate the controlled delivery of GnRH to the portal vasculature, a process that is also subject to sex steroid regulation.

Pituitary Gonadotropins

GnRH stimulates the production and secretion of LH and FSH from the gonadotrophs by binding to a cell-surface receptor, which triggers increased intracellular calcium concentration and phosphorylation of protein kinase C. LH is released from readily releasable pools, which lead to a rise in serum LH within minutes after a bolus of GnRH, but also from pools which take longer to mobilize. Whereas episodic stimulation by GnRH increases gonadotropin secretion, continuous infusion of GnRH decreases LH and FSH secretion and downregulates the pituitary receptors for GnRH—a phenomenon that is used in the medical treatment of central precocious puberty. Alterations in the GnRH receptor have other important roles in regulating gonadotroph function as estrogens increase and androgens decrease GnRH receptors.

FSH and LH are glycoproteins composed of two subunits, an α subunit that is common for all the glycoprotein hormones and distinct β subunits that confer receptor specificity. The β subunits are 115 amino acids long with two carbohydrate side chains. hCG produced by the placenta is almost identical in structure to LH except for an additional 32 amino acids and additional carbohydrate groups. Rare cases of mutations in the β subunit of gonadotropin molecules that cause pathological effects have been reported: a single case of an inactivating mutation of LHB caused absence of Leydig cells and lack of puberty in a male and two cases of inactivating mutations of FSHB led to lack of follicular maturation and amenorrhea and, in two males, azoospermia. In addition, a woman with a homozygous mutation in a 5' splice-donor site in the noncoding region of LHB displayed impaired LH secretion, normal pubertal development, secondary amenorrhea, and infertility. What is conceptually important from these observations is that normal pubertal maturation in women, including breast development and menarche, can occur in a state of LH deficiency, whereas normal LH secretion appears obligatory for ovulation. This implies that although LH is essential for the normal maturation of Leydig cells and steroidogenesis in men, its primary role in women is to induce ovulation.

The same gonadotroph cell produces both LH and FSH in the pituitary. Gonadotroph cells that are not stimulated, for example, because of disease affecting GnRH secretion, are small in diameter, while the gonadotroph cells in primary hypogonadism, stimulated by large amounts of GnRH, are large in diameter and demonstrate prominent rough endoplasmic reticulum.

Gonadal Hormone Production

Putting It All Together: the Pubertal Reactivation of the Gonadotropic Axis

Hormonal Changes

The gonadotropic axis undergoes complex cycles of activation and inhibition from fetal life to puberty ( Fig. 18.3 ).

Fetal testosterone secretion early in pregnancy is caused by placental hCG stimulation. The fetal hypothalamus contains GnRH-containing neurons by 14 weeks of gestation, and the fetal pituitary gland contains LH and FSH by 20 weeks. The hypothalamopituitary portal system develops by 20 weeks of gestation, allowing hypothalamic GnRH to reach the pituitary gonadotrophs. At midgestation, there is a striking rise of circulating gonadotropin levels in both male and female fetuses, which reaches its peak at 34 to 38 weeks in the male fetus, and then falls to low levels at birth. This change in gonadotropin secretion results from the development of a negative feedback system through sex steroids, as well as from the development of inhibiting influences from the CNS on GnRH neurons.

LH and FSH secretion rise during the first month after birth, probably because the negative feedback effect of placental estrogens is withdrawn. LH is secreted in pulses during this postnatal period, often termed the minipuberty . After this postnatal activity, the HPG axis becomes relatively dormant in children between the age of 2 and 8 to 9 years.

The transition from the relative prepubertal quiescence to the adolescent pattern of GnRH secretion is a gradual rather than abrupt process. LH and FSH pulsatility has been detected in normal children as young as 4 years of age. Throughout childhood, GnRH secretion appears to undergo small but progressive increases until the onset of puberty, when GnRH secretion increases, first at night and eventually throughout the day. Because of the episodic nature of gonadotrophin secretion, a single gonadotrophin determination is not informative regarding the secretory dynamics of these hormones. However, modern third-generation assays are sufficiently sensitive to indicate the onset of puberty in single basal unstimulated samples.

The first biological change demonstrating that the HPG axis is being reactivated at puberty is the augmentation of nocturnal LH pulses in children; this reactivation is subclinical and begins before clinical development of Tanner genital stage 2. This period may thus be seen as the hormonal onset of puberty. The differences between daytime and nighttime levels of LH remain until late stages of puberty but disappear by early adulthood. During this reactivation of the axis, there is a gradual development of a dynamic interplay between the central production of GnRH and gonadotropins and gonadal sex-steroid production. A progressive maturation of negative-feedback loops occurs but so too the prepubertal suppressant drive from the CNS gradually abates, and transient intensifying positive feedback results from increasing gonadal sex-steroid production.

During the period of relative quiescence, testosterone and estrogen are also measurable in the circulation using sensitive assays, demonstrating low but definite activity of the prepubertal gonads. However, gonadal contribution to the inhibition of the hypothalamic-pituitary system occurs later, becoming operative only at midpuberty, and eventually becomes dominant over the central inhibitory feedback drive ( Fig. 18.4 ). Both mean LH and FSH levels increase through pubertal development, although LH rises to a greater extent, probably because of differences in feedback mechanisms for these two hormones. These rises are the result of both an increase in basal levels of LH and FSH, and a greater number and amplitude of LH peaks.

In boys during puberty, plasma testosterone levels increase dramatically. The pubertal increase in testis size results primarily from an increased number of proliferating and differentiating germ cells and, to a lesser extent, an increase in Sertoli cells. In early and midpuberty, there is a pronounced diurnal rhythm with a morning peak in measurable testosterone, but this is less pronounced in later puberty and declines gradually with age, probably because of decreased day-night ratios of gonadotropins.

The biological reactivation of the gonadotropic axis occurs earlier in girls than boys, and the dynamic of the reactivation of the gonadotropic axis is not identical in the two sexes. The secretion of testosterone increases shortly after the increase in the plasma concentration of LH and FSH. In girls, estradiol increases together with increasing LH and FSH. Then, a hormonal dialogue between gonads, the hypothalamus, and pituitary contributes to the progressive activation of the gonadotropic axis, until the occurrence of the LH ovulatory pulse in females. The basis for this sexual dimorphism is unclear and may be related to differences in gonadal hormone production and could also be a feature of the sexual dimorphism of the brain. There are also some data suggesting that although most genes regulate the timing of puberty similarly in boys and girls, there is a small subset that may exert sex-specific effects.

Physiological Mechanisms

Our understanding of the activation of the gonadotropic axis at the end of the prepuberty remains incomplete but has advanced in some areas (see discussion later). GnRH neuronal activity is under the control of several neurotransmitters and neuropeptides, as described earlier, and the onset of puberty is triggered by a decline in these inhibitory signals and amplification of the excitatory inputs, leading to increased frequency and amplitude of GnRH pulses. The neuroendocrine mechanisms that control the activation of the gonadotropic axis in the fetus, as well as during the “minipuberty” are less well understood. Likewise, the exact nature of the brake that functions during prepuberty remains unknown.

One change at puberty is a shift in the balance of GABA-glutamate signaling in the brain. Another is an increase in dendritic spine density and a simplification of the dendritic architecture of GnRH neurons. A third is an increase in kisspeptin signaling in the hypothalamus, which is caused by an increase of kisspeptin synthesis, as well as an increased responsiveness of GnRH neurons to kisspeptin stimulation. Although mainly described in mice, this paradigm, which is well conserved in evolution, is probably true in monkeys as well in humans.

The mechanisms responsible for the increased biosynthesis of kisspeptins in the hypothalamus at the end of the juvenile period remain unknown. Data pointing to hypothalamic regulation via a hierarchical network of genes (see Fig. 18.2 ) have mainly come from a systems biology approach and animal models, with little data from human subjects to date; however, integration of findings from human genome-wide association studies will likely enrich these models. Candidate transcriptional regulators that have been identified via these approaches include octamer transcription factor 2 ( Oct-2 ), thyroid transcription factor-1 ( TTF-1 ), and enhanced at puberty 1( EAP1 ). TTF-1 is a homeobox gene that enhances GnRH expression. TTF-1 expression is increased in pubertal rhesus monkeys. Oct-2 is a transcriptional regulator of the POU-domain family of homeobox-containing genes. Oct-2 messenger RNA (mRNA) is upregulated in the hypothalamus in juvenile rodents; blockage of Oct-2 synthesis delays age at first ovulation; and hypothalamic lesions, which induce precocious puberty (e.g., hypothalamic hamartomas), activate Oct-2 expression. EAP1 mRNA levels also increase in the hypothalamus of primates and rodents during puberty, EAP1 transactivates the GnRH promoter, and EAP1 knockdown with small interfering RNA (siRNA) causes delayed puberty (DP) and disrupts estrous cyclicity in rodents.

Recent data have highlighted the importance of a genetic program that controls the expression of Kiss1. The intervention of the polycomb complex proteins, EED and Cbx7, in the transcriptional repression of Kiss1 has recently been revealed. The expression of these genes in the prepubertal period progressively decreases with increasing methylation of their promoters. Thus the binding of EED on the Kiss1 promoter decreases at puberty. The inhibition of the repression of Kiss1 is also correlated with a decrease in the expression of transcription factors with zinc-finger motifs. In addition, a microRNA switch was proposed to regulate the rise of GnRH1 synthesis, which occurs during the juvenile period in GnRH neurons. The initiation of puberty from the hypothalamus therefore results from a complex network of transcription factors mainly acting as repressors of Kiss1 and GnRH1 transcription.

The concept that puberty results from the disappearance of gonadotropic axis repression is also supported by the description of loss-of-function mutations of MKRN3 and DLK1 in familial central precocious puberty (CPP, discussed in detail in “Precocious Puberty” later). Where these factors function in the hierarchical network of genes controlling kisspeptin has yet to be determined.

Somatic Changes

In boys, the first physical finding that marks the onset of puberty is the change from Tanner genital stage G1 to stage G2, including enlargement of the testes (i.e., achievement of volume ≥ 4 mL or testicular length ≥ 25 mm). Originally Marshall and Tanner reported the mean (standard deviation [SD]) onset of puberty in boys to be 11.64 (1.07) years. These pubertal stages ( Fig. 18.5 ) were based on longitudinal photographic observations of genital development of a relatively small sample of 228 boys living in a children’s home. Despite the probably poor representative nature of this sample, studies in Switzerland, the United States, and Denmark have reported roughly similar mean ages of puberty onset.

Although the mean age of onset may be fairly uniform across populations, the range of ages for onset of puberty in normal, healthy adolescents varies widely. Several pathologic states may further influence the timing of puberty either directly or indirectly and may contribute to this splay, but the great majority of the variation in pubertal timing cannot be attributed to clinical disorders. Some 95% of boys experience the onset of genital development between 9.5 and 13.5 years, and these data have led to the traditional definition of sexual precocity in boys as development of secondary sexual characteristics before age 9 years and DP as lack of testicular enlargement by age 14 years.

Development of secondary sexual characteristics results from both gonadarche and adrenarche. Adrenarche refers to the maturation of the zona reticularis of the adrenal gland, resulting in increased production of the adrenal androgens dehydroepiandrosterone (DHEA) and androstenedione, as well as the relatively inactive metabolite DHEA sulfate (DHEA-S). These adrenal androgens, along with testicular androgens, contribute to secondary sexual characteristics, such as pubic hair (pubarche), axillary hair, apocrine body odor, and acne. Like gonadarche, the onset of adrenarche appears to be a gradual, progressive maturational process that begins in early childhood and is marked by the further increases of production of adrenal androgens around the time of puberty. Although adrenarche and gonadarche typically overlap, they are separate processes that are independently regulated. The triggers for adrenarche remain unknown; however, alterations in body weight and body mass index (BMI), as well as in utero and neonatal physiology, likely modulate this developmental process, perhaps along with intraadrenal cortisol production.

Secular Trends in the Timing of Puberty

The mean age of menarche in mid-nineteenth century Europe is reported to be between 17 and 18 years. Starting from the late nineteenth century to the mid-twentieth century, a gradual decline in age at puberty has been reported, after which this trend appears to have slowed. Much of this change in the timing in puberty has likely been the result of better hygiene and nutrition related to increased socioeconomic stability.

More recent declines in the age at puberty are more convincingly demonstrated among girls than in boys, In the mid-1990s, data from the Third National Health and Nutrition Examination Survey (NHANES III), where genital ratings were performed by visual inspection, reported earlier age at puberty in both boys and girls, than what previously had been reported from the United States. Using the traditional cutoff of 9 years, these data suggest that an increased number of boys would be classified as having precocious puberty. However, because of lack of data on pubertal onset in the previous population-based study (Third National Health Examination Survey [NHES III]), some controversy remained as how to interpret the NHANES III findings.

Furthermore, questions have been raised regarding the criteria used for genital staging in NHANES III. A subsequent secular trend analysis between NHES III (which lacked data from the early pubertal stages) and NHANES III did not find clear evidence supporting earlier age at puberty, although some indications were present in non-Hispanic white boys. These data were also reviewed by an expert panel, which concluded that the available data are insufficient in quality and quantity to confirm a change in pubertal timing in US boys. At the same time in Europe, in comparison with NHANES III studies, some data even reported suggesting older ages at pubertal onset in boys.

Secular trends in the timing have been assessed in a few European studies within specific populations. Earlier studies do not support a substantial enough change in the age at pubertal onset in boys from the mid-1960s to the late 1990s to warrant a change in the age definitions for precocious and DP. However, the most recent data from Copenhagen, comparing the timing of puberty in boys assessed between 2006 and 2008 compared with a group assessed between 1991 and 1993, does report earlier mean age of onset of puberty (11.62 vs. 11.92 years).

In summary, data are conflicting but there is likely an overall suggestion that puberty may be occurring earlier in boys than in the past. However, the change is not as dramatic as in girls, and age cutoffs used to define disorders of puberty have not been uniformly changed.

Genetic Contributions to Variation in Normal Puberty

Although the precise mechanisms that trigger puberty remain unknown, it is well established that the timing of puberty is influenced by both genetic and environmental factors. Evidence for the genetic contribution is provided by correlations in timing within families and between monozygotic twins. It is important to note that this genetic component does not preclude a significant role for environmental influences; indeed, genetics alone cannot explain secular trends in the timing of puberty. However, the strong genetic component provides an opportunity to identify factors that modulate the timing of normal puberty in the general population. Approaches used to identify specific genetic factors include candidate-gene studies and genome-wide association (GWA) studies.