Reproductive physiology

Genetic sex

Genetic sex is determined by the sex chromosomes—XY in males and XX in females. During the first 5 weeks of gestational life, the gonads are indifferent or bipotential—they are neither male nor female. At approximately gestational week 7 in genetic males, the gene product of the s ex-determining r egion of the Y chromosome (SRY gene) encodes a transcription factor that causes the testes to begin developing; at gestational week 9 in genetic females (in the absence of the SRY gene) the ovaries begin to develop. Therefore, genetic sex normally determines gonadal sex, and the gonads appear in males slightly before they appear in females.

Gonadal sex

Gonadal sex is defined by the presence of either male gonads or female gonads, namely, the testes or the ovaries. The gonads comprise germ cells and steroid hormone–secreting cells.

The testes, the male gonads, consist of three cell types: germ cells, Sertoli cells, and Leydig cells. The germ cells produce spermatogonia, the Sertoli cells synthesize a glycoprotein hormone called antimüllerian hormone, and the Leydig cells synthesize testosterone.

The ovaries, the female gonads, also have three cell types: germ cells, granulosa cells, and theca cells. The germ cells produce oogonia. The meiotic oogonia are surrounded by granulosa cells and stroma, and in this configuration, they are called oocytes. They remain in the prophase of meiosis until ovulation occurs. The theca cells synthesize progesterone and, together with the granulosa cells , synthesize estradiol.

There are two key differences between the male and female gonads that influence phenotypic sex. (1) The testes synthesize antimüllerian hormone (müllerian-inhibiting substance), and the ovaries do not. (2) The testes synthesize testosterone, and the ovaries do not. Antimüllerian hormone and testosterone are decisive in determining that the fetus will be a phenotypic male. If there are no testes and therefore no antimüllerian hormone or testosterone, the fetus will become a phenotypic female by “default.”

Phenotypic sex

Phenotypic sex is defined by the physical characteristics of the internal genital tract and the external genitalia. In males , the internal genital tract includes the prostate, seminal vesicles, vas deferens, and epididymis. The external genitalia in males are the scrotum and the penis. In females , the internal genitalia are the fallopian tubes, uterus, and upper one-third of the vagina. The external genitalia in females are the clitoris, labia majora, labia minora, and lower two-thirds of the vagina. As previously noted, phenotypic sex is determined by the hormonal output of the gonads as follows:

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  Male phenotype. Gonadal males have testes that synthesize and secrete antimüllerian hormone and testosterone , both of which are required for the development of the male phenotype. Embryologically, the wolffian ducts give rise to the epididymis, vas deferens, seminal vesicles, and ejaculatory ducts. Testosterone, which is present in gonadal males, stimulates the growth and differentiation of the wolffian ducts. Testosterone from each testis acts ipsilaterally (same side) on its own wolffian duct. In this action on the wolffian ducts, testosterone does not have to be converted to dihydrotestosterone (discussed later in chapter). At the same time, antimüllerian hormone produced by testicular Sertoli cells causes atrophy of a second set of ducts, the müllerian ducts. (The müllerian ducts would have become the female genital tract if they had not been suppressed by antimüllerian hormone.) The external male genitalia, the penis and scrotum, differentiate at gestational weeks 9 to 10. Growth and development of the external male genitalia depend on conversion of testosterone to dihydrotestosterone and the presence of androgen receptors in the target tissues. The congenital absence of androgen receptors in target tissues causes androgen insensitivity syndrome in which the external genitalia develop into the female phenotype and the wolffian and müllerian ducts regress ( Box 10.1 ).

  BOX 10.1

  Clinical Physiology: Androgen Insensitivity Syndrome

  Description of case.

  A girl who is apparently normal begins to develop breasts at age 11, and at age 13, she is considered to have larger-than-average breasts among her peers. However, by age 16, she has not begun to menstruate and has scant pubic and axillary hair. Upon pelvic examination, a gynecologist notes the presence of testes and a short vagina but no cervix, ovaries, or uterus. Chromosomal evaluation reveals that the girl has an XY genotype. Suspecting a form of androgen insensitivity syndrome (a testicular feminization), the physician orders androgen-binding studies in genital skin fibroblasts. The studies show no binding of testosterone or dihydrotestosterone, suggesting that androgen receptors in the tissue are absent or defective. She has mildly elevated levels of plasma testosterone and elevated levels of luteinizing hormone (LH). The young woman’s testes are removed, and she is treated with intermittent estrogen replacement therapy. She is advised, however, that she will never have menstrual cycles or be able to bear children.

  Explanation of case.

  This girl has a female phenotype with female external genitalia (lower vagina, clitoris, and labia). At puberty, she develops breasts. However, she has a male genotype (XY) and male gonads (testes).

  The basis for her disorder, a form of androgen insensitivity syndrome, is lack of androgen receptors in target tissues, which results in resistance to androgens. Her testes, which are normal, secreted both antimüllerian hormone and testosterone in utero. As in normal males, antimüllerian hormone suppressed development of the müllerian ducts in utero; therefore the girl has no fallopian tubes, uterus, or upper vagina. The testes also secreted testosterone in utero, which should have stimulated growth and differentiation of the wolffian ducts into the male genital tract and development of the male external genitalia. The male genital tract and external genitalia did not develop, however, because the target tissues lack androgen receptors. Thus although the testes secreted normal amounts of testosterone, testosterone could not act on the tissues of the male genital tract. (Lack of androgen receptors also explains the girl’s scant body hair at puberty.) The female phenotype (short vagina, labia, and clitoris) is present because, in the absence of testosterone receptors, the fetus became a phenotypic female by “default.”

  The girl’s breasts developed at puberty because her testes are producing estradiol from testosterone, stimulated by the high-circulating levels of LH. The estradiol then promotes breast development.

  Treatment.

  In androgen insensitivity syndrome, because the testes can develop a neoplasm, they are removed. Following removal of the testes (and therefore removal of the testicular source of estradiol), the girl is treated with estrogen therapy to maintain her breasts. She will not be able to bear children, however, because she lacks ovaries and a uterus.

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  Female phenotype. Gonadal females have ovaries that secrete estrogen , but they do not secrete antimüllerian hormone or testosterone. Thus in females no testosterone is available to stimulate growth and differentiation of the wolffian ducts into the internal male genital tract, and no antimüllerian hormone is available to suppress differentiation of the müllerian ducts. Consequently the müllerian ducts develop into the internal female tract (fallopian tubes, uterus, cervix, and upper one-third of the vagina). Like the internal genital tract, the development of the external female genitalia (clitoris, labia majora, labia minora, and lower two-thirds of the vagina) does not require any hormones, although growth of these structures to normal size depends on the presence of estrogen.

  • If a gonadal female is exposed to high levels of androgens in utero (e.g., from excessive androgen production by the adrenal cortex) when the external genitalia are differentiating, then a male phenotype results. If such exposure occurs after differentiation of the external genitalia, the female phenotype is retained, but perhaps with enlargement of the clitoris ( Box 10.2 ).

    BOX 10.2

    Clinical Physiology: Congenital Adrenal Hyperplasia

    Description of case.

    At birth, a baby is found to have ambiguous external genitalia. There is no penis, and a clitoris is significantly enlarged. Chromosomal evaluation reveals that the baby has an XX genotype. The baby is found to have ovaries but no testes. Tests confirm that the baby has a form of adrenal hyperplasia in which there is congenital lack of the adrenal cortical enzyme 21β-hydroxylase. Treatment involves surgical reconstruction of the external genitalia to conform to the female phenotype and the administration of glucocorticoids and mineralocorticoids. The child will be raised as a female.

    Explanation of case.

    The baby has a congenital absence of 21β-hydroxylase, the adrenal enzyme that normally converts steroid precursors to mineralocorticoids and cortisol (see Chapter 9 , Fig. 9.23 ). As a result of this defect, steroid precursors accumulate behind the enzyme block and are directed toward the production of the adrenal androgens dehydroepiandrosterone and androstenedione. The high levels of androgens caused masculinization of the external genitalia (enlargement of the clitoris) in utero. The genotype is XX (female), and the internal organs are female including ovaries, fallopian tubes, uterus, and upper vagina. The fallopian tubes, uterus, and upper vagina developed because, without testes, there was no source of antimüllerian hormone to suppress differentiation of müllerian ducts into the female genital tract. There is hyperplasia of the adrenal cortex because the absence of cortisol increases secretion of adrenocorticotropic hormone (ACTH), which then has a trophic effect on the adrenal cortex.

    Treatment.

    Surgical correction of the ambiguous external genitalia involves reconstruction to conform to a phenotypic female. Because the baby has normal ovaries, fallopian tubes, and uterus, she should begin normal menstrual cycles at puberty and have a normal reproductive capacity. Hormone replacement therapy has two goals: (1) to replace the missing adrenal glucocorticoids and mineralocorticoids and (2) to suppress ACTH secretion (by the negative feedback of glucocorticoids on the anterior pituitary) to reduce the adrenal output of androgens and prevent further masculinization.

Gonadotropin secretion over the lifetime

In both males and females, gonadal function is driven by the hypothalamic-pituitary axis, whose activity varies over the life span, as shown in Figure 10.2 .

Secretion of gonadotropin-releasing hormone (GnRH), the hypothalamic hormone, begins at gestational week 4, but its levels remain low until puberty. Secretion of follicle-stimulating hormone (FSH) and luteinizing hormone (LH), the anterior pituitary hormones, begins between gestational weeks 10 and 12. Like GnRH, the levels of FSH and LH remain low until puberty. During childhood, FSH levels are relatively higher than LH levels.

At puberty and throughout the reproductive years, the secretory pattern changes: Secretion of GnRH, FSH, and LH increases and becomes pulsatile. The relative levels of FSH and LH reverse, with LH levels becoming higher than FSH levels. In addition, in females, there is a 28-day cycle of gonadotropin secretion called the menstrual cycle.

Finally, in senescence, gonadotropin secretion rates increase further, with FSH levels becoming higher than LH levels, as they were in childhood.

Pulsatile secretion of GnRH, FSH, and LH

The primary event at puberty is the initiation of pulsatile secretion of GnRH . This new pattern of GnRH secretion drives a parallel pulsatile secretion of FSH and LH by the anterior lobe of the pituitary. One of the earliest events of puberty is the appearance of large nocturnal pulses of LH during rapid eye movement (REM) sleep. Another significant event early in puberty is increased sensitivity of GnRH receptors in the anterior pituitary. Thus at puberty, GnRH up-regulates its own receptors in the anterior pituitary, and a given concentration of GnRH produces greater stimulation of FSH and LH secretion. In addition, there is a shift in the relative secretion rates of the two anterior pituitary hormones; at puberty and throughout the reproductive period, LH levels are greater than FSH levels (compared with childhood and senescence, when FSH is greater than LH).

Pulsatile secretion of FSH and LH stimulates secretion of the gonadal steroid hormones, testosterone and estradiol. Increased circulating levels of the sex steroid hormones are then responsible for the appearance of the secondary sex characteristics at puberty.

The onset of the maturational process at puberty is genetically programmed, and familial patterns are evident. For example, the age at menarche (the onset of menses) is similar between mothers and daughters. The mechanisms underlying the onset of pulsatile GnRH secretion, however, remain a mystery. There may be gradual maturation of the hypothalamic neurons that synthesize and secrete GnRH. The central nervous system and nutritional status may alter the process; for example, extreme stress or caloric deprivation in girls delays the onset of puberty. It has been suggested that melatonin plays a role in the onset of puberty. Melatonin, secreted by the pineal gland, may be a natural inhibitor of GnRH release. Melatonin levels are highest during childhood and decline in adulthood, and this decline may release an inhibition of GnRH secretion. In support of a role for melatonin is the observation that removal of the pineal gland precipitates early puberty.

Characteristics of puberty

As noted, the biologic events at puberty are set in motion by the onset of pulsatile activity in the hypothalamic–anterior pituitary axis. In turn, this pulsatile, or bursting, activity causes the testes and ovaries to secrete their respective sex hormones, testosterone and estrogen, that are responsible for the development of the secondary sex characteristics. Pulsatility of the hypothalamic-pituitary axis is required for normal reproductive function, as illustrated by the treatment of persons with delayed puberty caused by GnRH deficiency. If the person is treated with a GnRH analogue in intermittent pulses (to replicate the normal pulsatile secretory pattern), puberty is initiated and reproductive function is established. However, if a long-acting GnRH analogue is administered, puberty is not initiated. The events of puberty and their timing are illustrated in Figure 10.3 .

In boys , puberty is associated with activation of the hypothalamic-pituitary axis, Leydig cell proliferation in the testes, and increased synthesis and secretion of testosterone by the Leydig cells. There is growth of the testes, largely because of an increased number of seminiferous tubules. There is growth of the sex accessory organs such as the prostate. There is a pronounced linear growth spurt, and the epiphyses close when adult height is attained. As plasma levels of testosterone increase, facial, pubic, and axillary hair appears and there is growth of the penis, lowering of the voice due to increased size of the larynx and vocal cords, and initiation of spermatogenesis ( spermarche ).

In girls , puberty also is associated with the activation of the hypothalamic-pituitary axis, which drives the synthesis of estradiol by the ovaries. The first observable sign of puberty in girls is budding of the breasts ( thelarche ), which is followed in approximately 2 years by menarche , the onset of menstrual cycles. The growth spurt and closure of the epiphyses typically begin and end earlier in girls than in boys. The appearance of pubic and axillary hair ( pubarche ) is due to increased production of adrenal androgens, or adrenarche . Pubarche and adrenarche precede menarche.

Structure of the testes

The male gonads are the testes , which have two functions: spermatogenesis and secretion of testosterone. Normally, the testes occupy the scrotum, which lies outside the body cavity and is maintained at 35° C to 36° C, or 1° C to 2° C below body temperature. This lower temperature, essential for normal spermatogenesis, is maintained by a countercurrent arrangement of testicular arteries and veins, which facilitates heat exchange.

Eighty percent of the adult testis is composed of seminiferous tubules , which produce the sperm. The seminiferous tubules are convoluted loops, 120 to 300 μm in diameter, which are arranged in lobules and surrounded by connective tissue. The epithelium lining the seminiferous tubules consists of three cell types: spermatogonia, which are the stem cells; spermatocytes, which are cells in the process of becoming sperm; and Sertoli cells, which support the developing sperm.

The Sertoli cells lining the seminiferous tubules have four important functions that support spermatogenesis. (1) The Sertoli cells provide nutrients to the differentiating sperm (which are isolated from the bloodstream). (2) Sertoli cells form tight junctions with each other, creating a barrier between the testes and the bloodstream called the blood-testes barrier . The blood-testes barrier imparts a selective permeability, admitting “allowable” substances such as testosterone to cross but prohibiting noxious substances that might damage the developing sperm. (3) Sertoli cells secrete an aqueous fluid into the lumen of the seminiferous tubules, which helps to transport sperm through the tubules into the epididymis. (4) Sertoli cells secrete androgen-binding protein into the lumen of the seminiferous tubule, near the developing sperm cells. Androgen-binding protein helps to keep local levels of testosterone high.

The remaining 20% of the adult testis is connective tissue interspersed with Leydig cells . The function of the Leydig cells is synthesis and secretion of testosterone, the male sex steroid hormone. Testosterone has both local (paracrine) effects that support spermatogenesis in the testicular Sertoli cells and endocrine effects on other target organs (e.g., skeletal muscle and the prostate).