The SRY gene triggers development of the testis, which makes the androgens and AMH necessary for male sexual differentiation
As noted above, the female pattern of sexual differentiation is the default program and the male pattern of sexual differentiation is directed by endocrine and paracrine factors produced by the testes. Therefore, we will successively examine the control of testicular development, the development of the male internal system of genital ducts, and the development of the male urogenital sinus and external genitalia.
Figure 53-9 summarizes the regulatory cascade that determines the pathway for sex determination. The indifferent gonad develops into a testis in response to TDF—the product of the SRY gene (see p. 1075)—before week 9 of development. If TDF is not present or if TDF is present only after the critical window of 9 weeks has passed, an ovary will develop instead of a testis. Further male-pattern sexual differentiation depends on the presence of three hormones: testosterone, dihydrotestosterone (DHT), and AMH. The testis directly produces both testosterone and AMH. Peripheral tissues convert testosterone to DHT.
FIGURE 53-9 Pathways for the hormonal control of sex determination.
Both the fetal and the postnatal testis produce testosterone, the primary male sex steroid (see pp. 1097–1100), and, in lesser amounts, DHT and estradiol. The Leydig cells (see p. 1095) are the source of testosterone production in the testes. The early increase in the number of Leydig cells and secretion of testosterone in humans could depend on either maternal human chorionic gonadotropin (hCG) or fetal luteinizing hormone. The human testis has its greatest abundance of side-chain-cleavage enzyme—which catalyzes the first committed step in steroid synthesis (see Fig. 50-2)—at 14 to 15 weeks of gestation and low values by 26 weeks of gestation. Because hCG follows a similar temporal pattern, it may be hCG that supports early testosterone production. Late regulation of testosterone production by fetal luteinizing hormone is supported by the finding that the testes of anencephalic fetuses, which lack a functional pituitary gland (see Box 10-2) at term, have few Leydig cells.
The androgenic steroid hormones, principally testosterone and DHT, passively move into target cells and act by binding to the androgen receptor (AR), which functions as a ligand-activated transcription factor to modulate the expression of genes whose products affect cell function and phenotype (see pp. 71–72). ARs are present in cells of the wolffian ducts and urogenital sinus tissues.
Congenital absence or abnormality of AR leads to androgen insensitivity syndrome, one kind of testicular feminization in which the external genitalia develop the female phenotype, and the wolffian and müllerian ducts regress (Box 53-3).
Impaired Androgen Action in Target Tissues
In the absence of androgens, male embryos follow a typically female pattern of sexual development. Any defect in the mechanisms by which androgens act on target tissues—in genotypic males—may lead to a DSD (see Box 53-2) that had been referred to as male pseudohermaphroditism. Affected individuals have a normal male karyotype (46,XY) and unambiguous male gonads but may have ambiguous external genitalia or may phenotypically appear as females. In principle, impaired androgen action could result from a deficiency of the enzyme that converts testosterone to DHT in target tissues, absence of androgen receptors, qualitatively abnormal receptors, a quantitative deficiency in receptor levels, or postreceptor defects. The two major forms that have been identified clinically are defects in the conversion of testosterone to DHT (5α-reductase deficiency) N53-5 and androgen-receptor defects. N53-6
A classic example of pseudohermaphroditism is a condition known as guevedoces (Dominican Spanish slang for penis at 12), first identified is isolated villages of the Dominican Republic in the 1970s. In those communities, ~2% of children who appear as girls at birth and through childhood undergo a remarkable masculinization at puberty. They do not develop breasts or menstruate, but instead become male: the clitoris enlarges to the size of a normal penis and is capable of an erection; the hitherto unrecognized and fully developed testes descend into their labia majora; the individual is capable of ejaculation of mature sperm, albeit through a urethral opening at the base of the enlarged clitoris/penis; and the individuals develop typical male secondary sexual characteristics, including facial hair, increased muscle growth, and a sexual attraction to females. These individuals are actually 46,XY males. At birth they have undescended testes, developed wolffian duct derivatives, no müllerian duct structures, but normal female external genitalia. They appear female at birth and through childhood because they have a 5α-reductase deficiency and, thus, insufficient androgen to masculinize the tissues of the urogenital sinus. The sudden masculinization at puberty is due to increased testosterone levels, which become high enough to exert androgenic action on the genital tubercle and urogenital sinus. These individuals spend the rest of their lives as men.
Contributed by Ervin Jones
The activity of 5α-reductase, which is necessary for the conversion of testosterone to DHT (see Fig. 54-6), is either low or absent in fibroblasts taken from the genital region and other tissues of individuals with one form of DSD (see Box 53-2). The testosterone concentration is either normal or increased. On the other hand, the concentration of DHT is low, which explains why testosterone-dependent development (e.g., differentiation of the wolffian duct) occurs whereas DHT-dependent development (e.g., male-pattern development of the urogenital sinus and external genitalia) does not. This disorder is transmitted via an autosomal recessive gene that is phenotypically manifested only in genetic males. At birth, these children have ambiguous genitalia, with a blind vaginal pouch and a small phallus that is bound ventrally and covered by a hood; they also have hypospadias (i.e., the urethral orifice is located too low on the undersurface of the penis). Wolffian duct development (which depends on testosterone) occurs, and müllerian regression (which depends on AMH) is normal. These individuals may be raised as females.
At the time of puberty, marked, though selective, virilization occurs. The testes descend from their intra-abdominal position. These individuals also undergo other signs of masculinization, including deepening of the voice, muscular development, and phallic enlargement. The cause of virilization at puberty in these individuals is unknown. However, it may be that the large quantities of testosterone produced at puberty are able to sufficiently stimulate androgen receptors in DHT-dependent tissues or that even relatively low levels of 5α-reductase activity are able to generate sufficient DHT in the presence of high levels of substrate (i.e., testosterone). However, masculine features that depend on DHT—such as beard growth, acne, and temporal balding—fail to occur.
Contributed by Ervin Jones
Individuals with complete androgen resistance (also known as androgen insensitivity syndrome or testicular feminization) have normal levels of testosterone and DHT. Moreover, the pathways of testosterone metabolism (see Fig. 54-6) and half-life are normal. However, androgen receptors are defective or absent. As a result, either androgens do not bind to receptors at high levels, or the androgen-receptor complex does not bind properly to DNA. Because target cells behave as though androgens were not present at all, androgen-dependent sexual development does not occur. As a result, the wolffian ducts degenerate, and the urogenital sinus and external genitalia all develop according to the female pattern. However, the normal levels of AMH suppress müllerian development. Although a rudimentary uterus and fallopian tubes are sometimes found, their growth and development have obviously been inhibited.
The term testicular feminization, which is no longer used, was introduced by J.M. Morris in 1953 to describe a group of patients with a distinctive form of male pseudohermaphroditism. This X-linked disorder has a prevalence of 1 in 20,000 live male births. These individuals are genetic males—their sex chromosome complex is XY—but phenotypic females. They appear normal at birth with the exception of the presence of an inguinal hernia. The testis is either intra-abdominal, located in an inguinal hernia, or in the labia majora. They have unambiguous female external genitalia. However, the labia majora are underdeveloped, the vagina is shallow and ends blindly, and the uterus is at most rudimentary. The clitoris is normal or small. Development and growth of individuals with complete androgen resistance usually follow the normal female pattern. At puberty, breast development occurs at the expected time. However, these individuals have a significant deficiency or total lack of pubic and axillary hair and fail to menstruate.
In certain target tissues, cytoplasmic 5α-reductase converts testosterone to DHT (see Fig. 54-6), which binds to the same AR as does testosterone. However, DHT binds to AR with an affinity that is ~100-fold greater than the binding of testosterone. Moreover, the DHT-AR complex binds to DNA targets more tightly than does the testosterone-receptor complex. Thus, a relatively low circulating level of testosterone—not enough to produce secondary masculinization—can profoundly affect cells of the urogenital sinus that express AR and 5α-reductase. Conversely, a deficiency in 5α-reductase is another cause of testicular feminization (see Box 53-3). N53-2
DHT and Intracrine Control of Hormone Action
Contributed by Sam Mesiano
Steroid hormone activity in some target cells is modified by specific enzymes that convert the incoming steroid to a more or less active form before it interacts with its cognate receptor. This is referred to as intracrine control of hormone action. In some androgen-responsive cells the cytoplasmic 5α-reductase enzyme converts testosterone to DHT (see Fig. 54-6), which binds to the same AR as testosterone but with a much higher affinity, ~100-fold greater than testosterone. Moreover, the DHT-AR complex binds to DNA targets more tightly than does the testosterone-AR complex. Thus, androgen target cells have the capacity to markedly increase the potency of testosterone by converting it to DHT. Via this mechanism a relatively low circulating level of testosterone that is not enough to produce secondary masculinization can have a profound effect on cells of the urogenital sinus that express AR and the 5α-reductase enzyme.
Other examples of intracrine effects are the intracellular conversion by 11β-hydroxysteroid dehydrogenase 2 of the active cortisol to the inactive cortisone in principal cells of the cortical collecting duct (see p. 766) and the conversion of the less-active thyroxine (T4) to the more-active triiodothyronine (T3) by deiodinase in many targets of thyroid hormones (see pp. 1009–1010).
As noted on page 1080, the Sertoli cells of the testis produce AMH, which promotes the regression of müllerian ducts. The cranial end of the müllerian duct becomes the vestigial appendix testis at the superior pole of the testis. The müllerian ducts are responsive to AMH at around the eighth week of development and undergo apoptosis in a craniocaudal direction. The ducts completely regress within 1 week of exposure to AMH. AMH deficiency in males, which is usually caused by mutations in the genes encoding AMH or its receptor, can lead to persistent müllerian duct syndrome. These males have undescended testes and a small rudimentary uterus and other müllerian-duct derivatives. AMH also appears to be involved in the regulation of sex hormone (testosterone and estradiol) production as well as in puberty and gender-specific behaviors in both sexes via targets in the brain.
Androgens direct the male pattern of sexual differentiation of the internal ducts, the urogenital sinus, and the external genitalia
Androgens play two major roles in male phenotypic differentiation: (1) they trigger conversion of the wolffian ducts to the male ejaculatory system, and (2) they direct the differentiation of the urogenital sinus and external genitalia. The wolffian phase of male sexual differentiation is regulated by testosterone itself and does not require conversion of testosterone to DHT. In contrast, virilization of the urogenital sinus, the prostate, the penile urethra, and the external genitalia during embryogenesis requires DHT, as does sexual maturation at puberty.
Differentiation of the Duct System
Shortly after the Sertoli cells initiate AMH production, the fetal Leydig cells begin producing testosterone. The embryonic mesenchyme contains androgen receptors and is the first site of androgen action during formation of the male urogenital tract. The Sertoli cells also produce a substance referred to as androgen-binding protein (ABP). It is possible that ABP binds and maintains a high concentration of testosterone locally. These high local levels of testosterone stimulate growth and differentiation of the medulla of the gonad into the rete testes, as well as differentiation of the wolffian ducts into the epididymis, the vas deferens, the seminal vesicles, and the ejaculatory duct. Testosterone also promotes development of the prostate from a series of endodermal buds located at the proximal aspect of the urethra. In the absence of testosterone, the wolffian system remains rudimentary, and normal male internal ductal development does not occur. Cells of the wolffian ducts lack 5α-reductase and therefore cannot convert testosterone to DHT. Thus, the internal male ducts respond to testosterone per se and do not require the conversion of testosterone to DHT.
Differentiation of the Urogenital Sinus and External Genitalia
Development of the male external genitalia and internal derivatives of the urogenital sinus (prostate and bulbourethral glands) commences at around the 9th week of gestation—all under the influence of DHT—soon after maturation of the wolffian ducts and is completed by 13th week of gestation. If androgens are absent (in females, or in males in which androgen production or action is disrupted) the indifferent external genitalia remain unfused and follow the female pattern of differentiation. In female fetuses exposed to abnormally high levels of extragonadal androgens (e.g., in congenital adrenal hyperplasia; Box 53-4) virilization of the urogenital sinus occurs; the extent of the virilization is more pronounced the earlier in development the exposure occurs. Once the labioscrotal swellings have established the female form, androgen-induced virilization affects mainly the size of the clitoris. However, if androgen exposure occurs earlier in development, partial fusion of the labioscrotal swellings can occur. The cells of the urogenital sinus and external genitalia, unlike those of the wolffian duct, contain 5α-reductase and are thus capable of converting testosterone to DHT. Indeed, conversion of testosterone to DHT is usually required for normal male development of the external genitalia.
Congenital Adrenal Hyperplasia
Ambiguous genitalia in genotypic females may result from disorders of adrenal function. Several forms of congenital adrenal hyperplasia have been described, including the deficiency of several enzymes involved in steroid synthesis (see Fig. 50-2): the side-chain-cleavage enzyme, 17α-hydroxylase, 21α-hydroxylase, 11β-hydroxylase, and 3β-hydroxysteroid dehydrogenase. Deficiencies in 21α-hydroxylase, 11β-hydroxylase, and 3β-hydroxysteroid dehydrogenase all lead to virilization in females—and thus ambiguous genitalia—as a result of the hypersecretion of adrenal androgens. 21α-hydroxylase deficiency, by far the most common, accounts for ~95% of cases. Some of the consequences of this deficiency are discussed in Box 50-2.
As indicated by Figure 50-2, 21α-hydroxylase deficiency reduces the conversion of progesterone to 11-deoxycorticosterone—which goes on to form aldosterone—and also reduces the conversion of 17α-hydroxyprogesterone to 11-deoxycortisol—which is the precursor of cortisol. As a result, adrenal steroid precursors are shunted into androgen pathways. Because negative feedback by cortisol to the hypothalamic-pituitary-adrenal axis (see Fig. 50-3) is lacking in the fetus, the adrenal gland experiences high levels of adrenocorticotropic hormone (ACTH), which induce the production of the weak androgen DHEA by the fetal zone of the adrenal cortex. The abnormally high levels of DHEA interact with the androgen receptor in tissue of the urogenital sinus to induce masculinization. In female infants, the result is sometimes called the adrenogenital syndrome. The external genitalia are difficult to distinguish from male genitalia on visual inspection. The clitoris is enlarged and resembles a penis, and the labioscrotal folds are enlarged and fused and resemble a scrotum. The genitalia thus have a male phenotype in an otherwise normal female infant.
In the presence of high intracellular concentrations of DHT, the genital tubercle, the bipotential predecessor to either a clitoris or a penis, elongates to become the glans penis, the corpus spongiosum, and the two corpora cavernosa. Formation of the penis and scrotum is complete by ~13 weeks of gestation. Congenital absence of 5α-reductase (see Box 53-3) is associated with normal development of the wolffian duct system but impaired virilization of the external genitalia. Even extremely high concentrations of testosterone after this time fail to cause midline fusion of the urethral groove or scrotum, although the clitoris will enlarge.
In the absence of androgen secretion by the fetal testis—or abnormal extragonadal sources—the indifferent external genitalia remain unfused and follow the female pattern of differentiation (Box 53-5).
Androgen Dependence of Testicular Descent
In preparation for descent, the testes enlarge. In addition, the mesonephric kidneys and wolffian (mesonephric) ducts atrophy. This process frees the testes for their future move down the posterior abdominal wall and across the abdomen to the deep inguinal rings. Testicular descent occurs in three phases during the last two thirds of gestation. During the first stage of testicular descent, rapid growth of the abdominopelvic region causes relative movement of the testes down to the inguinal region (Fig. 53-10A). The role of the gubernaculum—the ligament attaching the inferior part of the testis to the lower segment of the labioscrotal fold—is uncertain. However, the gubernaculum shortens and appears to guide the testis to its place of ultimate functional residence in the scrotum. The second stage of testicular descent is herniation of the abdominal wall adjacent to the gubernaculum (see Fig. 53-10B). This herniation, which occurs as a result of increasing abdominal pressure, forms the processus vaginalis; the processus vaginalis then folds around the gubernaculum and creates the inguinal canal. In the third stage, the gubernaculum increases to the approximate diameter of the testis. As its proximal portion degenerates, the gubernaculum draws the testis into the scrotum through the processus vaginalis (see Fig. 53-10C).
FIGURE 53-10 Testicular descent.
The testes usually complete their descent by the seventh month of gestation; ~97.5% of full-term infants and 79% of premature infants have fully descended testes at birth. At 9 months of age, only 0.8% of male infants have undescended testes. The incidence of undescended testes in young men is 0.2%.
Testicular descent is an androgen-dependent process, and development of the structures involved in testicular descent depends on testosterone. Thus, in testosterone-deficient states caused by inadequate secretion or disordered androgen action, the testes of genetic males often fail to descend. This abnormality can be seen in individuals with both 5α-reductase deficiency and complete androgen resistance (i.e., testicular feminization syndrome).
Androgens and estrogens influence sexual differentiation of the brain
Gonadal steroids influence the development of sexually dimorphic nuclei in the diencephalons of rodents and lower primates. However, androgens do not act directly on the hypothalamus and other areas of the brain having to do with sex behavior and control of gonadotropin secretion. Rather, aromatase—which catalyzes the formation of estrone and estradiol (see Fig. 55-8)—converts androgens to estrogens in the brain. Thus, androgens in the brain serve as prohormones for estrogens. Therefore, estrogens are derived from androgens that appear to masculinize sexually dimorphic nuclei directly in the brain. It is not clear why, in females, estrogens do not masculinize the brain.
Gonadal steroids affect sex behavior in both males and females. In rodents, lordosis behavior in females and mounting behavior in males is one example of sex behavior. An example of functional sexual dimorphism in the human brain is the manner in which gonadotropin is released. Gonadotropin release has been described as cyclic in the female and tonic in the male inasmuch as females have midcycle cyclic release of gonadotropin before ovulation whereas males seem to have a continuous tonic pattern of gonadotropin release.
Although controversy continues over the role of prenatal virilization in the determination of sexual dimorphism, sex steroids clearly have an impact on sexual behavior and sexual reference in humans.