DEVELOPMENT OF THE GENITAL TRACTS AND FUNCTIONAL RELATIONSHIPS OF THE GONADS

GENETICS AND BIOLOGY OF EARLY REPRODUCTIVE TRACT DEVELOPMENT

Most living species have some form of sex-determination system that drives the development and expression of sexual characteristics in that organism. Sex determination can be genetic or can be a consequence of environmental or social variables. In humans, sex determination is genetic and is governed by specific genes and chromosomes. It is believed that the two human sex chromosomes (X and Y) evolved from other nonsex chromosomes (autosomes) 300 million years ago. Human females have two of the same kind of sex chromosome (XX), whereas males have two distinct sex chromosomes (XY). However, both male and female features can rarely be found in one individual, and it is possible to have XY women and XX men. Analysis of such individuals has revealed the genes of sex determination, including SRY (sex-determining region Y gene) on the short arm of the Y chromosome, which is important for maleness. The SRY gene product is a protein that harbors a high-mobility group box (HMG) sequence, a highly conserved DNA-binding motif that kinks DNA. This DNA-bending effect alters gene expression, leading to formation of a testis and subsequently to the male phenotype. Notably, XY individuals who lack the SRY gene on the Y chromosome are phenotypic females.

It is now clear that the SRY gene does not act in isolation to determine human sex. Other genes in other locations are also important for complete male sexual differentiation. DAX1, a nuclear hormone receptor, can alter SRY activity during development by suppressing genes downstream to SRY that would normally induce testis differentiation. A second gene, WNT4 , largely confined to the adult ovary, may also serve as an “antitestis” gene. Indeed, the discovery of these genes has significantly altered theories of sex determination. Previously, SRY gene presence was thought to determine male gonadal development from the bipotential gonad. The female genotype was considered the “default” developmental pathway for gonads. It is now clear that genes such as WNT4 and DAX1 can proactively induce female gonadal development, even in the presence of SRY .

Once gonadal sex is determined, several other events must occur for normal male sexual differentiation. Within the testis, Leydig cells make testosterone, a hormone that is critical for development of the internal genitalia, including the vas deferens, epididymis, and seminal vesicles through wolffian duct differentiation. Leydig cells also synthesize insulin-like-3 to promote transabdominal testis migration that begins testis descent into the scrotum. Dihydrotestosterone (DHT), a testosterone metabolite, masculinizes the genital anlage to form the external genitalia, including the penis and scrotum as well as the prostate. In addition, Sertoli cells within the developing testis synthesize anti-müllerian hormone (AMH or MIF) , which prevents the müllerian duct from developing into uterus and fallopian tubes and helps the early germ cells remain quiescent in the developing testis. Deficiencies in any of these developmental pathways generally results in either birth defects or intersex disorders. Such development disorders, formerly termed true or pseudo-hermaphroditism , can include chromosomal abnormalities, ambiguous genitalia, phenotypic sex anomalies, or true intersex states.

HOMOLOGUES OF THE INTERNAL GENITALIA

Although sex is determined at the time of fertilization, phenotypic gender is determined by a complex tissue differentiation process that begins in the medial genital thickening or ridges on the posterior surface of the embryonic body cavity. During the 5th fetal week, primordial germ cells migrate from the yolk sac to the posterior body wall and induce the formation of genital ridges on either side of the midline. Here, these migrating cells induce the formation of undifferentiated primitive sex cords.

Signaled by the arrival of primordial germ cells, two sets of paired genital ducts, the mesonephric or nephric (wolffian) ducts and the paramesonephric (müllerian) ducts also develop. The mesonephros is a prominent excretory structure that consists of a series of mesonephric tubules that connect with the elongating mesonephric (wolffian) ducts as the latter extend caudally until they terminate in the urogenital sinus on each side of the midline. The paramesonephric ducts develop lateral to each of the mesonephric ducts and are derived from the evagination of the coelomic epithelium. The cephalad ends open directly into the peritoneal cavity, whereas the distal ends grow caudally, fuse in the lower midline, form the uterovaginal primordium, and join the urogenital sinus as an elevation, the müllerian tubercle, which separates the urogenital area from the more posterior gut.

Under the influence of the SRY gene in the male primitive sex cord, the mesonephric (wolffian) ducts are maintained during development. As the developing male Sertoli cells begin to differentiate in response to SRY, they secrete a glycoprotein hormone, müllerian-inhibiting substance (MIS) or anti-müllerian hormone (AMH) that causes the paramesonephric (müllerian) ducts to regress rapidly between the 8th and 10th fetal weeks. Müllerian duct remnants in the male include the appendix testis and the prostatic utricle. In females, MIS is not present, so müllerian ducts remain, and the mesonephric tubules and ducts degenerate in the absence of androgens, often resulting in remnant epoöphoron and paroöphoron cystic structures within the ovarian mesentery and Gartner duct cysts within the anterolateral vaginal wall. These structures are clinically important because they may develop into sizable and symptomatic cysts (see Plates 8-13 and 9-13 ).

In the male, under the influence of testosterone secreted by Leydig cells at 9 to 10 weeks, the majority of the mesonephric ducts develop into the vas deferens and body (corpus) and tail (cauda) of the epididymis. The mesonephric tubules nearest to presumptive testis form the globus major or caput of the epididymis and the efferent ductules that connect to the testis, forming ducts to transport sperm. The more cranial mesonephric tubules develop into the vestigial appendix epididymis, and the more caudal tubules may develop into remnants called paradidymis. The seminal vesicles sprout from the distal ends of the mesonephric ducts, whereas the prostate and bulbourethral glands develop from the urogenital sinus, thus revealing different embryologic origins. In the fully developed male embryo, the distal orifice of the mesonephric duct (ejaculatory duct) terminates in the verumontanum on the floor of the prostatic urethra.

During the 10th week of gestation in females, in the absence of MIS and androgens, the primordial müllerian ducts remain separate and form the fallopian tubes superiorly. At their caudal ends, the ducts join, fuse, and form a common channel called the uterovaginal canal, which later develops into the uterus and proximal four-fifths of the vagina. The remainder of the distal vagina forms from paired thickenings on the posterior urogenital sinus called sinovaginal bulbs and the vaginal plate, whose origin is not clear.

Intersex disorders can result from failure of the müllerian or wolffian ducts to regress completely. An example of this is hernia uteri inguinale or persistent müllerian duct syndrome, in which MIS deficiency or receptor abnormalities cause persistence of müllerian duct structures in an otherwise phenotypically normal male. This is commonly diagnosed during exploration for an infant hernia or undescended testicle because the müllerian structures can tether the testis in the abdomen and restrict normal scrotal descent. Vestigial remnants of the wolffian duct can also exist in fully developed females. Vestiges of the male prostate may appear as periurethral ducts in the female (see Plate 7-5 ). In addition, homologues of male Cowper glands are the major vestibular glands (Bartholin glands) in the female (see Plate 6-16 ).

HOMOLOGUES OF EXTERNAL GENITALIA

Before 9 weeks of gestation, both sexes have identical external genitalia, characterized by a urogenital sinus. At this undifferentiated stage, the external genitalia consist of a genital tubercle above a urethral groove. Lateral to this are urethral or urogenital folds and even more lateral are the labioscrotal swellings or folds. The male and female derivatives from these structures are shown.

The bladder and genital ducts find a common opening in the urogenital sinus. This sinus is formed from the earlier urogenital slit, which is a consequence of the perineal membrane separating the urogenital ducts from the single cloacal opening.

In male development, the genital tubercle elongates, forming a long urethral groove. The distal portion of the groove terminates in a solid epithelial plate (urethral plate) that extends into the glans penis and later canalizes. The midline fusion of the lateral urethral folds is the key step in forming a penile urethra, but this fusion only occurs after the urethral plate canalizes distally. In the female, the primitive structures do not lengthen and the urethral folds do not fuse in the midline. Instead they become the labia majora.

The vagina develops as a diverticulum of the urogenital sinus near the müllerian tubercle. It becomes contiguous with the distal end of the müllerian ducts. Roughly four-fifths of the vagina originates from the urogenital sinus and one-fifth is of müllerian origin. In the male, the vaginal remnant is usually extremely small, as the müllerian structures atrophy before the vaginal diverticulum develops. In intersex disorders (formerly called pseudohermaphroditism and most recently termed disorders of sexual development [DSD]) such as androgen insensitivity syndrome, however, an anatomic remnant of the vaginal diverticulum may persist as a blind vaginal pouch.

In normal female development, the vagina is pushed posteriorly by a down growth of connective tissue. By the 12th week of gestation, it acquires its own, separate opening. In female intersex disorders, the growth of this septum is incomplete, thus leading to persistence of the urogenital sinus.

Male and female external genitalia in the first trimester of development appear remarkably similar. The principal distinctions between them are the location and size of the vaginal diverticulum, the size of the phallus, and the degree of fusion of the urethral folds and the labioscrotal swellings.

FACTORS DETERMINING DIFFERENTIATION OF THE EXTERNAL GENITALIA

Similar to the genital ducts, there is a tendency for the external genitalia to develop along female lines. Masculinization of the genital ducts is induced by androgenic hormones, principally testosterone from Leydig cells in the fetal testis during the differentiation process. More important than the source of androgens, however, is the timing and amount of hormone. Examples of this include inappropriate androgen exposure from congenital adrenal hyperplasia or from the maternal circulation, both of which can induce various degrees of masculinization of the female system characteristic of intersex disorders. By the 12th week, androgenic exposure will no longer cause fusion of the urethral and labioscrotal folds in the female, as the vagina has migrated fully posteriorly. Clitoral hypertrophy, however, may still result from such exposures at any time in fetal life or even after birth.

TESTOSTERONE AND ESTROGEN SYNTHESIS

Under the control of the anterior pituitary, three glands produce steroid hormones involved in reproduction: the adrenal cortex responding to adrenocorticotropic hormone (ACTH), and the ovary and testis, both under the influence of the gonadotropin luteinizing hormone (LH). For the majority of sex hormones that result from this stimulation, cholesterol is the precursor molecule.

In each of these organs, side chains are degraded from cholesterol to form pregnenolone and dehydroepiandrosterone (DHEA). In humans, DHEA is the dominant sex steroid and precursor or prohormone to all other steroid sex hormones, including testosterone and estrogens. In the blood, most DHEA is found in its sulfate-bound form, DHEAS, and not in the free form. DHEA supplements are often used as muscle-building or performance-enhancing drugs by athletes. However, randomized placebo-controlled trials have found that DHEA supplementation has no effect on lean body mass, strength, or testosterone levels. Pregnenolone is converted to progesterone, which by degradation of its side chain is converted to androstenedione and then to testosterone. The latter two of these hormones are the main products of testicular Leydig cells. Androstenedione, also termed “andro,” is an FDA-banned dietary supplement that is also taken by athletes to improve performance. In the ovary, synthesis of androstenedione by theca interna cells and its subsequent conversion to estrone in follicular granulosa cells, along with conversion of testosterone to estradiol by aromatases, comprise the main secretory products. With polycystic ovary syndrome, enzymatic conversion of testosterone to estradiol in the ovary is impaired and DHEAS levels are elevated, leading to an androgenized phenotype in affected women. Estriol, a product of estrone metabolism in the placenta during pregnancy, is the third major estrogenic hormone in the female but is the least potent biologically.

About 5% of normal daily testosterone product is derived from the adrenal cortex, and the remainder is secreted by the testis into the systemic circulation. In the plasma, testosterone is virtually entirely bound (98%) by proteins such as sex hormone binding globulin or albumin. The remainder of testosterone (2%) exists in a free or unbound form, which is the active fraction. Testosterone is conjugated in the liver and excreted by the kidney in this water-soluble form. Circulating estrogens have a similar bioavailability profile and are also carried on plasma proteins, notably albumin. Inactivation of estrogen occurs in the liver through conversion to less active metabolites (estrone, estriol), by conjugation to glucuronic acid, or by oxidation to inert compounds. There is also considerable enterohepatic circulation of estrogens in the bile. Estrogen, testosterone, and their metabolites are ultimately excreted by the kidney, for the most part in the form of 17-ketosteroids in which a ketone group is present on the steroid ring. Examples of 17ketosteroids include androstenedione, androsterone, estrone, and DHEA.

Although important for premenopausal women, the value of estrogen and progesterone supplementation in postmenopausal women is controversial. A randomized, controlled trial of 15,730 women in the Women's Health Initiative was stopped early, after 5.6 years, because of the finding that risks (including stroke, blood clots, and breast cancer) outweighed benefits (lower risk of hip fractures and colon cancer) among subjects taking hormone supplements. Similarly, the value of testosterone supplements in older men who have reached andropause (androgen deficiency with age) is even more controversial, as large, randomized, placebo controlled trials of sufficient duration to assess long-term clinical outcomes and events have not been undertaken.

HYPOTHALAMIC–PITUITARY–GONADAL HORMONAL AXIS

The hypothalamic–pituitary–gonadal (HPG) axis plays a fundamental role in phenotypic gender development during embryogenesis, sexual maturation during puberty, and endocrine (hormone) and exocrine (oocytes and sperm) function of the mature ovary and testis. Importantly, gonadal function throughout life, similar to the adrenal cortex and thyroid, is under the control of the adenohypophysis (anterior lobe of the pituitary) and hypothalamus.