Internal genitalia

Unlike the bipotential gonads and external genitalia, the male and female internal genitalia arise from separate duct systems (Fig. 6.1). Development of these structures occurs in parallel and in close physical proximity with the developing urinary system. Both begin to occur at about 4 embryonic (6 menstrual) weeks. The primordial kidney (mesonephros) is composed of tubules and a duct known as the mesonephric or Wolffian duct. The Wolffian duct grows out from the tubules toward the urogenital sinus. The mesonephric tubules make contact with the primitive sex cords just as the gonad begins to differentiate. Simultaneously, an inpocketing of the coelomic epithelium near the lateral edge of the mesonephric ridge forms the paramesonephric or Müllerian duct. As kidney development proceeds (metanephric stage), the mesonephric structures will become totally incorporated into the reproductive tract and lose their urinary function. The Wolffian and Müllerian ducts are primordia for the internal organs of reproduction in the male and female, respectively. In each sex, the other duct system typically disappears by the 3rd fetal month, leaving behind vestiges that are usually unimportant clinically.

In the normal male embryo, the secretion of a peptide called Müllerian-inhibiting substance (MIS; also known as anti-Mullerian hormone or AMH) occurs under the direction of sex-determining region of the Y chromosome (SRY). MIS is secreted by cells that will become Sertoli cells in the adult testis. MIS causes the Müllerian duct to degenerate. Testosterone is produced by those testicular cells destined to become Leydig cells in the adult. Testosterone directs development of the Wolffian duct system to form the epididymis, vas deferens and seminal vesicles. In contrast to the adult, testosterone production by the embryonic testes is controlled not by the hypothalamic–pituitary system, but by the placental hormone human chorionic gonadotropin (hCG).

The absence of MIS in the female embryo permits the Müllerian system to persist. Upon reaching the urogenital sinus, the Müllerian ducts induce the formation of a vaginal plate. Contact of the Müllerian ducts with the vaginal plate also initiates the fusion of the ducts to form the body of the uterus. The Müllerian ducts will form the fallopian tubes, uterus and the upper portion of the vagina. Failure of the Müllerian ducts to develop or fuse completely can cause uterine and cervical anomalies. In the absence of testosterone, the Wolffian system regresses. A vestige of the Wolffian duct, known as Gartner’s duct, persists in its length from the ovary to the hymen. Clinically apparent cysts may form anywhere along Gartner’s duct.

Most of the prostate gland develops from the same primordial area of the urogenital sinus that forms the vaginal plate in the female, making the prostate a homolog of the upper vagina. Mesenchyme in this tissue differentiates into the peripheral zone of the prostate, under the influence of dihydrotestosterone (DHT). In the presence of a functional fetal testis, DHT is produced locally from testosterone by the enzyme 5α-reductase. The more central tissue in this area, which may be of Wolffian derivation, forms the central and transition zones of the prostate. Cancers of the prostate are most likely to arise from the peripheral zone (Chapter 41).