Development of the indifferent gonad

The gonads develop from the intermediate mesoderm that is situated in the paired longitudinal urogenital ridges, the more medial part of these ridges being the gonadal ridges. In the sixth week primordial germ cells migrate from the wall of the yolk sac via the dorsal mesentery of the hindgut to occupy the gonadal ridges. The arrival of these cells induces the cells in the ridges to form primitive sex cords (derived from the mesonephros and overlying coelomic epithelium) (Fig. 9.1A). At this stage the gonad is indifferent or uncommitted (Fig. 9.2), and consists of an outer cortex and an inner medulla. During the next week the male and female gonads begin to differentiate (Fig. 9.1B-D). If germ cells fail to migrate to the gonadal ridges the gonads do not develop.

Fig. 9.1 Formation of gonads during the fifth week. The migration of primordial germ cells to the gonadal ridges is shown in (A). (B) Indifferent stage at 7 weeks. (C) Male genital system in a newborn. (D) Female genital system in a newborn.

Fig. 9.2 Indifferent genital system at 6 weeks.

Development of the testis (Fig. 9.3)

The sex-determining gene in the Y chromosome produces a protein (testis-determining factor) that promotes the development of a testis: the primitive sex cords proliferate and penetrate into the medulla to form the testicular cords. Some of those cells differentiate into Sertoli cells, whilst the remainder become incorporated into seminiferous tubules. The latter are solid cylinders until after puberty at which time they canalize. The testicular cords anastomose to form the rete testis, which becomes continuous with 15–20 persisting mesonephric tubules, the ductuli efferentes. The testis-determining factor also induces differentiation of gonadal mesenchymal cells into the interstitial Leydig cells.

Fig. 9.3 Development of the testis and male genital ducts.

Development of the ovary (Fig. 9.4)

In embryos where no testis-determining factor is produced the primitive sex cords extend into the medulla of the gonad, degenerate and form a vascular stroma, resulting in an ovary. The surface epithelium of the female gonad, unlike that of the male, gives rise to second generation cortical cords. In the fourth month these cords invest the primordial germ cells to form the ovarian follicles.

Fig. 9.4 Differentiation of the indifferent gonad into an ovary.

Genital ducts

Initially, two pairs of genital ducts arise in both male and female: the mesonephric and paramesonephric ducts (Fig. 9.1B–D).

In the male, the mesonephric duct loses its urinary function once the mesonephros is superseded by the metanephros. The mesonephric duct becomes the ductus deferens and the epididymis (the main genital ducts in the male), opening into the urogenital sinus (the anterior part of the cloaca) (Fig. 9.1). The secretion of testosterone by the interstitial Leydig cells from week 8 stimulates the mesonephric ducts to differentiate into their adult derivatives. Each seminal vesicle buds out from the distal mesonephric duct. The prostate arises as a bud from the urethra (Fig. 9.5). The regression of the paramesonephric ducts in the male is stimulated by the anti-mullerian hormone, secreted by the Sertoli cells. The old name for the paramesonephric duct was mullerian duct; the old name for the mesonephric duct was wolffian duct.

Fig. 9.5 Development of the accessory glands of the male genital system at 10 weeks (A) and 12 weeks (B).

In the female, the mesonephric duct is not involved in genital duct formation. In the absence of anti-mullerian hormone and testosterone the mesonephric ducts regress in the female. The paramesonephric duct system arises as a pair of longitudinal invaginations (Figs 9.1 and 9.2