In the chapter on renal development (see Chapter 36) we talked about the development of the gonadal ridge from intermediate mesoderm, an important source of cells for the reproductive system and the location for the beginning of the development of the gonads.
Gonads are formed from three sources of cells: the intermediate mesoderm, the mesodermal epithelium that lines the developing urogenital ridge and germ cells.
Germ cells originate in the extra‐embryonic endoderm of the yolk sac near the allantois and migrate along the dorsal mesentery of the hindgut to reach the gonadal ridge at the beginning of week 5 (Figure 38.1). By the sixth week they invade the gonadal ridge (see Figure 36.2). Also at this time the epithelium overlying the mesoderm begins to proliferate, penetrating the mesoderm and forming cords that are continuous with the surface epithelium (Figure 38.2).
This indifferent gonad has a discernible external cortex and internal medulla. If the migrating germ cells fail to arrive the gonads will not develop because of the absence of reciprocal interactions between germ cells and surrounding epithelia.
In the early female gonad the cortex develops and the medulla regresses. The primitive sex cords dissociate and form irregular cell clusters containing germ cells (Figure 38.3). These cords and clusters disappear and are replaced with blood vessels and connective tissue.
Surface epithelia continue to proliferate and produce a second wave of sex cords that remain close to the surface. In the fourth month of development these also dissociate and form cell clusters surrounding one or more germ cells. This is the primitive follicle and the sur-rounding epithelial cells develop into follicular cells (see Figure 9.1). Each primitive germ cell becomes an oogonium. Oogonia divide significantly before birth but there is no division postnatally.
A part of peritoneum attached to the gonad develops into the gubernaculum. This structure passes through the abdominal wall (the future inguinal canal) and attaches to the internal surface of the labioscrotal swellings (see Figure 37.8). The ovaries descend into the pelvis, and the gubernaculum becomes attached to the uterus. In the adult the gubernaculum remains as the round ligament (passing through the inguinal canal) of the uterus and the ovarian ligament.
The cortex regresses and the medulla develops (Figure 38.4). Testes develop quicker than ovaries, and the primitive sex cords do not degenerate but continue to grow into the medulla.
Testosterone producing cells, called Leydig cells, develop from mesoderm of the gonadal ridge and are located between the developing sex cords. They produce testosterone by week 8.
The primitive sex cords break up and form two networks of tubes: the rete testis and the seminiferous tubules. The tunica albuginea (thick fibrous connective tissue) develops to separate the networks from the surface epithelia. The rete testes are the connection between the seminiferous tubules and the efferent ducts of the testes (see Figure 8.1), which are derived from the mesonephric tubules (see Chapter 36).
In the fourth month the seminiferous tubules contain two important cell types: primitive germ cells that form spermatogonia, and Sertoli cells that have support roles for the cells passing through spermatogenesis. The male gubernaculum runs from the inferior pole of the testis to the labioscrotal folds (see Figure 37.9) and guides the testis into the scrotum, along with the ductus deferens and its blood vessels, as the foetus becomes longer and the pelvis becomes larger. The inguinal canal normally closes behind the testis, but failure of this process increases the risk of an indirect inguinal hernia.
The gonads develop in the abdomen and hence receive their blood supply directly from the abdominal aorta. The male arteries are called the testicular arteries and the female arteries the ovarian arteries. During the descent of the testes their blood vessels are pulled behind them as they pass through the inguinal canal and into the scrotum. The lymphatic system of both gonads also follows these pathways.
Undescended testes (cryptorchidism) describes the failure of the testes to descend normally into the scrotum by birth. This may occur bilaterally or unilaterally, and is more common in premature males. The testes may remain in the abdominal cavity, at a point along their normal route of descent or within the inguinal canal (Figure 38.5). Often, the testes will have descended to the scrotum by the end of the first year, but testes that remain undescended are likely to cause fertility problems. Undescended testes, even if they later descend, are linked to an increased risk of testicular cancer.
Hormonal imbalances can result in a varied range of developmental abnormalities to the reproductive system. Chromosomal defects are also responsible for many genital abnormalities, often presenting with other congenital defects. Those with gonadal dysgenesis have male chromosomes but no testes. Patients can have female external genitalia and underdeveloped female internal genitalia or ambiguous external genitalia and a mixture of both sexes internally, but are often raised as girls.
Ovarian and testicular cancers are relatively common forms of cancer. If testicular cancer is suspected it is often from a lump found in one testis and diagnosed through an ultrasound scan. It is important to remember that lymph drainage is to the retroperitoneal para‐aortic lymph nodes rather than pelvic nodes, and these are involved in the staging of testicular cancer. Affected nodes must also be removed surgically together with the testis. The prognosis for testicular cancers is generally good. Ovarian cancer symptoms are often absent and if present, unspecific. An increase in abdominal size and urinary problems are possible. Surgical treatment is often required but because of the lack of e diagnosis the prognosis is generally poor.