DEVELOPMENT OF THE MALE REPRODUCTIVE ORGANS

Although the genetic sex of the embryo is decided when the male and female gametes combine, the early stages of morphological differentiation of the reproductive organs follow an indifferent pattern that is common to the two sexes.

An early indication that the gonad will become a testis is provided by a marked mesenchymal condensa­tion (tunica albuginea) below the celomic epithelium. Now isolated from the surface epithelium, the cords increase in size and in complexity of arrangement (Figure 5-8/5). They connect to a plexus or network (rete) within the testis. On the other side the plexus makes contact with the blind ends of the few tubules that have survived the general regression of the meso­nephros (Figure 5-8, B/5-5). Differentiation within the cell cords permits recognition of two cell lineages. One provides the sustentacular (Sertoli) cells of the seminif­erous tubules; the second, contributed by the primordial germ cells, provides the germinal epithelium. During fetal development the primordial germ cells differenti­ate into gonocytes, which after birth give rise to

Figure 5-7 A, Early development of the indifferent gonad.

spermatogonia. At puberty, the spermatogonia prolifer­ate and differentiate to supply cells that undergo meiosis and spermiogenesis to form male gametes (see Figure 5-39). Sections through the adult testis show seminifer­ous tubules cut in various planes. The walls of the highly convoluted tubules are lined by a stratified ger­minal epithelium consisting of cells in various stages of differentiation. Supporting Sertoli cells nourish the germ cells. Cells of an additional type can be identified. These, the Leydig cells, produce the steroid testosterone that is essential if spermatogenesis is to continue. Their progenitors, like those of Sertoli and primordial germ cells, presumably migrate from the mesonephros during fetal development to become embedded in a mesenchy­mal interstitium, and around puberty, when the process of spermatogenesis is initiated, a second generation of Leydig cells develops. The initial formation of the semi­niferous cords is followed in later fetal life by canaliza­tion of the cords to create a series of passages leading to the mesonephric duct, which thus becomes the outlet for the gamete products of the testis. The peripheral parts of the cords become seminiferous tubules, the central parts become the rete testis, and the mesoneph­ric tubules become the efferent ductules (Figure 5-8, C). The first part of the mesonephric duct convolutes and forms the duct of the epididymis within the dense con­nective tissue of that organ; the remaining part retains a straighter course, and as the deferent duct (Figure 5-5/3), it opens into that part of the cloaca that becomes the urogenital sinus (Figure 5-5/6). Glandular prolifera­tion of the lining of the duct toward its termination produces the ampullary thickening, while in most species, but not in carnivores, a subterminal budding enlarges as the vesicular gland (Figure 5-9/5). In some species a final short passage, the ejaculatory duct, per­sists, but in others later adjustments cause the deferent and vesicular ducts to open separately.

The division of the cloaca has been described (p. 147). The caudal part of the sinus constitutes the pelvic part of the urethra. Outgrowths from its lining differ­entiate into the prostate and bulbourethral glands in a species-characteristic fashion (see Figure 5-9). The greater part of the male urethra lies within the penis and has a different origin. Thickenings appear around the margin of the urogenital membrane in the indiffer­ent stage (Figure 5-10). One, ventral and median, con­stitutes the genital (phallic) tubercle or swelling (Figure 5-10/7), which gives rise to the greater part of the penis; other thickenings that are more lateral in position con­tribute the scrotum. A further urogenital fold that appears medial to each scrotal swelling makes an addi­tional contribution to the penis. A groove extends along the (initially) dorsal surface of the genital tubercle; it is gradually closed by the approach and mergence of these urogenital folds. This process is rather complex as the lining of the penile urethra is provided by an extension of the endoderm of the urogenital sinus, although the initial swellings have ectodermal coverings. The corpus spongiosum (spongy body) of the penile urethra directly continues the bulbar tissue of the pelvic urethra, while

Figure 5-8 Three stages in the development of the testis. A, The epithelial cords are isolated from the surface epithelium by the formation of the tunica albuginea. B, The epithelial cords, rete, and mesonephric tubules have interconnected.

the corpus cavernosum penis forms within the genital swelling. The lateral swellings grow and join together to form the scrotum, which retains evidence of its bilateral origin in a median raphe and septum.

Differentiation of the male efferent duct system, accessory glands, and external genitalia depends on the presence of testosterone, the male sex hormone pro­duced by the developing testes. The testes also produce several other hormones, for example, the antimullerian hormone (AMH) and insulin-like factor 3 (descendine), respectively responsible for the disappearance of the mullerian duct and the outgrowth of the gubernaculum. Without exposure to these three hormones the genital tract would develop in the female direction. Removal of the pituitary by decapitation in the fetal period does not disturb the production of these hormones by the testis (Figure 5-11, A-B).