The development of the reproductive systems begins soon after fertilization of the egg, with primordial gonads beginning to develop approximately one month after conception. Reproductive development continues in utero, but there is little change in the reproductive system between infancy and puberty. In both male and female embryos, the same group of cells has the potential to develop into either the male or female gonads; this tissue is considered bipotential. The SRY gene actively recruits other genes that begin to develop the testes, and suppresses genes that are important in female development. As part of this SRY-prompted cascade, germ cells in the bipotential gonads differentiate into spermatogonia. Without SRY, different genes are expressed, oogonia form, and primordial follicles develop in the primitive ovary.
do not develop. Hence, the primordial germ cells have an inductive influence on development of the gonad into ovary or testis. Shortly before and during arrival of primordial germ cells, the epithelium of the genital ridge proliferates, and epithelial cells penetrate the underlying mesenchyme. Here they form a number of irregularly shaped cords, the primitive sex cords (Fig. 16.19). In both male and female embryos, these cords are connected to surface epithelium, and it is impossible to differentiate between the male and female gonad. Hence, the gonad is known as the indifferent gonad.
Testis:
If the embryo is genetically male, the primordial germ cells carry an XY sex chromosome complex. Under infl uence of the SRY gene on the Y chromosome, which encodes the testis determining factor, the primitive sex cords continue to proliferate and penetrate deep into the medulla to form the testis or medullary cords (Figs. 16.20A and 16.21). Toward the hilum of the gland, the cords break up into a network of tiny cell strands that later give rise to tubules of the rete testis. During further development, a dense layer of fibrous connective tissue, the tunica albuginea, separates the testis cords from the surface epithelium (Fig. 16.20). In the fourth month, the testis cords become horseshoe-shaped, and their extremities are continuous with those of the rete testis. Testis cords are now composed of primitive germ cells and sustentacular cells of Sertoli derived from the surface epithelium of the gland. Interstitial cells of Leydig, derived from the original mesenchyme of the gonadal ridge, lie between the testis cords. They begin development shortly after onset of differentiation of these cords. By the eighth week of gestation, Leydig cells begin production of testosterone and the testis is able to influence sexual differentiation of the genital ducts and external genitalia. Testis cords remain solid until puberty, when they acquire a lumen, thus forming the seminiferous tubules. Once the seminiferous tubules which in turn enter the ductuli efferent. These efferent ductules are the remaining parts of the excretory tubules of the mesonephric system. They link the rete testis and the mesonephric or wolffian duct, which becomes the ductus deferens (Fig. 16.20B).
Ovary:
In female embryos with an XX sex chromosome complement and no Y chromosome, primitive sex cords dissociate into irregular cell clusters. These clusters, containing groups of primitive germ cells, occupy the medullary part of the ovary. Later, they disappear and are replaced by a vascular stroma that forms the ovarian medulla. The surface epithelium of the female gonad, unlike that of the male, continues to proliferate. In the seventh week, it gives rise to a second generation of cords, cortical cords, which penetrate the underlying mesenchyme but remain close to the surface. In the third month, these cords split into isolated cell clusters. Cells in these clusters continue to proliferate and begin to surround each oogonium with a layer of epithelial cells called follicular cells. Together, the oogonia and follicular cells constitute a primordial follicle .It may thus be stated that the genetic sex of an embryo is determined at the time of fertilization, depending on whether the spermatocyte carries an X or a Y chromosome. In embryos with an XX sex chromosome configuration, medullary cords of the gonad regress, and a secondary generation of cortical cords develops.In embryos with an XY sex chromosome complex, medullary cords develop into testis cords, and secondary cortical cords fail to develop
Genital Ducts
Indifferent Stage:Initially, both male and female embryos have two pairs of genital ducts: mesonephric (wolffian) ducts and paramesonephric (müllerian) ducts. The paramesonephric duct arises as a longitudinal invagination of the epithelium on the anterolateral surface of the urogenital ridge. Cranially, the duct opens into the abdominal cavity with a funnel-like structure. Caudally, it fi rst runs lateral to the mesonephric duct, then crosses it ventrally to grow caudomedially. In the midline, it comes in close contact with the paramesonephric duct from the opposite side. The two ducts are initially separated by a septum but later fuse to form the uterine canal. The caudal tip of wall of the urogenital sinus, where it causes a small swelling, the paramesonephric or müllerian tubercle. The mesonephric ducts open into the urogenital sinus on either side of the müllerian tubercle.
Genital Ducts in the Male:
As the mesonephros regresses, a few excretory tubules, the epigenital tubules, establish contact with cords of the rete testis and finally form the efferent ductules of the testis. Excretory tubules along the caudalpole of the testis, the paragenital tubules, do not join the cords of the rete testis Their vestiges are collectively known as the paradidymis Except for the most cranial portion, the appendix epididymis, the mesonephric ducts persist and form the main genital ducts Immediately below the entrance of the efferent ductules, the mesonephric ducts elongate and become highly convoluted, forming the (ductus epididymis. From the tail of the epididymis to the outbudding of the seminal vesicle, the mesonephric ducts obtain a thick muscular coat and form the ductus deferens. The region of the ducts beyond the seminal vesicles is the ejaculatory
duct. Under the influence of antimullerian hormone (AMH) produced by sertoli cells, paramesonephric ducts in the male degenerate except for a small portion at their cranial ends, the appendix testis.