The testes differentiate themselves earlier than the ovaries, namely in the course of the 7th week at about 44 days (stage 18).
Responsible for this is the SRY gene on the Y chromosome that induces the development of the testes through the activation of a series of further genes (sex-determining genetic factors and hormones).
Development of the parenchyma
The differentiation of Sertoli's supporting cells form the first step in the organogenesis of the testes. These cells come - in any case in mice - from pluripotent coelomic epithelial cells of the gonadal ridge. In the gonadal anlage, through the influence of genetic products that are activated by the SRY, they form intercellular membrane connections and in this way surround more and more the primordial germ cells, while growing at the same time as gonadal cords into the medulla.
In addition, in a male embryo, cells of mesonephric origin are involved as well in forming the gonadal cords, by accumulating on the outside of the gonadal cords and forming the peritubular myoblasts. From the gonadal cords the testicular cords form that then differentiate to become the convoluted seminiferous tubules (500 to 1000) and straight seminiferous tubules of the mature testicles.
Until puberty the coiled testicular cords are filled. During puberty they form lumens and are from then on called convoluted seminiferous tubules. The germ cells on the other hand divide mitotically, but their meiosis begins only with puberty. The deep portions of the coiled testicular cords, which are delimited by septa, are stretched and are called straight seminiferous tubules. These last go over into the rete testis, which is a labyrinth of small passages in the tunica albuginea. The thin wall possesses a cubic epithelium. During the 9th week, from 5-12 mesonephric tubules the efferent ductules form that bind with the rete testis in the 3rd month.
The efferent ductules form the connection between the rete testis and mesonephric duct. Towards the end of the 8th week, under the influence of testosterone, the cranial part of the mesonephric duct gets to be tightly coiled and so forms the ductus epididymidis which, outside the epididymis, continues as the deferent duct. (see: the inner sex passages).
After the 8th week certain mesenchymal cells between the testicular cords differentiate to become interstitial cells (Leydig), which produce testosterone. The testes thus represent an endocrine gland that produces androgens. The origin of these cells is still unclear – one suspects that a steroid-producing population of cells in the ventral part of the mesonephros differentiate and form both the origin of the adrenal cortex cells and also interstitial cells (Leydig).
Development of the stroma
The mesenchyma between the testicular cords congeals and forms connective tissue septa that subdivide the testicles into lobules (ca. 250-370).
In stage 22 (ca. 53 days) this mesenchyma also forms a taut connective tissue layer between the testicular cords and the coelomic epithelium as well as the future tunica albuginea. Finally, the coelomic epithelium transforms itself into a mesothelium, just like the coelomic epithelium around the other serous cavities (peritoneum, pleura, pericardium).
- In the testicular cords PGC (future spermatozoa) are to be found. The somatic cells differentiate themselves into Sertoli's supporting cells, responsible for nourishing the spermatozoa and secreting the antimüllerian hormone (AMH), which promotes the atrophy of the paramesonephric duct (Müller).
- The rete testis forms the continuation of the centrally-lying testicular cords or the straight seminiferous tubules.
- The efferent tubules connect the rete testis with the mesonephric duct (Wolff), the future epididymis, which continues with the deferent duct.
- The interstitial mesenchymal cells of the testes develop into Leydig's interstitial cells. They are responsible for the production of testosterone that, among other things, assures that the mesonephric duct (Wolff) does not atrophy.
- The stroma, made of connective tissue, subdivides the testes into lobules and forms the tunica albuginea.