99. Development of The Testis

  1. Testicular differentiation: The male gonad develops into the testis toward week 7 of development. Until then, it is undifferentiated. The differentiation is determined by the XY genetic constitution and proceeds as follows
    1. THE PRIMARY SEX CORDS, proliferating from the coelomic epithelium, condense and extend into the medulla of the gonad.
      1. In the medulla, the cords branch, their deep ends anastomose, and they form the rete testis
      2. The prominent sex cords become the seminiferous or testicular cords which soon lose their connections with the germinal epithelium because of the development of a thick fibrous capsule, the tunica albuginea
        1. The tunica albuginea is a layer of connective tissue which is interposed early, between the coelomic epithelium (parietal peritoneum) and the rest of the glan It produces partitions which compartmentalize the gland, closing off the seminiferous ducts, about day 50, into testes cords
        2. Development of the tunica albuginea is a characteristic and diagnostic feature of testicular development
      3. The seminiferous or testicular cords develop into the seminiferous tubules, whose deep portions narrow to form the tubuli recti, which converge on the rete testis
      4. The seminiferous tubules become separated by mesenchyme which gives rise to the interstitial cells of Leydig
        1. It is here that the androgenic hormones are secreted which help in the differentiation of the genital tract and the external genital organs
        2. The interstitial cells of Leydig reach their maximum development between 3 1/2 and 4 months
      5. The walls of the seminiferous tubules, as a result of their cellular duality of origin, are composed of 2 types of cells: supporting or sustentacular cells of Sertoli, derived from the germinal epithelium, and the spermatogonia, derived from the primordial germ cells (unlimited in number, in contrast to the oogonia)
        1. The cells of Sertoli make up most of the seminiferous epithelium in the fetal testis
      6. Gradually, the enlarging testis separates from the regressing mesonephros and is suspended by its own mesentery, the mesorchium
    2. IN LATER DEVELOPMENT, THE GERMINAL EPITHELIUM flattens to form the mesothelium on the surface of the testis and the rete testis becomes continuous with the 15 to 20 adjacent persistent mesonephric tubules
      1. The persistent mesonephric tubules, after regression of the mesonephric (wolffian) body, participate in the formation of the excretory tracts of the testis, forming the vasa efferentia or efferent ductules
      2. The efferent ductules open into the adjacent mesonephric duct which becomes the ductus epididymidis (epididymis)
      3. Thus, the vasa efferentia and the epididymis are of mesonephric origin

development of the testis: image #1