Gonads: indifferent stage

The gonads arise from two very different kinds of cells that originate in the embryo:

• The primordial germ cells (PGC) will form the gametes (sperm cells and oocytes). These cells come from the ectoderm, but they separate themselves from it at a very early stage in the development.
• The somatic cells with nourishing functions surround the primordial germ cells and form the somatic gonadal blastema. In the testes the supporting cells (Sertoli) and the interstitial cells (Leydig) are involved, in the ovary the follicle cells and the theca cells.
  Their origin is still a matter of discussion, however, whereby three possible sources come into question: mesonephros, local mesenchyma, as well as superficial epithelial cells (coelomic epithelium).

The primordial germ cells (PGC) already appear at the time of gastrulation (stage 6) in the epiblast and complete an emigration out of the embryo into the wall of the yolk sac (umbilical vesicle). Thanks to the collaboration of three factors - the folding of the embryo, chemotactic factors and ameboid movements - they get from here into the wall of the intestinal tract within the embryo again and, via the dorsal mesentery, into the gonadal ridge (stage 14). During their migration, which takes place between the 4th and 6th weeks (stages 11 - 14), they multiply through mitosis.

Like the nephrogenic cord the gonadal ridge extends from the heart region to the location near the cloaca. In the time between the 4th and 6th week (stages 11 - 14) the middle section of this gonadal ridge develops into a gonad anlage in that cells of the coelomic epithelium proliferate there. The immigrated PGC (stage 14) penetrate into this thickened zone of the coelomic epithelium. The indifferent gonads thus assemble themselves from cells of various origins, whereby, as a result, the primordial germ cells and the local somatic blastema influence each other reciprocally.

The local coelomic mesenchyma underneath also multiplies. The coelomic epithelium, which now becomes multi-layered, loses for now its basal membrane. Gonadal cords arise that surround the PGC and extend into the depths. One knows that - in the male - mesonephric cells are also involved, but it is unknown whether in females the cells immigrate as far as the gonadal ridge.

Up to the 6th week male and female gonads cannot be distinguished. The gonadal cords and the PGC can be found both in the cortical as well as in the medullar zones of the future gonads.