Germ cells in the ovarian surface during fetal development in humans. A three-dimensional microanatomical study by scanning and transmission electron microscopy.

Observations by correlated scanning and transmission electron microscopy during human ovarian development (from 7 weeks to delivery) have revealed germ cells in the surface epithelium of the ovary. During embryonal differentiation (7/8 weeks) and in early stages of fetal development (9-12 weeks) the ameboid features of some primitive germ cells occurring in the cortical areas of the ovary, would suggest their capability to move and possibly migrate to the superficial coelomic layer covering the gonad. Subsequently the germ cells, gradually transformed in clusters of dividing oogonia and further meiotic oocytes, were closely associated to proliferating somatic cells of sexual cords and often resulted passively pushed and consequently dislocated in the most superficial areas of the ovary. As a result of these intense morphogenetic processes--even when the ovary was fully developed--the germ cells which were early in these superficial areas became included in the surface epithelium of the ovary. Therefore, through all stages of ovarian development and from these areas, germ cells were extruded on the surface of the gonad and ultimately liberated in the peritoneal cavity. Since germ cells were found within the surface epithelium of the ovary even in two newborns, and since some of these oocytes were eliminated from the ovary, it is possible that this extrusion mechanism might even continue to operate during the period between birth and puberty. On the other hand germ cells that in early stages had been more deeply located in the ovarian tissues, closely associated with the proliferating sex cords, maintained this relationship and became later incorporated in primitive follicles. Present evidence suggests that the above morphodynamics can be most likely depending upon a special tropism of germ cells towards the epithelioid components of the surface epithelium and mesonephric cords of the developing ovary. This close and early relationship between somatic and germ components is recognized as crucial to further development and fate of germ cell population in human ovary.