Ectopic germ cells: natural model for the study of germ cell sexual differentiation.

In the course of a study on the morphogenesis of the adrenal gland in random-bred Swiss albino mice, we noted the presence of ectopic germ cells in the adrenal cortexes and medullas in animals of both sexes, from day 12 1/2 of fetal development to postnatal day 12. Up to day 15 of fetal development, the cells exhibited characteristics of primordial germ cells. At day 17, and irrespective of the sex of the fetus, they all entered meiosis in synchrony with those in the ovary. Postnatally, in females as well as males, all ectopic germ cells displayed morphologic characteristics identical to those of young oocytes in unilaminar ovarian follicles. No germinal elements were seen in the adrenal glands past day 12 of life. Our study shows that mammalian germ cells are capable of undergoing sustained differentiation outside the gonads and that, in ectopic sites, they all differentiate into oocytes as they normally would in the ovary, even in males.