Formation of male and female sex cords in gonadal development of C57BL/6 mouse.

The sex cords of male and female fetal C57BL/6 mice were studied by light and transmission electron microscopy to elucidate the origins of Sertoli cells (male) and follicle cells (female) in detail. In the testes of fetal mice from day 12 to day 14 post coitum (p.c.), PAS-positive substances were detected exclusively throughout the cytoplasm of Sertoli cells. On day 12 p.c., pre-Sertoli cells, identified by PAS-reaction, formed irregular and cord-like arrangements around germ cells. The arrangements were not associated with the coelomic epithelium. Thus, it is suggested that Sertoli cells are originated from the mesonephric tissues. In the ovaries, germ cell cords were still not observed at a stage immediately following the gonadal sex differentiation in male. On about day 15 p.c., connective tissues including many capillaries penetrated into the ovaries, resulting in obvious formation of germ cell cord-like arrangements. At the same time, the coelomic epithelium-derived cells (CEd cells) as well as the mesonephros-derived cells (Md cells) invaded the adjacent arrangements. It is concluded that the follicle cells are originated most from Md cells and some from CEd cells.