Meiosis and differentiation of mouse germ cells.

The female pathway of germ cell development is characterized by early entry into meiotic prophase, before birth in the mouse. This pathway is followed by all germ cells in the ovary and in the mesonephric region of the urogenital ridge in female embryos, and by all germ cells in the adrenals of both sexes. The male pathway, with meiosis delayed until well after birth, is taken by all germ cells within the testis cords, all or almost all of those within the testis but outside the cords, and most of those in the mesonephric region. Results of culture and co-culture experiments are described. No conclusive evidence has yet been obtained to discriminate between the hypothesis that all germ cells spontaneously enter meiosis before birth unless prevented from doing so, and the alternative hypothesis that germ cells only enter meiosis under the inducing influence of somatic cells. The pathways of development that radiate from an egg cell all lead to a state of terminal differentiation, with the single exception of the pathway that leads to another egg cell. This pathway is capable of cyclic repetition, even without the intervention of a fertilizing sperm: if a normal mouse embryo is aggregated with one derived from a parthenogenetically activated egg, the parthenogenetic component may contribute to the oocyte population as well as to all other cell types examined (Stevens, 1978).