Cytological evidence for an inactive X chromosome in murine oogonia

Purified fetal mouse germ cell preparations have been analyzed for the presence of a heterochromatic X chromosome at cell division. A single large heterochromatic chromosome is found in oogonia stage germ cells, but such a chromosome is not present in 16-day fetal oocytes. These results support the idea that mitotic stage germ cells are subject to X-chromosome inactivation and that they go through a reactivation step at meiosis.