X chromosome reactivation in oocytes of Mus caroli.

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Abstract
Mature mammalian oocytes have both of their X chromosomes active, but somatic cells from the same individual have 1 of their X chromosomes in an inactive state. Whether the X chromosomes of the germ cells never undergo inactivation in their ontogeny or whether inactivation of an X chromosome occurs but is followed by a subsequent reactivation event was asked. An electrophoretic polymorphism was used for the X-linked enzyme glucose-6-phosphate dehydrogenase (G6PD) in the mouse species M. caroli. G6PD is dimeric and a heterodimer is produced in cells from heterozygous females if both X chromosomes are active. Ovaries from heterozygous fetuses at different gestational ages were dissected and either studied cytologically or pressed between microscope slides to obtain germ cell-rich and germ cell-poor preparations. No heterodimer band was detected on the 10th day of development in germ cell-rich preparation. On subsequent days, an increasingly intense heterodimer band was detected which by the 13th day, was approximately twice as intense as the corresponding homodimer bands. The G6PD activity/germ cell and per somatic cell and the percentage of germ cells in the germ cell-rich preparations was considered. A heterodimer band apparently would be visible on the 10th day if both X chromosomes were active. Cytological examinations showed that the earliest germ cells enter meiotic prophase on the 11th day. Oogonia evidently have a single active X chromosome and the inactive X chromosome is reactivated at or, more likely, shortly before entry into meiotic prophase.