Changes in protooncogene expression correlated with general and sex-specific differentiation in murine primordial germ cells.

Primordial germ cells (PGCs) of the mouse undergo key developmental transitions during embryonic days 12-15. On day 12 they complete migration into the gonads. They cease mitotic proliferation on day 13 and subsequently enter sex-specific pathways of development. The molecular mechanisms controlling these transitions are poorly understood, yet they are crucial to production of normal gametes later in life. We have used the polymerase chain reaction (PCR) to directly compare levels of expression of several protooncogenes proposed to be involved in control of cell proliferation and differentiation in proliferating and differentiating PCGs of both sexes over a 4 day time course. We report here that mRNA levels for nuclear protooncogenes c-myc, c-fos, and c-jun increase dramatically in both sexes from little or no detectable expression on day 12 to high expression on days 13-15. We observe c-kit message on day 12 in combined PGCs of both sexes, in female but not male PGCs on day 13, and in both sexes on day 14, c-kit mRNA is undetectable on day 15 in either sex, c-mos is not expressed at detectable levels on day 12 in either sex, but increases gradually in female PGCs to very high levels on day 15. In male PGCs, c-mos is expressed at high levels on days 13-15. Our results are consistent with a role for protooncogenes c-myc, c-fos and c-jun in mediating the initial differentiation of PGCs of both sexes that occurs upon colonization of the gonad. Because c-kit and c-mos are expressed differentially in male and female day 13-15 germ cells, they may play roles in initiating or mediating progress along the sex-specific pathways of development that PGCs embark upon at this time.