Retinoic acid appears to control the timing and perhaps the choice for germ cells in the developing mouse to begin changing into eggs or sperm, scientists report in the March 30 online edition of Science. Their paper, and the recent findings of another group appearing in the Proceedings of the National Academy of Sciences, could force a rethinking of prevailing theories that suggest germ cells have genetically programmed fates.Instead, germ cells may be "a blank slate that await chemical instructions to tell them which way to go," Science study coauthor Peter Koopman at the University of Queensland in Brisbane told The Scientist.Whether developing germ cells become male or female depends on when they enter meiosis. If meiosis begins during fetal development, oogenesis is triggered, while delayed meiosis spurs spermatogenesis. It is widely thought that fetal germ cells in both males and females are intrinsically programmed to enter...

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