In mice, female germ cells can develop into viable ova in male testes, Japanese researchers report this week. Their paper in PNAS suggests that sex determination occurs early in embryonic development before genomic imprinting takes effect.

"[Our] paper indicated that once the direction of the sex is determined in germ cells, the following influence from the environment seems not to be significant," coauthor Masaru Okabe told The Scientist in an E-mail. The research showed that female germ cells encompassed by male tissue with XY gene imprinting continue their programmed development into eggs.

Okabe, from Osaka University's Genomic Information Research Center, and colleagues combined male embryos with female embryos minus the zona pellucida in vitro to create XX-XY chimeric embryos.

Growing the embryos to term, researchers found that the seminiferous tubules of 3-week-old male mice produced meiotic XX germ cells with a normal zona pellucida structure that could fuse with...

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