Males can grow egg cells, too

"[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 sperm. The so-called "testicular eggs" were smaller and grew more slowly than normal ovarian eggs, and they were present only in anterior and posterior parts of the testis nearby resident Sertoli cells.Like normal ova, the testicular eggs were heavily methylated and in some cases expressed SCP3, a primary meiotic complex protein. Methylation patterns in cells are characterized as sex specific, according to the authors, and have been shown to turn off gene expression."It is difficult to elucidate the role of genomic imprinting in germ cell development, but we consider the imprint as a result of sequential chain events after the initial sex determination of germ cells," Okabe told The Scientist.Previous research has shown that XX spermatogonia-like cells are normally found within the testis but disappear a few days after formation for unknown reasons. Okabe says sex determination in germ cells likely occurs before genomic imprinting because inside the testes, genomic imprinting did not always follow cues from the environmental conditions in which the germ cells were found.Earlier work in sex-reversed mice by Anne McLaren from the United Kingdom's Cambridge University showed that large, egg-like cells could develop in the testes. The new fi