Testicular eggs show role of early genomic imprinting

Development of immature germ cells into sperm or eggs is mainly determined by sex chromosomes, XX for female and XY for male. Previous studies suggest that the body’s environment can also direct germ cell differentiation. To determine how the environment of the testes impacts germ cell development, Ayako Isotani et al. fused female and male embryos to create XX–XY chimeric embryos containing both female and male cells. One X chromosome of the female cells was labeled with GFP to track cellular development in the testes. The researchers observed that most female germ cells developed into sperm progenitor cells, demonstrating that the cells underwent paternal imprinting despite their chromosomal makeup. On the other hand, a small number of female germ cells developed into ‘‘testicular eggs’’ that formed zona pellucida and could fuse with sperm, but these eggs, when fertilized, were not able to develop into embryos. Isotani et al. hypothesize that early molecular events initiated egg formation in these cells via maternal imprinting, which could not be overridden by the testicular environment. These results indicate that germ cell differentiation is not always determined by chromosomes or environmental conditions, but may be defined by the sex that was chosen by the germ cells during early embryonic stages of development.