Helen Pearson writes in Nature:
To say that eggs grow only in females and sperm grow only in males seems a pretty uncontroversial statement. But Japanese researchers have shown that it’s not as simple as that, by nurturing female eggs in the testes of male mice.
In a growing mouse embryo, the cells that will become the testes or ovaries, known as germ cells, start out the same in both sexes. In males, a gene on the Y chromosome called Sry switches on about halfway through gestation and prompts these undecided cells to develop into testes containing sperm. Females lack Sry and, by default, develop ovaries and eggs.
But what happens if you have a female germ cell surrounded by male cells? Will it be influenced by the male signals around it and become a sperm, or will it follow its own genetic path and become an egg?