BioEd Online

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?

Masaru Okabe at Osaka University and his colleagues expected the former, but to find out for sure they sandwiched together cells from male and female embryos and allowed the 'chimeric' embryos to grow into mice.

As suggested by previous studies, most of the female cells growing in the testes of the male mice abandoned their genetic legacy and went through the early stages of sperm development. Okabe found that signals from surrounding cells with an active Sry gene triggered these female cells to start up a pattern of gene activity that is normally only found in male cells.

But some of the female cells that lodged in the testes developed partly into eggs, the researchers report in the Proceedings of the National Academy of Sciences¹. The eggs were able to fuse with sperm, but did not develop into embryos. "It was a big surprise," says Okabe, who dubbed the cells 'testicular eggs'.

Cell pockets

Testicular eggs are not entirely new. A study 25 years ago reported eggs that seemed to grow in male mice². But that was simply based on the shape and size of the cells. Okabe's study is the first to use modern genetic techniques to confirm that such cells are genetically female.

The idea of eggs growing in the testes is "a nice concept", says Wolf Reik, who studies egg and sperm development at The Babraham Institute in Cambridge, UK. He suspects that the eggs are able to develop because they grow in a little of pocket of female cells within the testis.

The researchers hope their studies will help scientists to understand what goes wrong when the testes develop in patients with chromosomal sex disorders. In Klinefelter syndrome, for example, males carry an additional X chromosome and budding sex cells disappear. Okabe says that the chimeric mice could help explain exactly what happens to these cells and why.