Should We Really Be Tinkering with Human Egg Stem Cells?

How glowing green mice could keep you fertile

It’s been all over the news lately that a Massachusetts General Hospital researcher, Jonathan Tilly, is claiming that contrary to the common belief that women are born with a set number of eggs in their ovaries, we may instead be equipped with “egg stem cells” that continue to develop into eggs as we grow and age. The science behind how Tilly is proving his claim is as basic as simple addition and subtraction and as otherworldly as glowing green mice.

First, the addition and subtraction. For more than half a century, the prevailing wisdom has been that baby girls are born with about a million immature eggs, or oocytes, apiece. Once they reach puberty, they shed about a thousand matured eggs per month (though in each cycle, only one egg goes into the fallopian tubes to be fertilized). Tilly actually counted the number of egg cells shed by mice, and in 2004 showed there were simply too many to allow the mice to remain fertile throughout their reproductive lives. His conclusion? Something was making more egg cells. That something turned out to be fetal germ cells, which in a fetus develops into either sperm or eggs, depending on the mouse’s gender.

Not everyone bought into this theory. But in 2009, Chinese researchers reported that they’d successfully isolated these “female germline stem cells” from mice. How did they prove it? They isolated the stem cells, injected them with green fluorescent protein (GFP), and then reimplanted them into the ovaries of sterilized mice. Not only did the mice give birth to healthy mouse pups; they gave birth to healthy mouse pups that glowed fluorescent green.

Granted, mice aren’t people. But Tilly, using the Chinese researchers’ method, has successfully grown the stem cells in vitro and established them in human tissue, where they generated new oocytes when reimplanted into mice.

This is simultaneously scary and cool. The ethics of tinkering with human egg stem cells are troubling, especially since such cells almost always develop mutations when cultured. But the implications for fertility treatments for women whose ovaries have been damaged by cancer treatments or who’ve suffered from premature menopause are vast. Like so much cutting-edge science, the new research answers questions even as it raises more.