For example, a study found Y chromosomes in 63 percent of the brains of 59 women — the oldest of whom was That means these weren't women who just gave birth. But the fact that they do stay suggests that there could be an adaptive explanation for it. A century later, interest in the phenomenon re-emerged, when scientists realized that fetal microchimerism could explain how Y chromosomes — only passed down from father to son — sometimes ended up in women's cells. The fact that they can be found in so many different tissue types indicates that they're probably stem cells, or cells that can differentiate into any type of cell, she said. This transformation doesn't result in a part-lion, part-goat fire-breathing monster of Greek mythology. Knowing how the fetal cells get into the mother's tissues is one thing.