When the scientists examined the female mice’s heart tissue two weeks after the heart attacks, they found lots of glowing green tissue—cells that came from the fetus—in the mom’s heart. Mice who had heart attacks had eight times as many cells from the fetus in their hearts as mice who hadn’t had a heart attack did, meaning the high volume of fetal cells was a response to the heart attack.

What’s more, the embryo’s stem cells had differentiated into various types of heart tissue, including cardiomyocytes, the rhythmically contracting muscle cells that produce a heartbeat.
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The hearts of two women who suffered from severe heart weakness were later found to contain cells derived from the cells of a male fetus years after they gave birth to their sons.

The same thing seems to hold true for other organs. When pregnant women have damage in other organs, including the brain, lung, and liver, earlier studies have shown, fetal cells show up there, too.