Initial studies focussed on females who had carried male babies - it's possible to discriminate the male cells against the female tissue background.
This is not even the coolest part yet....
And it's still not the coolest part....
If you induce a heart attack in a (rodent) pregnant Mum, you can find large numbers of these fetal cells clustering around the damaged area....
Babies might keep their Mums alive.
This shot of stem cells in adulthood might underpin some of the reason that females live longer.
My jaw was dragging. He got great marks ;)
Developmental biologists have the privilege of seeing pr learning about so many amazing things in the course of their careers, but this is a strong contender for "Best Ever".