A recent paper in Cell Stem Cell provides some interesting new information about the origin of hematopoietic stem cells (HSCs), arguably the best characterized population of stem cells in the organism, and the one population that has been successfully used in regenerative medicine for some time.
We already knew that, in the mouse, the placenta acts as a very early reservoir for HSCs. But do they come from the circulation or are they born there? In the new study, Katrin Rhodes and her colleagues looked in mouse embryos that lack a functional heart and have therefore no circulation, and found that bona fide, multipotential HSCs develop in the placental vasculature in the absence of blood flow.
The authors admit that there were fewer HSCs in the placentas of mutant mice than in the placentas of wild-type controls, indicating that blood circulation may after all make a contribution to the total number of HSCs, but these observations do provide good evidence that the placenta is more than a mere reservoir of stem cells, simply waiting for the liver to become the first true hematopoietic organ.