During mouse embryonic development, haematopoietic cells develop at several sites in the embryo, such as the yolk sac. The mid-term placenta also contains many haematopoietic cells but, until now, it has been unclear whether it is a source of haematopoietic cells or a stem cell niche. On , Zeigler and co-workers help resolve this issue by demonstrating that the placenta has haematopoietic potential. The chorio-allantoic placenta of mammals forms through the fusion of the allantois (the precursor of the umbilical cord) and the chorion, which forms the chorionic disc. By isolating the allantois and chorion from mouse embryos before the establishment of the embryonic circulation, the researchers show that both tissues contain cells that express Runx1, a transcription factor that is required for haematopoietic stem cell formation. Furthermore, both tissues form myeloid and erythroid cells in explant cultures, even before chorio-allantoic fusion. The researchers conclude that both of the tissues that form the mammalian placenta, like the allantois in avian embryos, have intrinsic haematopoietic properties.
