PARAAORTIC SPLANCHNOPLEURA FROM EARLY MOUSE EMBRYOS CONTAINS B1A CELLPROGENITORS

DEFINITIVE erythropoiesis in birds originates from stem cells that eme rge in the splanchnopleural mesoderm near the embryonic aorta1-4. The yolk sac is still generally held to be the unique provider of haematop oietic stem cells during mammalian ontogeny5, although there may be an alternative intraembryonic source of stem cells in the mouse fetus6,7 . Here we search for a possible non-yolk-sac source of stem cells by g rafting intraembryonic splanchnopleura from 10- to 18-somite mouse emb ryos into adult immunodeficient SCID mice. We find significant amounts of donor-derived serum IgM, normal numbers of IgM-secreting plasma ce lls, and the B1a (IgM(bright)(a)B220(dull)CD5+) cell subset to be full y reconstituted by donor progenitors 3 to 6 months after engraftment. The haematogenic capacity revealed in our experiments is present in a previously unrecognized site, the earliest described in the embryo, 12 hours before fetal liver colonization.