Antigenic profiles of endothelial and hemopoietic lineages in murine intraembryonic hemogenic sites.

Two hemogenic sites are present in mouse embryos before the onset of fetal liver hemopoiesis. While the yolk sac provides for immediate erythropoiesis, an intraembryonic region encompassing the dorsal aorta produces definitive hematopoietic stem cells, as shown experimentally. At early developmental stages this region, that we named paraaortic splanchnopleura, produces multipotent progenitors. At the time of fetal liver colonisation, the paraaortic splanchnopleura further evolves into aorta, gonads and mesonephros (AGM) and contains progenitors capable of long term multilineage reconstitution. Only then are cytologically identifiable collections of early hemopoietic cells present in various arteries and in the mesentery. The present report focuses on the antigenic characterisation of immature hemopoietic progenitors in order to trace back the intraembryonic precursors at earlier developmental stages. CD34, an antigen expressed by immature progenitors and endothelial cells, labels all potential hemopoietic sites. Markers, supposed to counterstain endothelial cells and spare CD34+ hemopoietic cells, also stain various hemopoietic cells. The meaning of these shared antigenic expressions between cells of the endothelial and hemopoietic lineages in the early embryo is discussed.