It is widely thought that haemangioblasts are precursors for both blood cells and vascular endothelial cells. However, haematopoiesis and angiogenesis have not been analysed in detail in mouse embryos before embryonic day (E) 7.0 when haemangioblasts appear. Furuta and co-workers now report that the angioblast lineage diverges from mesoderm before, and independently of,haemangioblast formation (see ). Using in-vitro culture to evaluate vasculogenesis and haematopoiesis in cells taken from early mouse embryos, the researchers show that endothelial progenitors are present by E5.50 but stroma-dependent haematopoietic progenitors are not seen until E6.75; colony-forming units (the precursors of macrophages,erythrocytes and megakaryocytes) appear at E7.50. At this time, vasculogenic and haematopoietic precursors both express the transcription factor Oct3/4,which maintains the pluripotency of embryonic stem cells, but expression of CD31 (platelet endothelial cell adhesion molecule 1) divides them into angioblast (Oct3/4+, CD31-) and haemangioblast (Oct3/4+,CD31+) lineages. Thus, the authors conclude, there are distinct pathways for vasculogenesis and haematopoiesis that are independent of haemangioblasts.
