Ontogenic emergence of a quail leukocyte/endothelium cell surface antigen

The ontogenic emergence of MB1, a quail cell surface antigen expressed by endothelial and hemopoietic cells but not erythrocytes, was followed by direct immunofluorescent staining of transverse sections of the developing blastodisc, from the stage of the cephalic fold until 22 pairs of somites. Along the developmental sequence that leads from hemangioblasts, the mesodermal precursors of both endothelium and hemopoietic cells, to vessels containing blood cells, MB1 is first expressed by arising endothelial cells. These first emerge as flattened cells at the periphery of hemangioblastic clusters in the area opaca from the stage of one pair of somites and slightly later as unicellular angioblasts in the area pellucida and in the embryo. MB1 expression is then maintained on endothelium as vessels develop, in contrast with extraembryonic blood islands in which primitive erythroblasts remain MB1-negative. A small proportion of blood island cells and budding of endothelium contribute a population of MB1-positive hemopoietic cells appearing soon after the onset of angiogenesis.