CD34 EXPRESSION PATTERNS DURING EARLY MOUSE DEVELOPMENT ARE RELATED TO MODES OF BLOOD-VESSEL FORMATION AND REVEAL ADDITIONAL SITES OF HEMATOPOIESIS

CD34 is a cell surface glycoprotein that is selectively expressed with in the human hematopoietic system on stem and progenitor cells, and in early blood vessels, To elucidate its functions during early blood ve ssel formation and hematopoiesis, we analyzed the expression patterns, in day 8 to day 10 mouse embryos, of CD34 RNA by in situ hybridizatio n and protein by immunohistochemistry using the monclonal antibody RAM 34. Levels of expression in embryonic blood vessels were correlated w ith the mode of vessel formation, being high in pre-endothelial cells and in vessels forming by vasculogenesis (particularly the dorsal aort ae) or angiogenesis, but low in vessels forming by coalescence (the ca rdinal veins). CD34(+) erythroid cells, presumably of yolk sac origin, were present in the liver of day 10 embryos; at the same stage, putat ive definitive hematopoietic cells, strongly CD34(+), were present in the para-aortic mesenchyme. Possible sites of hemangioblastic differen tiation were detected in the form of CD34(+) endothelium-attached hema topoietic cells in the dorsal aorta and in two previously unreported l ocations, the proximal umbilical and vitelline arteries. These observa tions suggest functions for CD34 in relation to specific modes of bloo d vessel formation, and a hemangioblastic role in both embryonic and e xtraembryonic sites. (C) 1997 by The American Society of Hematology.