Background: Immunohistochemical studies have demonstrated that CD34 antigen is present on the surface of vascular endothelial cells and hematopoietic cells. Roles of CD34 for angiogenesis and its function as a homophilic adhesion molecule between endothelial and hematopoietic cells have been speculated.
Experimental design: Northern blotting was used to examine the expression levels of CD34 mRNA, and in situ hybridization was used to localize CD34 mRNA. First, we investigated changes in CD34 mRNA expression in developmental processes of mice. Second, we investigated the changes in skin tissues of adult mice in the process of wound healing and tumor growth.
Results: CD34 mRNA was strongly expressed by most of the vascular endothelial cells in developing organs. The magnitude of the expression decreased after birth but increased again in the process of wound healing and tumor growth. Although CD34 mRNA signals were observed in hematopoietic cells in the yolk sac and fetal liver, the endothelial cells of these tissues did not express CD34 mRNA signals. CD34 mRNA signals were detectable in neither hematopoietic nor endothelial cells of the bone marrow. In tissues other than hematopoietic ones, however, blood cells within the vessels did not express CD34 mRNA, although the endothelial cells expressed CD34 mRNA.
Conclusions: The expression pattern of CD34 mRNA suggested its significant role in development of blood vessels not only in embryos but also in adults. Although CD34 mRNA was expressed both by endothelial cells and hematopoietic cells, the expression did not occur within the same organ, suggesting that the CD34 molecule may not be used as a homophilic adhesion molecule between endothelial and hematopoietic cells.