Diagrammatic summary indicating the intensity of staining for CD34 RNA transcripts and protein in developing blood vessels, indicating the relationship between staining intensity (as indicated by depth of shading) and the mode of blood vessel formation. In general, CD34 expression is strong in pre-endothelial cells, in vessels formed by condensation of pre-endothelial cells (ie, by vasculogenesis), particularly the dorsal aortae. Equally strong staining is observed in vessels forming by capillary sprouting, but the larger vessels of the aortic arch region are not strongly stained during their formation even though their origin can be described as angiogenetic. Vessels formed by the coalescence of networks of smaller vessels showed low or undetectable levels of CD34 staining.