Endothelial cells of hematopoietic origin make a significant contribution to adult blood vessel formation

Granulation tissue formation is an example of new tissue development in an adult. its rich vascular network has been thought to derive via angiogenic sprouting and extension of preexisting vessels from the surrounding tissue. The possibility that circulating cells of hematopoietic origin can differe ntiate into vascular endothelial cells (ECs) in areas of vascular remodelin g has recently gained credibility. However, no quantitative data have place d the magnitude of this contribution into a physiological perspective. We h ave used hematopoietic chimeras to determine that 0.2% to 1.4% of ECs in ve ssels in control tissues derived from hematopoietic progenitors during the 4 months after irradiation and hematopoietic recovery. By contrast, 8.3% to 11.2% of ECs in vessels that developed in sponge-induced granulation tissu e during 1 month derived from circulating hematopoietic progenitors. This r ecruitment of circulating progenitors to newly forming vessels would be dif ficult to observe in standard histological studies, but it is large enough to be encouraging for attempts to manipulate this contribution for therapeu tic gain.