Scientists from the Finnish University of Helsinki have discovered the origin of a stem cell that is critical for forming new blood vessels. Such vessels are lined with a layer of endothelial cells, and the origin of new endothelial cells during angiogenesis has been unclear. In their studies, the authors identified a rarely occurring stem cell in the vascular lining itself, and showed that such cells could generate tens of millions of offspring, and make blood vessels that could connect to existing circulation.