Molecular Origin of Blood Stem Cells (IMAGE) University of Pennsylvania School of Medicine Caption During development, a small number of endothelial cells express Runx1 (blue cells), signaling the production of grapelike clusters of hematopoietic (or blood) stem cells along the interior walls of several major blood vessels in the mouse embryo. This cluster of endothelial cells is in the lumen of the vitelline artery. Credit Nancy Speck, PhD, University of Pennsylvania School of Medicine. Usage Restrictions None License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.