Abstract
Vascular structures in the developing brain are thought to form via angiogenesis from preformed blood vessels in the cephalic mesenchyme. Immunohistochemical studies of developing mouse brain from E10.5 to E13.5 revealed the presence of avascular blood islands of primitive erythroid cells expressing hemangioblast markers (Flk1, Tal1/Scl1, platelet endothelial cell adhesion molecule 1, vascular endothelial-cadherin, and CD34) and an endothelial marker recognized by Griffonia simplicifolia isolectin B4 (IB4) in the cephalic mesenchyme. These cells formed a perineural vascular plexus from which angiogenic sprouts originated and penetrated the neuroepithelium. In addition, avascular isolated cells expressing primitive erythroid, hemangioblast and endothelial makers were visible in the neuroepithelium where they generated vasculogenic and hemogenic foci. From E10.5 to E13.5, these vasculogenic foci were a source of new blood vessel formation in the developing brain. In vitro, cultured E13.5 brain endothelial cells contained hemogenic endothelial cells capable of generating erythroid cells. Similar cells were present in primary cultures of dissociated cells from E10.5 embryonic head. Our results provide new evidence that the brain vasculature, like that of the yolk sac and the eye choriocapillaris and hyaloid vascular systems, develops at least in part via hemovasculogenesis, a process in which vasculogenesis and hematopoiesis occur simultaneously.