The endocardium is a single layer of squamous epithelial cells that lines the heart. In mammals, the layer originates from clusters of migrating angioblasts derived from pre-somitic cranial mesoderm. The clusters enter the pericardial area to form a vascular plexus adjacent to the developing myocardium. The endocardium is unique among endothelial tissues in that it arises through de novo vasculogenesis, from a population of precardiac progenitor cells. These progenitor cells are characterized by Flk- 1 expression in the late primitive streak, which also give rise to the future cardiomyocytes.