The vascular system develops from fixed primitive angiogenic cells that are mesodermal in origin. This occurs in the middle of week 3 of fetal development. The process begins with hemangioblasts, which divide into primitive hematopoietic progenitor cells (HPCs) and endothelial precursor cells (EPCs, otherwise known as angioblasts). Hematopoiesis begins at the yolk sac of the extraembryonic mesoderm, and flattened EPCs initiate vasculogenesis at the embryonic discs. EPCs form the vessels that eventually become the arteries and veins through which blood circulates (Ilina and Lilley 2019; Sadler 2018).