Model of the roles of EPCs, CECs, and EOCs in angiogenesis. Hematopoietically derived EPCs circulate and are the first cells to move into a site to facilitate the initiation and propagation of an angiogenic response. These angiogenic cells recruit LPP- or HPP-ECFCs that are residing in the endothelial intima in nearby vessels through the degraded vessel wall to form the endothelial sprouts. The roles played by the CECs and circulating EOCs are less clear, but these cells possess proliferative potential (may possess any level of progenitor activity from cluster to HPP-ECFCs, depending on the age of the host and whether the host is healthy or ill) and may well participate in neoangiogenesis.