Model of an endothelial progenitor cell hierarchy based on the proliferative and clonogenic potential of discrete populations of progenitor cells. HPP-ECFCs are large colonies that form secondary and tertiary colonies on replating. HPP-ECFCs give rise to all subsequent stages of endothelial progenitors in addition to replating into secondary HPP-ECFCs. LPP-ECFCs form colonies that contain more than 50 cells but do not form secondary colonies or LPP-ECFCs on replating. Endothelial cell clusters can arise from a single cell but contain fewer than 50 cells that are typically larger than the smaller cells found in HPP-ECFC and LPP-ECFC colonies. Mature, terminally differentiated endothelial cells do not divide.