Endopoiesis: the process of endothelial cell development. We propose that high-proliferative potential–endothelial colony-forming cells (HPP-ECFCs) are the most proliferative EPCs and that these cells demonstrate high replating potential. HPP-ECFCs can give rise to at least secondary HPP-ECFCs and all other EPCs and mature ECs. Low-proliferative potential–endothelial colony-forming cells (LPP-ECFCs) do not replate into secondary LPP-ECFCs but do form endothelial cell clusters and mature ECs. Endothelial cluster-forming cells give rise only to mature nondividing ECs. Mature ECs do not divide. Microscope, objective, and camera details are as in Figure 1. Cells were infected with a retrovirus encoding green fluorescence protein and thus appear green in the micrographs.