Quantitation of the clonogenic and proliferative potential of single cord blood endothelial cells derived from EPC colonies, HUVECs, and HAECs. (A) The percentage of single cord blood (CB) EPC-derived ECs, HUVECs, or HAECs undergoing at least one cell division after 14 days of culture. Results represent the average of 5 independent experiments using single ECs derived from different donors. (B) Number of cell progeny derived from a single CB EPC-derived EC (□), HUVEC (▦), or HAEC (□) in an individual well after 14 days of culture. *P < .01 by Student paired t test for comparison of a single CB-derived EC versus either a single HUVEC or HAEC. (C) Representative photomicrographs (× 50 magnification) of the different EC clusters (less than 50 cells) or colonies (more than 50 cells) derived from a single cord blood EPC-derived EC, HUVEC, or HAEC. Results are representative of 4 other independent experiments utilizing cells from different donors. Scale bar in photomicrographs represents 100 μm. (D) Percent of the cell progeny derived from a single cord blood EPC-derived EC, HUVEC, or HAEC that formed secondary colonies or rapidly grew to cell confluence after 7 days of culture in a 24-well tissue culture plate. Results represent the average ± SEM of 4 independent experiments using cells derived from 4 different donors. *P < .01 by Student paired t test for comparison of a single CB-derived EC versus either a single HUVEC or HAEC. (E) Representative photomicrographs (× 50 magnification) of the secondary EC colonies or confluent cell monolayers derived from the cell progeny of a single plated cord blood EPC-derived EC, HUVEC, or HAEC in a 24-well plate after 7 days in culture. Scale bar in photomicrographs represents 100 μm. (F) Telomerase activity in an HPP-ECFC (HPP) and LPP-ECFC (LPP) colony derived from HUVECs. “Pos” indicates telomerase activity in HeLa cells, which were used as a positive control, and “neg” indicates a negative control. Results are representative of 4 other independent experiments. Similar differences in telomerase activity were observed between HPP-ECFC and LPP-ECFC colonies isolated from cord blood ECs and HAECs (data not shown).