Replating potential and long-term culture of the cell progeny derived from a single cord blood or adult EPC-derived endothelial cell. (A) Percentage of the cell progeny derived from a single adult peripheral blood (AB) or umbilical cord blood (CB) EPC-derived endothelial cell, which formed secondary colonies or rapidly grew to cell confluence after 7 days of culture in a 24-well tissue culture plate. Results represent the mean ± SEM of 4 independent experiments using cells derived from 4 different donors. ^*P < .01 by Student paired t test. (B) A representative photomicrograph (50 × magnification) of the primary colony (left panel) or the secondary endothelial cell colonies or confluent cell monolayers (right panels) derived from the cell progeny of a single plated cord blood EPC-derived endothelial cell in a 24-well plate after 7 days in culture. Arrows indicate either secondary colonies (top right panel) or a confluent monolayer of ECs (bottom right panel) derived from the primary colony. Scale bar represents 100 μm. (C) Growth kinetics of the cell progeny of 11 single-plated EPC-derived endothelial cells isolated from 3 different cord blood donors in long-term culture. ▪ indicates the total number of cells at each passage. (D) Telomerase activity of early-passage adult (lanes 1-4) and cord (lanes 5-8) blood EPC-derived endothelial cells isolated from different donors. Adult and cord cells were tested at a CPDL of 15. P indicates telomerase activity in HeLa cells, which were used as a positive control, and N indicates a negative control. (E) Comparison of telomerase activity of early- and late-passage adult (Ad) and cord (Co) blood EPC-derived endothelial cells. PD indicates the cumulative population doubling level of the cells tested. P indicates telomerase activity in HeLa cells, which were used as a positive control. N indicates a negative control. Three other experiments using early- and late-passage cord blood and adult EPC-derived endothelial cells from 3 different donors showed similar results.