Growth kinetics and in vitro expansion of EPCs. (A) In vitro expansion of cbEPCs and adult blood EPCs isolated from mononuclear cells and purified by CD31⁺ selection. (B) Growth curves of cbEPCs at different passage numbers (P4, P6, P9, P12, and P15). Each data point represents the mean of 3 separate cultures ± SD. (C) Doubling time profiles of cbEPCs at different passage numbers. Values were calculated from the mean values of cell number obtained at specific time points after plating. (D) Morphologic differences of cbEPCs at increasing passage. Each bar represents the mean area ± SD obtained from randomly selected fields. All values were normalized to the total cell area occupied by HDMECs. *P < .05 compared with HDMECs.