Clonogenicity of bone marrow derived hMSC determined in CFU-F assay. hMSCs grown and assayed under normal O2 (21%) - Normoxic MSC in normoxia, grown and assayed in low O2 (5%) - Hypoxic MSC in hypoxia, grown in normoxia and assayed in hypoxia - Normoxic MSC in hypoxia. The assay was performed in 9 cm tissue culture dishes, five dishes per experimental group. The cultures were terminated 14 days after initiation. The dishes were stained with Giemsa and the colonies (CFU-F) were manually counted. Each experiment was repeated three times. Columns represent the average number of colonies per plate from all three experiments combined (n = 15). The bars represent 1 SEM. × - P < 0.05, ×× - P < 0.0001. CFU-F, colony forming unit fibroblasts; hMSC, human mesenchymal stem cells; SEM, standard error of the mean.