Evaluation of hSSC cultures on htECM and ptECM through flow cytometry: (A–C) Flow cytometry was used to quantify the number of SSEA4+cKIT− (quadrant D) and SSEA4+cKIT+ (quadrant B) cells at the beginning of culture, day 7 and day 14. The population of cells expressing Ki-67 overlaid on top of the graph is indicated in green. (D–F) Plots show the populations of SSEA4+ cells that are annexin V− (quadrant P) and apoptotic SSEA4+ cells that bind annexin V (quadrant N). (G) SSEA4+cKIT− cells decline significantly over the duration of culture at days 7 and 14 on both ECMs relative to day 0. (H) The proportion of SSEA4+ cells expressing cKIT did not change significantly in htECM-based cultures over 2 weeks. ptECM-based cultures, however, had a lower number of SSEA4+ spermatogonia that were cKIT+ cells compared with day 0. (I) The proportion of SSEA4+ cells undergoing annexin V was significantly higher at day 14 of cultures on htECM and at days 7 and 14 of cultures on ptECM. (J) Quantification of Ki-67 expression in SSEA4+ cells showed a significant increase in proliferating cells within the SSEA4+ population at day 14 on htECM and ptECM. Bar graphs are represented as mean ± SEM. *Indicates p < 0.05, **indicates p < 0.01, ***indicates p < 0.0005, and ****indicates p < 0.0001. SSEA4, stage-specific embryonic antigen 4. Color images are available online.