Figure 4.

Structure of hESC colonies maintained at ambient air oxygen concentration or exposed to moderate hypoxia during short‐ or long‐term culture. CLS1 line was routinely grown in the presence of 20% oxygen, and for short‐term hypoxic exposure it was cultured in 5% of oxygen atmosphere for 3 weeks. CLS1‐LT cell line, that has previously been maintained in long‐term (>18 months) hypoxic conditions corresponding to 5% oxygen, has also been sub‐cultured in 5% oxygen for 3 weeks. The nuclei are rendered in blue (Hoechst 33342), undifferentiated areas in green (Oct4⁺), and differentiated areas in red (SSEA1⁺). Arrowheads indicate accumulations of cells negative for both Oct4 and SSEA1. The overview images are mosaics, entailing from 30 to up to 120 single fields, at 10‐fold magnification. SSEA, stage‐specific embryonic antigen; hESC, human embryonic stem cell.