Morphological evaluation of MSC under different oxygen conditions. Photomicrographs of two representative MSC cultures from separate donors. In one culture no significant morphological difference after 3 weeks between 21% O2 (A) and 1% O2 (B) were found. In a second culture, MSC under 1% O2 did not adapt the typical triangular shape and became plastic-adherent, but died after 1 week (D), while the control cells maintained under 21% O2 (C) exhibited the characteristic MSC morphology. Obviously, donor-dependent factor greatly impact the growth and differentiation behavior of MSC under these conditions. Out of 10 cultures 3 showed growth under both oxygen tensions, whereas 7 displayed less tolerance to low oxygen tension of 1%.