In vitro aging of human bone marrow derived stromal cells.

We have studied the in vitro cellular aging process in human bone marrow derived stromal cells. Two cell types, here called type I and type II, can be distinguished in the primary cell population. During subcultivation, resulting in aging in vitro, the type I cells gradually disappear, whereas the type II cells form an increasing fraction of the population. This observation is confirmed by cloning experiments, revealing that type I cells act as progenitor cells, giving rise to non-dividing type II cells. These results are best explained by the terminal differentiation theory as proposed by Martin et al.