Figure 1. Long-term growth curves of MSC. Mesenchymal stem cells were isolated from human adipose tissue of 14 donors and culture expanded until the cells reached a senescent state. Every cell passage is indicated by a point and the number of cumulative population doublings (PD) was calculated based on the ratio of cells seeded versus cells harvested per passage (A). In parallel, MSC were seeded in limiting dilution to determine the fibroblastoid colony forming unit (CFU-f) frequency for subsequent passages (B). To account for the fact, that the progeny of each passage is based on a decreasing percentage of highly proliferative cells, we have recalculated long-term growth curves on the basis of the number of CFU-f seeded versus cells harvested per passages (C; black: conventional PD; grey: CFU-f-adopted growth curves).