Long-term growth curves of nucleus pulposus chondrocytes (NPCs) obtained from donors of various ages.

Plots of cumulative population doubling levels (PDLs) against time in culture (A) and passage number (B). The cumulative PDLs of NPCs from the young group (patients in their 30s, 40s, and 50s) were relatively higher than those from the old group (patients in their 60s and 70s). Plots of cumulative PDLs against time in culture showed a steep curve in the early passages and a flattening with increasing time. The early steepness was longer in cultures from the young group than in cultures from the old group. Plots of cumulative PDLs against passage number showed a reverse slope at the end, indicating a decrease in cumulative PDL in the last passage just after the maximum cumulative PDL. Cellular death presumably account for these population drops.