Gene expression of chondrocytic sub-populations dependent on the population doublings. Primary expanded chondrocytes and disc cells were MACS separated and expanded in monolayer cultures over several passages. Gene expression was determined at the end of the first passage after MACS sorting (MACS P0), when cells had undergone 4 ± 1 pds after MACS, and after cells had undergone more than 8 pds after MACS. Expression of collagen type I (COL1), collagen type II (COL2), and the transcription factors SRY (sex determining region Y)-box 9 (SOX9) and core-binding factor subunit alpha-1 (CBFA1) were analyzed and expressed relative to the reference gene GAPDH. *: significant difference (p < 0.05) between all chondrocyte populations compared to all disc populations. (For mean expression values see Additional file 1: Table S4).