Fig. 4. Impaired differentiation and delayed senescence of primary Fosl2^–/– rib cage chondrocytes. (A) Formation of cartilage nodules by primary chondrocytes. Primary chondrocyte cultures stained with Alcian Blue (left panel). Quantification of Alcian Blue bound to sulfated proteoglycans in the chondrocyte ECM (right panel). Bars represent mean value±s.d.; n=4; ^*P<0.05. (B) Cumulative cell number assay of primary rib cage chondrocytes. Experiments were carried out in triplicate and repeated three times. One representative experiment is shown. (C) BrdU-labeling of proliferating primary chondrocytes after 2 days in culture. (D) TUNEL staining of apoptotic primary chondrocytes after 2 days in culture. (E) Semi-quantitative RT-PCR analysis of cartilage markers and Fosl2 in primary rib cage chondrocytes at day 3, 6 and 9 of differentiation. Tubulin was used as a loading control (n=3).