Abstract
Fibroblast growth factor‐2 and parathyroid hormone are strong modulators of the maturation process of chondrocytes during endochondral ossification. To clarify whether and how these agents may exert stage‐specific effects during this process, we analyzed the responsiveness and phenotypic consequences of treatment with fibroblast growth factor‐2 or parathyroid hormone on chondrocytes at different stages of maturation. Populations of immature lower sternal, maturing upper sternal, and hypertrophic tibial growth plate chondrocytes were isolated from day 18–20 chick embryos and were allowed to resume the maturation process by growth in standard monolayer cultures. Treatment of immature lower sternal cultures with as little as 0.1 ng/ml of fibroblast growth factor‐2 or 10^−10^ M parathyroid hormone prevented both the emergence of mature type‐X collagen‐synthesizing chondrocytes and the ensuing enlargement of cells that occurred in control (untreated) cultures. Similarly, the treatment of cultured early maturing upper sternal cells with these factors severely reduced the synthesis of type‐X collagen and alkaline phosphatase activity and the levels of their respective mRNAs. In sharp contrast, when the cultured upper sternal cells were allowed to grow and mature further before treatment, the responsiveness to fibroblast growth factor‐2 was markedly reduced and the responsiveness to parathyroid hormone remained strong and largely unchanged. Cultures of hypertrophic tibial growth plate cells displayed a similar reduced sensitivity to fibroblast growth factor‐2, as also indicated by the lack of mitogenic effects, and strong sensitivity to parathyroid hormone. The phenotypic changes induced by treatment with either of these factors were fully reversible when cultures that had been treated were placed in control medium. The results demonstrate that fibroblast growth factor‐2 and parathyroid hormone are equally potent in affecting the early stages of maturation but exert differential effects as the cells progress along the maturation pathway. The factors appear to be part of sequentially acting mechanisms to ensure normal progression of chondrocyte maturation during endochondral ossification.