Terminal differentiation of chondrocytes in culture is a spontaneous process and is arrested by transforming growth factor-beta 2 and basic fibroblast growth factor in synergy

At Day 17 of in ovo development, chondrocyte hypertrophy including synthesis of collagen X takes place in a limited region within the cranial part of chick embryo sternum. Here we analyze in suspension culture the differences in response to single growth factors of chondrocytes derived from the cranial part versus cells derived from the caudal part. Cells from either part were cultured separately without serum in the presence of insulin-like growth factor-1, transforming growth factor beta 2, basic fibroblast growth factor, or thyroid hormones. In culture, chondrocytes derived from the cranial part of sterna from 14- to 18-day-old chicken embryos become hypertrophic and initiated the synthesis of collagen X and alkaline phosphatase. These processes were enhanced by anabolic diffusible signals, such as those contained in fetal bovine serum, insulin-like growth factor-1, or thyroxine. Cells derived from the caudal part lack this capacity and, instead, prevented hypertrophy of cranial cells in cocultures, presumably by secreting diffusible signals. As candidate molecules, we have identified transforming growth factor beta 2 and basic fibroblast growth factor, which both were released by chondrocytes. Synergistic action of transforming growth factor beta 2 and basic fibroblast growth factor was required to suppress insulin-like growth factor-1-stimulated maturation of cranial chondrocytes in culture.