A comparison of the morphological, histochemical and biochemical features of embryonic chick sternal chondrocytes in vivo with chondrocytes cultured in three-dimensional collagen gels.

Disaggregated chondrocytes from embryonic chick whole sterna proliferate in three-dimensional collagen gels forming mixtures of cartilage nodules (chondroids) and columns of cells. A typical matrix elaborated by the cells is composed of collagen type II and chondroitin and keratan sulphate proteoglycans. The cells of the chondroids are small and uniform whilst those in the columns are larger (hypertrophic) and have a vacuolated cytoplasm. Disaggregated cells from the caudal portion of the sternum (which in vivo contains small uniform cells and does not calcify) tend to form chondroids in culture whilst cells from the cephalic portion (which in vivo contains large hypertrophic cells and whose matrix calcifies) tend to form columns of hypertrophic cells. Only the cultures containing hypertrophic cells synthesise collagen type X and only the hypertrophic cells contain alkaline phosphatase. Collagen type X and alkaline phosphatase may therefore be coregulators of the calcification of hypertrophic cartilage matrix.