Late events in chondrocyte differentiation: hypertrophy, type X collagen synthesis and matrix calcification.

During endochondral bone formation chondrocytes pass through several stages of differentiation which are characterized by cell proliferation, matrix synthesis and cell hypertrophy. Type X collagen is synthesized in vivo after chondrocytes have become hypertrophic, but before abundant mineral accumulates in the cartilage extracellular matrix. The molecule is also present in the uncalcified membranes of the avian eggshell. Type X collagen synthesis increased with the concentration of calcium phosphate deposited in the cell layer of chondrocyte cultures. The addition of calcium chloride to chondrocyte cultures increased the synthesis of type X collagen in a dose-and time-dependent manner.