In vitro formation of mineralized cartilagenous tissue by articular chondrocytes.

Study of the deep articular cartilage and adjacent calcified cartilage has been limited by the lack of an in vitro culture system which mimics this region of the cartilage. In this paper we describe a method to generate mineralized cartilagenous tissue in culture using chondrocytes obtained from the deep zone of bovine articular cartilage. The cells were plated on Millipore CMR filters. The chondrocytes in culture accumulated extracellular matrix and formed cartilagenous tissue which calcified when beta-glycerophosphate was added to the culture medium. The cartilagenous tissue generated in vitro contains both type II and type X collagens, large sulfated proteoglycans, and alkaline phosphatase activity. Ultrastructurally, matrix vesicles were seen in the extracellular matrix. Selected area electron diffraction confirmed that the calcification was composed of hydroxyapatite crystals. The chondrocytes, as characterized thus far, appear to maintain their phenotype under these culture conditions which suggests that these cultures could be used as a model to examine the metabolism of cells from the deep zone of cartilage and mineralization of cartilagenous tissue in culture.