AN ANTIGEN OF CHONDROITIN SULFATE PROTEOGLYCAN; A MARKER FOR CARTILAGE DIFFERENTIATION

Antisera to the chondroitin sulfate proteoglycan complex (CPG) of cartilage were used to study the specificity of the CPG-associated antigen as a biochemical marker for cartilage differentiation and to study the expression of differentiation by cultured chondrocytes. Of 7 tissues tested, antigen giving an identity reaction with this protein could be detected by the Ouchterlony double diffusion test in extracts of sternum and brain of 14-day chick embryos. Extracts of 2 non-cartilage tissues gave a reaction indicating that they contain a related, but not identical antigen. Ouchterlony double diffusion tests showed that extracts of morphologically differentiated chondrocytes cultured in vitro contain the CPG-associated antigen. The radio-precipitin test, used to quantitate the rate of synthesis of this antigen, provided a measure of cartilage phenotype expression in culture. The cultured chondrocytes synthesized antigenic protein at a rate similar to that of 14-day sternum. In contrast to intact cartilage, however, the cultured chondrocytes released much of the newly synthesized antigen into the medium. The possibility was explored that synthesis of the CPG-associated antigen might be characteristic of all cells in culture, and not a specific expression of the cartilage phenotype. However, skin fibroblast cultures only contained detectable antigen of the "partially identical" type.