Chondrocytes dedifferentiated by serial monolayer culture form cartilage nodules in nude mice

Chondrocytes isolated from rabbit costal growth cartilage grew rapidly and expressed the differentiated phenotypes of chondrocytes in primary monolayer cultures. They had a typical polygonal shape, synthesized glycosaminoglycan (GAG) actively and responded to parathyroid hormone (PTH). However, during successive passages they lost the abilities to proliferate and express differentiated phenotypes. They changed from polygonal to fibroblast-like morphology during the third to fourth passage. Their GAG synthesis rapidly decreased in the 4th passage to 10% of that during primary culture. The increase in GAG synthesis and intracellular cyclic AMP induced by PTH also became smaller and were lost in the 4th passage. From these findings, cells in the 4th passage appeared to be dedifferentiated. However, subcutaneous transplantation of cells at the 5th passage into nude mice resulted in the formation of cartilaginous nodules. Moreover, cells isolated from these nodules and cultured in vitro had a typical polygonal shape and synthesized GAG actively and PTH increased their metachromatic matrix and intracellular cyclic AMP level. These findings suggest that chondrocytes that appear to be dedifferentiated after serial passages still have the potential to differentiate.