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Expression of Type X Collagen and Matrix Calcification in Three‐Dimensional Cultures of Immortalized Temperature‐Sensitive Chondrocytes Derived from Adult Human Articular Cartilage

Chondrocytes isolated from normal adult human articular cartilage were infected with a retroviral vector encoding a temperature‐sensitive mutant of the simian virus 40 large tumor antigen and a linked geneticin (G418)‐resistance marker. G418‐resistant colonies were then isolated, ring‐cloned, and expanded in serum‐containing media. Several immortalized chondrocyte cell lines were established from the clones that survived, some of which have been maintained in continuous culture for over 2 years. Despite serial subcultures and maintenance as monolayers, these cells retain expression of markers specific for cells of the lineage, namely type II collagen and aggrecan, detected immunocytochemically. We also examined the phenotype of three of these immortalized cell lines (designated HAC [human articular chondrocyte]) using a pellet culture system, and in this report, we present evidence that a prototype of these lines (HAC‐F cells) expresses markers normally associated with hypertrophic chondrocytes. When HAC‐F cells were cultivated in centrifuge tubes, for periods of up to 63 days, at 39°C with mild and intermittent centrifugation they continued to express both lineage markers; total type II collagen/pellet remained stable, whereas there was a temporal decrease in cartilage‐specific glycosaminoglycans content. In addition, in the presence of ascorbate but in the absence of a phosphate donor or inorganic phosphate supplement, the cells also begin to express a hypertrophic phenotype characterized by type X collagen synthesis and extensive mineralization of the extracellular matrix in late stage cultures. The mRNA encoding type X collagen was detected in the cell pellets by reverse transcriptase polymerase chain reaction as early as day 2, and anti‐type X collagen immunoreactivity was subsequently localized in the matrix. The mineral was characterized by energy‐dispersive X‐ray microanalysis as containing calcium (Ca) and phosphorus (P) with a Ca:P peak height ratio close to that of mineralized bone tissue. The unexpected phenotype of this human chondrocyte cell line provides an interesting opportunity for studying chondrocyte maturation in vitro.