Gene and Protein Expression During Differentiation and Matrix Mineralization in a Chondrocyte Cell Culture System

Endochondral bone formation occurs through a series of developmentally regulated cellular stages, from initial formation of cartilage tissue to calcified cartilage, resorption, and replacement by bone tissue. Nasal cartilage cells isolated by enzymatic digestion from rat fetuses were seeded at a final density of 10⁵ cell/cm² and cultured in Dulbecco's modified Eagle medium (DMEM) supplemented with 10% fetal calf serum in the presence of ascorbic acid and β-glycerophosphate. First, cells lost their phenotype but in this condition they rapidly reexpressed the chondrocyte phenotype and were able to form calcified cartilaginous nodules with the morphological appearance of cartilage mineralization that occurs in vivo during endochondral ossification. In this mineralizing chondrocyte culture system, we investigated, between day 3 and day 15, the pattern expression of types II and X collagen, proteoglycan core protein, characteristic markers of chondrocyte differentiation, as well as alkaline phosphatase and osteocalcin associated with the mineralization process. Analysis of labeled collagen and immunoblotting revealed type I collagen synthesis associated with the loss of chondrocyte phenotype at the beginning of the culture. However, our culture conditions promoted extracellular matrix mineralization and cell differentiation towards the hypertrophic phenotype. This differentiation process was characterized by the induction of type X collagen mRNA, alkaline phosphatase, and diminished expression of type II collagen and core protein of large proteoglycan after an increase in their mRNA levels before the mineralizing process. These results revealed distinct switches of the specific molecular markers and indicated a similar temporal expression to that observed in vivo recapitulating all stages of the differentiation program in vitro. Received: 12 December 1996 / Accepted: 26 June 1997