Hypertrophy and calcification of rabbit permanent chondrocytes in pelleted cultures: synthesis of alkaline phosphatase and 1,25-dihydroxycholecalciferol receptor.

Cartilage calcification at specific sites is a key event that leads to skeletal development and growth. To obtain insights into the control of cartilage calcification, we examined whether cells distributed in permanent cartilage regions might have the ability to express the calcification-related phenotype in a permissive environment. Chondrocytes were isolated from the permanent and growth plate cartilages of 4-week-old rabbit ribs. They were seeded as a pelleted mass in a centrifuge tube and cultured in Eagle's minimum essential medium supplemented with 10% fetal bovine serum. These cells proliferated for several generations, and then synthesized large amounts of proteoglycans, yielding a cartilage-like tissue in 16 days. Cultures from the permanent and growth plate cartilages showed similar time courses for increases in DNA synthesis and proteoglycan production that reached similar maximal levels. Thereafter, they initiated the syntheses of alkaline phosphatase and 1,25-dihydroxycholecalciferol receptor and induced matrix calcification without additional phosphate. The increases in alkaline phosphatase, 1,25-dihydroxycholecalciferol receptor, and calcium contents in cultures from the permanent cartilage were consistently delayed for 4-7 days relative to the growth plate-derived cells, but caught up by Day 28. The maximal levels of alkaline phosphatase and 1,25-dihydroxycholecalciferol receptor in the cultures from the permanent cartilage were 40- to 100-fold higher than that of the in vivo permanent cartilage. These results provide evidence that permanent cartilage cells in postnatal young rabbit ribs have the capacity to express alkaline phosphatase and 1,25-dihydroxycholecalciferol receptor and induce calcification in a permissive environment, although they never express these calcification-related phenotypes in vivo.