Hypertrophy is not a prerequisite for type X collagen expression or mineralization of chondrocytes derived from cultured chick mandibular ectomesenchyme

Meckel's cartilage in the avian mandible is a neural crest-derived permanent cartilage. To investigate whether chondrocytes that form Meckel's cartilage can be induced to undergo maturation and mineralization by manipulating the environment, we used in vitro micromass culture in which young embryonic mandibular ectomesenchymal cells were maintained at a high cell density (2 x 10(7) cells/ml) and treated with ascorbic acid (AA) or with dexamethasone and ascorbic acid (DEX + AA). Chondrogenesis and chondrocyte maturation were analyzed by histological, immunohistochemical and SDS/PAGE techniques. Chick mandibular ectomesenchymal cells from Hamburger and Hamilton (J. Morphol. 88:49-92, 1951) stage 21 (HH stage 21) chick embryos undergo chondrogenesis forming cartilage nodules when maintained under micromass culture conditions. These chondrocytes undergo maturation in response to AA but not DEX. Addition of AA to culture medium induced type X collagen expression by these chondrocytes. With prolonged culture, chondrocytes began to mineralize turning cartilage nodules into completely mineralized structures. There was no correlation between chondrocyte size and type X collagen expression. Small chondrocytes as well as large (hypertrophic) chondrocytes expressed type X collagen and then underwent mineralization. Co-treatment of cultures with DEX + AA caused reduction of chondrogenesis and inhibited chondrocyte maturation and mineralization seen with AA alone.