Chondrogenic potential of chick embryonic calvaria: I. Low calcium permits cartilage differentiation

Calvaria from day-14 calcium-deficient chick embryos produced by long-term maintenance in shell-less culture, exhibit a cartilage-like phenotype (Jacenko and Tuan [1986] Dev. Biol. 115:215-232), which is restored to an osteogenic phenotype upon calcium repletion to the embryo. The expression of cartilage markers in a typically osteogenic tissue under calcium deficiency implies the presence of chondrogenic cells, and questions the conditions associated with calcium deficiency which may cause their divergent pathway of differentiation. In the present study, by explanting normal and shell-less embryonic calvarial pairs in organ culture in vitro and experimentally regulating their calcium supply, the calcium status of the calvaria was modulated as a function of medium calcium. Histological and immunoblotting analyses demonstrated for the first time that calvaria possess cells which can form genuine cartilage. This chondrogenic potential is expressed only in a low-calcium environment, where cartilage forms in both normal and shell-less calvarial pairs; their calcium-supplemented counterparts, however, develop as fully osteogenic tissues. Furthermore, chondrogenesis in both normal and shell-less calvaria indicates that the chondrogenic cells must be endogenous constituents of the calvaria, rather than being derived elsewhere in response to systemic calcium deficiency. Finally, the correlation between matrix under-calcification and cartilage expression in the embryonic calvarium suggests that calcium, perhaps in the form of matrix mineral, may modulate cell differentiation during skeletogenesis.