EFFECTS OF CALCIUM DEFICIENCY ON CHONDROCYTE HYPERTROPHY AND TYPE-X COLLAGEN EXPRESSION IN CHICK EMBRYONIC STERNUM

Maintenance of chick embryos in long-term culture without their calcar eous eggshell is a useful method for studying the relationship between calcium homeostasis and cell differentiation during skeletogenesis. P reviously, we have shown that in shell-less (SL) embryos, calcium defi ciency induces a cartilage-like phenotype in osteogenic tissues, such as calvaria (Jacenko and Tuan [1986] Dev. Biol. 115:215). In this inve stigation, we have studied the relationship between cartilage calcific ation and hypertrophy, and the expression of type X collagen, a specif ic product of hypertrophic chondrocytes. For this study, the cephalic (calcifying) and caudal (permanently cartilaginous) regions of sterna from day 18 and day 20 normal (NL) and SL embryos were metabolically l abeled with [C-14]-proline. Analysis of the biosynthetic products reve aled significant differences in type X collagen expression in the ceph alic region of sternal cartilage. In NL tissues, type X collagen produ ction increased from 13.1% of total collagen at day 18 to 43.7% at day 20. In contrast, in SL embryos, type X collagen was not detectable un til day 20, when it represented only 1% of total collagen. Comparison of the NL and SL embryos with respect to their serum calcium level and sternal calcium content and histology revealed a direct relationship between low systemic calcium and limited cartilage hypertrophy, underm ineralization, and decreased type X collagen production in the sternal cephalic cartilage. Supplementation of CaCO3 to SL embryos increased their serum and sternal calcium, and restored cartilage hypertrophy, m ineralization, and type X collagen synthesis in the cephalic portion o f the sterna. These findings confirm that a critical relationship exis ts between calcium homeostasis, chondrocyte hypertrophy, mineralizatio n, and type X collagen synthesis in the cephalic region of sternal car tilage. These results further demonstrate the importance of calcium in the morphogenetic events of endochondral ossification, in particular the transition from hyaline cartilage to hypertrophic cartilage, and e ventually to bone. (C) 1993 Wiley-Liss,Inc.