Quantitative analysis of collagen expression in embryonic chick chondrocytes having different developmental fates.

A quantitative determination of collagen expression was carried out in cultured chondrocytes obtained from a tissue that undergoes endochondral bone replacement (ventral vertebra) and one that does not (caudal sterna). The "short chain" collagen, type X is only expressed in the former while the other "short chain" collagen type IX, was primarily expressed in the latter. These two tissues also differ in that vertebral chondrocytes express moderate levels of both type I procollagen mRNAs which were translated into full length procollagen chains both in vivo and in vitro, while caudal sternal chondrocytes did not. The percent of collagen synthesis was about 50% in both cell types, but sternal cells expressed twice as much collagen as vertebral cells even though type II procollagen was more efficiently processed to alpha-chains in vertebral chondrocytes than in sternal chondrocytes. The number of type II procollagen mRNA molecules/cell was found to be about 2300 in vertebral chondrocytes and about 8000 in sternal cells, in good agreement with the results reported by Kravis and Upholt (Kravis, D., and Upholt, W. B. (1985) Dev. Biol. 108, 164-172). There were about 630 copies of type I procollagen mRNAs with an alpha 1/alpha 2 ratio of 1.6 in vertebral chondrocytes compared with 5100 copies and an alpha 1/alpha 2 ratio of 2.2 in osteoblasts, and less than 40 copies in sternal cells. Since the rate of type I collagen chain synthesis was 50 times greater in osteoblasts than in vertebral cells, type I procollagen mRNAs were about six times less efficiently translated in vertebral cells than in osteoblasts. The type I mRNAs in vertebral chondrocytes were polyadenylated and had 5' ends that were identical in osteoblasts, fibroblasts, and myoblasts. Moreover, type I mRNAs isolated from vertebral chondrocytes were translated into full length preprocollagen chains in vitro in rabbit reticulocyte lysates. Thus, chondrocytes isolated from cartilage tissues with different developmental fates differed quantitatively and qualitatively in total collagen synthesis, procollagen processing, and distribution of collagen types.