Abstract
We examined the spatio-temporal pattern of collagen types I, II, IX and X collagen mRNAs in embryonic chick vertebrae undergoing endochondral ossification by in situ hybridization. Before stage 35, non-hypertrophic chondrocytes expressed both type II and IX collagen genes in the vertebrae. At stage 36. hypertrophic chondrocytes having a moderate amount of type X collagen mRNA were first detected in a few vertebral body segments as an island of cells in the dorsal part of midvertebrae.. Hypertrophic chondrocytes were increased in number at stage 37 and 38, and spread to larger areas.. These cells contained high levels of type X mRNA and decreased levels of type II and IX mRNAs.. From stage 38, blood vessels invaded deeply into the midvertebral hypertrophic chondrocytes area. As. As the vertebrae grew further at stage 40, the leading front of active hypertrophic chondrocytes with high levels of type X mRNA shifted from the midvertebral perivascular area towards intervertebral borders. Later at stage 45, comparison of the localization of collagen types II, IX and X mRNAs in the vertebral segment revealed a clearly reversed pattern of distribution between type X collagen mRNA and types II and X mRNAs. Type I collagen started to deposit in the cartilage matrix adjacent to invading capillaries and in the lacunae enclosing terminal hypertrophic chondrocytes at stages 38 and 39. From stage 40, perivascular mesenchymal cells and/or osteoblasts, bordering between the endochondral bone and marrow cavities, expressed high levels of type I collagen mRNA.
