To investigate the temporal and topographic changes in the expression of various collagens during the process of endochondral bone formation, qualitative and quantitative analysis of the collagens synthesized by organ cultures from the separated presumptive calcification and permanent cartilaginous regions of embryonic chick sternum at various stages of development was performed. Special emphasis was placed on the study of Type X collagen, a recently described species that may play a role in tissue calcification. We found that Type X collagen is biosynthesized exclusively by cartilage from the zone of presumptive calcification and that its biosynthetic expression is acquired at stage 43 (day 17) of sternal development. Quantitative analysis indicated that Type X was the biosynthetic product which showed the most dramatic changes increasing markedly with increased sternal age. While no Type X collagen could be detected at stage 40, it represented about 12% of the total collagen synthesized at stage 43, further increasing to 45% at stage 46 of sternal development. The increase in Type X collagen in the presumptive calcification region was accompanied by a relative decrease in the proportion of 1 alpha, 2 alpha, 3 alpha, alpha 1(II), and Type IX collagens. In contrast, the permanent hyaline cartilage did not display detectable synthesis of Type X collagen at any sternal age. The strict topographic distribution and the temporal expression of Type X collagen biosynthesis coincident with the development of sternal calcification, confirm the notion that this collagen may play an important role in the extracellular matrix remodeling associated with the initiation and progression of tissue calcification.