Deposition of type X collagen in the cartilage extracellular matrix

In cultured chick embryo chondrocytes, type X collagen is preferentially deposited in the extracellular matrix, the ratio between type II and type X collagen being about 5 times higher in the culture medium than in the cell layer. When the newly synthesized collagens deposited in slices from the epiphyseal cartilage of 17-day-old embryo tibiae were isolated, type X collagen was always the major species. In agreement with this result the mRNA for type X collagen was the predominant mRNA species purified from the same tissue. When the total collagen (unlabeled) deposited in the epiphyseal cartilage was analyzed, it was observed that type X collagen represented only 1/15 of the type II collagen recovered in the same preparation. The possible explanations for these differences are discussed.