Segmental appearance of type X collagen in the developing avian notochord.

To determine whether short-chain cartilage collagen, collagen type X, is a component of notochordal matrix, we have performed immunohistochemistry with a monoclonal antibody (X-AC9) previously shown to be specific for this molecule (Schmid and Linsenmayer (1984). J. Cell Biol. 100, 598-605). We have also examined different stages of embryos to establish the temporo-spatial appearance of the molecule. The data show that type X collagen is indeed a component of notochordal matrix, but that its developmental appearance is quite late compared to that of type II collagen, another cartilage matrix molecule found in notochord. It is detected only in embryos older than 12 days. The appearance of type X collagen within the notochord is preceded by its appearance within the surrounding hypertrophic cartilage matrix of the vertebral bodies. Spatially, within the notochord the appearance of type X collagen is initially restricted to sites at which the surrounding vertebral cartilage matrix is also reactive for type X. With subsequent development, the notochordal reactivity extends, in a decreasing gradient, anteriorly and posteriorly toward the intervertebral zones. However, the brightest immunofluorescence of notochordal type X is maintained at the midvertebral sites. The ultimate fate of the notochordal tissue at such midvertebral sites is to be removed during endochondral bone formation within the vertebrae. We have observed that type II collagen is also found within notochordal tissue, but this molecule has a distribution which is the converse of that of the type X collagen. The type II collagen is preferentially deposited at intervertebral sites (i.e., the locations of future intervertebral discs). That the type X collagen within the notochord is preferentially deposited at sites destined to be replaced is consistent with one of the hypotheses we previously raised for the function of the type X collagen molecule--it may "target" skeletal tissues for eventual removal.