Mice lacking type II collagen fail to develop intervertebral discs. The present study describes the distribution of the developmentally expressed type IIA procollagen molecule, as well as types I and III collagens, in human IV disc specimens ranging from 42 to 101 days gestation. Type IIA procollagen contains the alternatively spliced exon 2 which encodes a 69-amino-acid cysteinerich domain. By radioactive in situ hybridization and fluorescence immunohistochemistry, we identified changes in the localization patterns of type IIA procollagen, particularly between days 54 and 101. At day 54, the developing disc was divided into the outer annulus containing types I and III collagens, the inner annulus containing type IIA procollagen and the notochord consisting of all three fibrillar collagens. Specifically, the IIA N-terminal propeptide was localized in the extracellular matrix at day 54 but, by day 101, was only observed in the cytoplasm of the inner annulus cells. A functional role for the IIA N-terminal propeptide during this specific stage of disc development seems apparent. This function may involve regulation of growth factors since the exon 2-encoded domain of type IIA procollagen has previously been shown to bind to bone morphogenetic protein-2 and transforming growth factor-β. We aim to explore this mechanism further.

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