Expression of a collagen gene in mesenchyme lineages of the Strongylocentrotus purpuratus embryo.

We have previously described cloning of an exon of a sea urchin collagen gene and shown that its expression is temporally regulated during embryogenesis, beginning during blastula formation. We have now localized the protein encoded by the gene and the sites of its mRNA synthesis in the developing embryo. Antibody to a synthetic peptide reacts with a 208,000 Mr protein that is digestible by collagenase. Fractionation of pluteus stage embryos demonstrates that the protein is localized primarily with cells that form the syncytium of primary mesenchyme that elaborates the larval endoskeleton; furthermore, immunofluorescence localizes the epitope to the periphery of the endoskeleton in situ. Transcripts of the gene accumulate only in mesenchyme cells, especially those of the primary mesenchyme lineage. Measurements of absolute transcript abundance show that collagen mRNA is present in blastula primary mesenchyme cells at 600-700 copies per cell and at about fourfold lower amounts in other mesenchyme cells.