Role of aggregation factor and cell type in sponge cell adhesion

A pair of sponge species, Microciona prolifera and Halichondria bowerbanki, which lack mutual species specificity in their aggregation "factor", are useful in establishing the mechanisms of action of these factors. These sponges were dissociated both mechanically, which leaves the factor on the cell surface, and by Humphrey's (1963) method, which isolates the factor from the cells. The adhesive specificities which arose, in the various combinations tested, point to an intercellular factor bridge consisting of a single symmetrical unit. An analysis of most other workers' results is consistent with this interpretation. However, MacLennan and Dodd's (1967) results using other species would require a bridge consisting of two or more asymmetrical units. Differences were found in the specificity of adhesion of various types of cells within a single species. This presents a heretofore unconsidered problem in assesing the adhesive factor's mechanism of action. Three structurally distinct cell types were separated from a suspension of dissociated Microciona cells by velocity sedimentation. These cells differ greatly in adhesiveness. The differences in adhesion are correlated with numbers and positions of cells incorporated into aggregates. Such differences are considered in explaining the mechanism of action of the factors.