Fibronectin-like protein in Porifera: its role in cell aggregation.

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Abstract
Experiments were carried out on a freshwater sponge (E. muelleri) in order to demonstrate the presence of fibronectin in Porifera. By using antibodies to highly purified human plasma fibronectin, the presence of a similar or identical protein could be demonstrated in the membranes of E. mulleri cells such as epithelial cells, fibroblast-like cells and choanocytes. The reaction was specific, could be abolished by the addition of excess fibronectin, and was not observed with nonimmune rabbit serum. The immune fluorescent reaction became stronger when the sponge cells were pretreated with acetone and could also be observed, although with a less intense staining, on the intercellular matrix. This shows the predominant presence of a sponge fibronectin-like protein in the cell membranes and also its presence to a lesser extent in the intercellular matrix. When dissociated sponge cells were led to reassociate under the microscope, reassociation could be completely inhibited by anti-human fibronectin antiserum up to a dilution of 1:120 and partially inhibited up to a dilution of 1:240. The reassociation of dissociated sponge cells could also be inhibited by the addition of purified gelatin but not with serum albumin or with a normal, nonimmune rabbit serum. A sponge cell fibronectin-like protein may play an important role as the (or one of the) recognition site(s) of the aggregation factor(s) and can therefore be directly involved in cell association, morphogenesis and differentiation.