AbstractWe examined the distribution of fibronectin‐like (FNL) immunoreactivity associated with intact buccal ganglia, cell‐cultured buccal ganglia neurons and nonneuronal cells, and brain‐conditioned medium from the snailHelisoma. In addition, the possible roles of fibronectin in the regulation of neurite outgrowth were studied. Immunofluorescent staining for FNL antigens revealed intense staining in patches and fibrous arrays over the connective tissue sheaths of buccal ganglia and nerve trunks. Within the ganglia, heavy staining was seen surrounding neurons and in track‐like arrangements. In cell cultures, specific staining was associated with nonneuronal cell surfaces and to a lesser degree with the surface of identified neurons. In addition, a noncellular, substrate‐bound component of brain‐conditioned medium displayed FNL immunoreactivity. Since culturedHelisomaneurons require a substrate‐associated, brain‐derived conditioning factor (CF) in order to elaborate neurites with motile growth cones, we tested whether the FNL immunoreactive substance might act as a neuritotropic agent. Fibronectin antiserum suppressed, in a dose‐dependent manner, the CF‐induced sprouting of identified neurons in isolated cell culture. When added at increasing concentrations to neurons already growing in response to CF, fibronectin antiserum exerted a biphasic effect on neurite elongation; outgrowth was accelerated at low, but inhibited at high, antiserum concentrations. In contrast, growth cone structures associated with motility (filopodia and lamellipodia) were progressively reduced by increasing levels of antiserum. A short peptide derived from fibronectin's cell‐binding domain (Arg‐Gly‐Asp‐Ser) also greatly reduced neurite outgrowth. The combined results of this study indicate an abundance of FNL immunoreactive molecules within the CNS ofHelisoma, their probable production by nonneuronal cells, and their function as a substrate‐associated component of CF which promotes growth cone filopodi
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