Abstract.We have developed an in vitro assay which measures the ability of growth cones to extend on an axonal substrate. Neurite lengths were compared in the presence or absence of monovalent antibodies against specific neural cell surface glycoproteins. Fab fragments of antibodies against the neural cell adhesion molecule, NCAM, have an insignificant effect on the lengths of neurites elongating on either an axonal substrate or a laminin substrate. Fab fragments of polyclonal antibodies against two new neural cell surface antigens, defined by mAb G4 and mAb FU, decrease the lengths of neurites elongating on an axonal substrate, but have no effect on the lengths of neurites elongating on a laminin substrate. (34 antigen is related to mouse L1, while Fll antigen appears to be distinct from all known neural cell surface glycoproteins.Our results suggest that the G4 and Fll antigens help to promote the extension of growth cones on axons.ONG the many cues which guide the axons of embryonic neurons to their proper destinations, neuronal cell surfaces may be an important source of information. Extending axons are frequently observed to elongate directly upon the surfaces of pre-existing axons. This has been described for developing optic axons of the crustacean Daphnia magna (Lopresti et al., 1973), for developing mandibular axons of Xenopus laevis (Davies et al., 1982), and for developing peripheral neurons in the grasshopper Schistocerca nitens (Ho and Goodman, 1982). In the chick embryo, motoneurons appear to prefer the surfaces of other motoneuron axons as opposed to non-neuronal or extracellular surfaces (AI-Gaith and Lewis, 1982;Tosney and Landmesser, 1985). In the central nervous system of the grasshopper embryo, elongating neurites choose to grow upon the surfaces of specific axons, and the ablation of these axons results in abnormalities in outgrowth (Raper et al., 1983(Raper et al., , 1984. Similar results have been obtained in the spinal cord of a fish (Kuwada, 1986).To identify axonal surface molecules that mediate neurite extension, we have developed an assay which measures the distance growth cones extend on an axonal substrate. Neurite lengths can then be compared in the presence or absence of Fab fragments of antibodies directed against specific neural cell surface molecules. The antibodies used in this study were directed against the neural cell adhesion molecule (NCAM) 1 (for reviews see Edelman, 1984; Rutishauser, 1. Abbreviations used in this paper: anti-Fll Fabs, Fab fragments of polyclonal antibodies raised against antigens immunopurified by mAb Fll; anti-G4 Fabs, Fab fragments of polyclonal antibodies raised against antigens immunopurified by mAb G4; anti-NCAM Fabs, Fab fragments of 1984); and against two recently purified neural cell surface antigens, defined by mAb's G4 and Fll (for full details, see Rathjen et al., 1987, in this issue). Polyclonal antisera against the G4 and FI1 antigens stain components of 135 kD on immunoblots of adult chick brain membrane proteins. G4 antigen is related to...
