Axons derived from young ganglion cells in the periphery of the retinae of larval and adult goldfish are known to fasciculate with one another and their immediate forerunners, creating the typical age-related order in the retinotectal pathway. Young axons express the E587 antigen, a member of the L1 family of cell adhesion molecules. Repeated injections of Fab fragments from a polyclonal E587 antiserum (E587 Fabs) into the eye of 3.4 cm goldfish disrupted the orderly fascicle pattern of RGC axons in the retina which was preserved in controls. Instead of bundling tightly, RGC axons crossed one another, grew between fascicles and arrived at the optic disk in a broadened front. When added to RGC axons growing in vitro, E587 Fabs neutralized the preference of growth cones to elongate on lanes of E587 protein, caused defasciculation of axons which normally prefer to grow along each other when explanted on polylysine, and prevented clustering of E587 antigen at axon-axon contact sites. Monoclonal E587 antibody disturbed axonal fasciculation moderately but led to a 30% reduction in growth velocities when axons tracked other axons. Therefore we conclude that E587 antigen mediates axonal recognition, selective fasciculation and the creation of the age-related order in the fish retina.

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