Target recognition and dynamics of axonal growth in the retinotectal system of fish

Embryonic and regenerating retinal axons in fish are able to seek out their retinotopic target sites in the tectum. Neither a specific preordering of axons in the retinotectal pathway nor activity-dependent axon-target interactions are required for appropriate axonal targeting. Axon-target recognition appears to be predominantly mediated by positional cell surface markers. The discrimination of position-dependent differences by retinal axons in a special in vitro assay is consistent with this concept. To understand retinal axonal regeneration we have analyzed the glial cells of the fish optic nerve and the expression of growth-associated cell surface molecules on the regenerating axons. The surfaces of the glial cells identified as oligodendrocytes are excellent substrates for the elongation of regenerating axons. Raising monoclonal antibodies we have found 3 cell surface proteins specific for growing axons. In the normal adult goldfish optic nerve, these proteins are only expressed by the few new axons from the newborn ganglion cells at the retinal margin. They are re-expressed on all axons during regeneration. A known cell surface molecule, NCAM, is expressed in a similar, specific spatiotemporal pattern on the fish retinal axons and may--in normal nerves--contribute to the establishment of the age-related fiber association. Whether the re-expression of NCAM and the antigens detected by the novel monoclonal antibodies are functionally involved in axonal growth and regeneration remains to be investigated.