In the central nervous system of fish and frogs, some, but not all, axons can regenerate. Retinal ganglion cells are among those that can. The retinae of fish and frogs produce new retinal neurons, including ganglion cells, for months or years after hatching. We have evaluated the hypothesis that retinal axonal regeneration is obligatorily linked to continued production of new ganglion cells. We used bromodeoxyuridine immunocytochemistry to assess retinal neurogenesis in juvenile, yearling, and 10 year old Xenopus laevis. Retinal ganglion cell genesis was vigorous in the marginal retina of the juveniles, but in the yearlings and the 10 year olds, no new ganglion cells were produced there. Cellular proliferation in the central retina was evident at all three ages, but none of the cells produced centrally were in the ganglion cell layer. Regeneration was examined in vivo by cutting one optic nerve and then, weeks later, injecting the eye with tritiated proline. Autoradiographs of brain sections showed that the optic nerves of all three ages regenerated. Regeneration in vitro was assessed using retinal explants from frogs of all three ages. In all cases, the cultures produced neurites, with some age-specific differences in the patterns of outgrowth. We conclude that retinal axonal regeneration is not linked obligatorily to maintained neurogenesis.

No abstract

Several lines of evidence suggest that glial cells have major effects on neuronal pathfinding. We have examined in vitro whether the outgrowth pattern of Xenopus retinal fibres is influenced by the glial cells encountered as they grow to the optic tectum. Strips of retina were cultured on monolayers of glial cells from the diencephalon and from the rostral and caudal ends of the optic tectum. On glia from the caudal end of the tectum the growth of fibres from the nasal and temporal ends of the strips was different: temporal fibres were shorter and more fasciculated than nasal fibres. This difference was still discernible on glia isolated from the rostral end of the tectum, but to a lesser extent. On glia from the diencephalon there was no difference between nasal and temporal fibres.