[Optic nerve regeneration by nerve transplantation]

The optic nerve fibers of adult mammals, once injured, can not regenerate spontaneously. However, from recent studies it has become clear that when their extracellular environment is replaced with that of the peripheral nervous system, namely surrounded with Schwann cells, they can regenerate their axons through the grafted nerve. Previous studies on the optic nerve regeneration by peripheral nerve transplantation have been done mostly in rats or hamsters. With an expectation of clinical application in future, we studied the optic nerve regeneration of adult cats because a great deal of knowledge on the optic nerve fibers and retinal ganglion cells had been accumulated. From recent series of studies we have obtained following results. 1. Retinal ganglion cells that regenerated axons constitute 2--4% of the total population, and among several types of ganglion cells, alpha cells have the greatest capacity for axonal regeneration. The ability of alpha cells for axonal regeneration is related to their relative resistance to axotomy. 2. Retinal ganglion cells with regenerated axons preserve their original dendritic fields, but their axons are thinner than normal, and mostly unmyelinated. 3. Single unit activities recorded from teased fibers of regenerated axons revealed that the units have mostly normal receptive field properties, enabling us to classify them into Y, X or W cells. 4. Amplitude reduction of pattern reversed electroretinogram (ERG) after the optic nerve section was slowed, to some extent, by the peripheral nerve transplantation. How we can reconnect these regenerated optic nerve fibers to the target neurons in the central visual system is a matter for future study.