The guidance of optic axons in the developing and adult mouse retina.

Previous studies in chick embryos (Goldberg, '77) indicated that unidirectional guidance of retinal axons toward the optic nerve is restricted to the vitread portion of the ganglion cell fiber layer (GCFL) of the retina; random fiber growth was noted after deflection of the optic axons sclerad to the GCFL. The present study on mice confirms these observations. Silver-stained flat mounts of retinal colobomas were examined. Many optic axons in colobomas do not exit normally from the eye, but travel randomly when deflected sclerad to the GCFL. Newborn mouse axons grew around retinal lesions in a highly directed manner. Such axons were always situated in the vitread portion of the GCFL. The unidirectional guidance found in newborn mice was absent in adults. Deflected adult axons traveled randomly regardless of their level within the GCFL. We propose that defective guidance largely accounts for failure of axonal regeneration in the adult mouse retina. The inability of the adult axons to fasciculate (adhere to one another and form fiber bundles) suggests that impaired cellular adhesivity may be part of the mechanism of regenerative failure.