ANOMALOUS AXONAL OUTGROWTH AT THE RETINA CAUSED BY INJURY TO THE OPTIC-NERVE OR TECTAL ABLATION IN ADULT XENOPUS

The retina and optic nerve head were examined by light microscopy and EM in adult X. laevis after injury to topic nerve fibers. Intraorbital resection, transection or crush of the optic nerve all resulted in the appearance at the retina of a mass of actively growing axons which formed a ring around the intraretinal and adjacent chloroidal portions of the optic nerve head. Formation of this heterotopic axon population was first noted at 2 wk after nerve and head by astrocytes within the retina or by blood vessles and fibroblasts of the leptomeninges at extraretinal locations. In general, the orientation of the ectopic fibers was perpendicular to the fibers of the optic nerve. Bundles of axons were found between the ring of ectopic fibers and the pigment epithelial layer of the retina or among the blood sinuses of the choroid. Similar ectopic fibers were seen following transection of the optic nerve at the chiasm and after tectal ablation although the onset of these changes was slower than that seen after nerve resection. Damage to visual pathways in the frog apparently induces dramatic morphological alterations in the optic nerve and retina far proximal to the site of injury in this regenerating system.