REGENERATIVE GROWTH OF ADULT RAT RETINOFUGAL AXONS: TRAUMATIC REACTION, NEUROMA FORMATION, AND THE REGENERATION OF AXONS INTO AND THROUGH HETEROTOPIC TRANSPLANTS OF EMBRYONIC NEOCORTICAL TISSUE

The response of adult rat retinofugal axons to axotomizing injuries and the ingrowth of retinal afferents to transplants of 17 day embryonic neocortical tissue in the superior colliculus and caudal diencephalon of adult rats were studied using the Fink-Heimer and HRP anterograde axonal tracing methods. Three phases in the metamorphosis of damaged retinofugal axons were recognized following partial transection of the optic tract and brachium of the superior colliculus: (1) a phase characterized by the formation of hypertrophied segments and swollen terminal clubs, defining the zone of traumatic reaction in the proximal stump of the damaged retinofugal projection, (2) a transient phase of apparent terminal and collateral sprouting from the damaged axons with the zone, and (3) a phase of sustained regenerative growth, characterized by the development of dense fascicle and neuroma-like tangled mass neoformations of retinofugal axons within the zone of traumatic reaction. In some cases fascicles of retinofugal axons crossed the margin of the lesion cavity and coursed up to 2.5 mm upon mesenchymal elements present in the lumen. The existence of this phase indicates that the adult rat retinal ganglion cells possess a significant capacity for axonal regeneration. The results of the transplantation studies suggested that this regenerative capacity may be expressed as retinal afferent ingrowth to the transplants. The necessary conditions for retinal afferent ingrowth to neural transplants appeared to be: (1) that host retinofugal axons be damaged at the transplantation site, (2) that the transplanted tissue becomes adherent to the damaged parenchymal surfaces of the lesion cavity, and (3) that the transplants develop well integrated interface regions with the zone of traumatic reaction in the host\u27s retinofugal projection. Retinal afferent ingrowth across transplant interface regions with periterminal regions of the retinofugal projection (LGN, optic layers of the superior colliculus) was characterized by patches of dense termination proximal to the interface. The magnitude of this form of ingrowth was highly dependent upon the relative number of damaged retinofugal axons in the zone of traumatic reaction at the transplant optic layer interface and was not dependent upon the area of the transplant\u27s entire optic layer interface. . . . (Author\u27s abstract exceeds stipulated maximum length. Discontinued here with permission of author.) UM