Retinal Afferent Ingrowth to Neocortical Transplants in the Adult Rat Superior Colliculus is due to the Regeneration of Damaged Axons

Retinal afferent ingrowth to embryonic neural transplants in the adult rat superior colliculus may represent either sprouting of intact axons or the regeneration of transected axons. If ingrowth represents regeneration of damaged retinofugal axons, then lesions that axotomize more retinofugal axons at the transplantation site should induce greater retinal afferent ingrowth. Alternately, if ingrowth represents terminal or collateral sprouting of intact retinofugal axons at or near the transplant/host optic layer interface, then the magnitude of retinal afferent ingrowth should be directly related to the total area of this interface. To test between these two hypotheses surgical knife wounds were made either parallel (in the sagittal plane) or perpendicular (in the transverse plane) to the course of axons in the stratum opticum, embryonic neocortical tissue was transplanted at the coordinates of these tectal slits, and retinal afferent ingrowth visualized 1-90 days after surgery using anterogradely transported HRP. A zone of traumatic reaction (ztr) in the optic layers was seen in every case, characterized by hypertrophied axons and swollen terminal clubs at 1 day. Between 30 and 90 days the damaged retinofugal axons in the zone formed dense fascicles and neuroma-like tangles. Retinal afferent ingrowth occurred only across transplant interface regions with the ztr. The magnitude of ingrowth was directly related to the area of the ztr interface and not the total optic layer interface area. Retinal afferent ingrowth appears to reflect the intrinsic regenerative capacity of adult mammalian retinal ganglion cells and not sprouting of undamaged axons.