REINNERVATION OF PACINIAN CORPUSCLES BY CNS AXONS AFTER TRANSPLANTATION TO THE DORSAL COLUMN - INCIDENCE AND ULTRASTRUCTURE

We have investigated the capacity of injured axons in the spinal dorsa l columns of young adult rats to reinnervate grafted Pacinian corpuscl es. A branch of the hindlimb interosseous nerve with a group of crural Pacinian corpuscles attached to it was autotransplanted to the surfac e of the spinal cord and the nerve stump was implanted into the dorsal column. Two to three months later 16 grafts were removed for examinat ion by light and electron microscopy. By 3 months after transplantatio n almost all Schwann cell columns of the grafted nerve branch were occ upied by regenerated myelinated and unmyelinated axons. Of 41 corpuscl es examined by electron microscopy 24 were reinnervated by 13 myelinat ed fibres which gave rise to multiple terminals in the inner core. The remaining corpuscles appeared to be denervated. Only two of the reinn ervated corpuscles contained regenerated endings which reiterated the distinct ultrastructure of normal presynaptic terminals of CNS axons, characterized by clusters of lucent vesicles and paramembranous densit ies. All other corpuscles were reinnervated by terminals which resembl ed peripheral mechanosensory endings as they contained mitochondria an d very few vesicles. One such corpuscle was coinnervated by small term inals filled with large dense cored vesicles. We assume that the major ity of grafted Pacinian corpuscles have been reinnervated by dorsal co lumn axons and that the regenerated terminals with the ultrastructure of peripheral mechanosensory endings derive from central axons of prim ary sensory neurons, which are apparently capable of constructing mech anosensory-like terminals in response to signals from the Pacinian cor puscles. The vesicle-filled endings are probably formed by second orde r sensory neurons, corticospinal neurons and small peptidergic neurons unable to adjust their terminals to the new target.