A morphological study of the collateral reinnervation of the cat's canine tooth.

We examined the morphologic characteristics of the collateral reinnervation of the cat's canine tooth pulp. Collateral reinnervation was stimulated in six adult cats by transecting the ipsilateral inferior alveolar nerve and preventing it from regenerating by sealing the ligated nerve inside a nylon tube. The apices of the canine teeth were examined after 15 weeks using electron microscopy and each axon was counted and measured. Compared with contralateral control teeth, the collateral reinnervation consisted of fewer myelinated fibers and in five animals the number of nonmyelinated fibers was also smaller. In one animal the nonmyelinated count on the operated side exceeded that on the control side. The total number of axons was 33.5% of the number in control teeth, considerably fewer than seen after reinnervation by regenerating fibers in previous experiments. The structural characteristics of the collateral fibers were similar to those of regenerating nerve fibers. Compared with controls the myelinated collateral fibers were smaller and had thinner myelin sheaths. There were more nonmyelinated fibers containing only one axon. More nonmyelinated fibers had exposure of part of their axolemma to the extracellular space and there was also an increase in axoaxonal apposition.