Structural and functional adaptation of the pulpal nerve fibers in the rat incisor.

Special fine structure of the pulpal nerve fibers adapted to the continuously growing incisor was studied on 20 rats (25-d-old-males) electron microscopically and was compared with that of the molars of limited growth. The incisal pulp was innervated by 2 kinds of the nerve fibers derived from the cervical sympathetic and the trigeminal ganglia: polyaxonal unmyelinated and monoaxonal myelinated. In the apical pulp, however, the pulpal nerve fibers were unmyelinated, showing 3 different profiles in the fine structure: 1. polyaxonal, 2. polyaxonal with dial plate-like profile and 3. monoaxonal. The monoaxonal unmyelinated fiber reflected the transverse profile of the unmyelinated part of the myelinated axon derived from the trigeminal ganglion. It can be concluded that the pulpal nerve fibers derived from the trigeminal ganglion maintain the immatured nature in the apical proliferating pulp to adapt to the continuous growth of the incisor.