Quantitative, morphological, and somatotopic nuclear changes after facial nerve regeneration in adult rats: A possible challenge to the “no new neurons” dogma

THE ANATOMIC REORGANIZATION of the subnucleus that controls the stylohyoid muscle (the stylohyoid subnucleus) within the brain stem facial nucleus was studied after regeneration of the facial nerve in adult rats. Horseradish peroxidase was injected into the right stylohyoid muscle 3 to 21 months after transection and repair of the right facial nerve at the level of the stylomastoid foramen. Position, number, and soma diameter of retrogradely horseradish peroxidase-labeled motoneurons were established, as well as the rostro-caudal extension of the stylohyoid subnucleus. In experimental rats, the stylohyoid subnucleus showed either an ipsilateral (50% of the rats) or a bilateral representation. In all of the experimental rats, the motoneurons composing the stylohyoid subnucleus had a more dispersed horizontal distribution pattern when compared with controls. More than 80% of the motoneurons were located outside the borders of the normal stylohyoid subnucleus, either ventrally or, especially in the rostral sections, dorsally closer to the floor of the fourth ventricle. The mean rostro-caudal length of the stylohyoid subnucleus was 2028.6 ± 152.7 μm. The mean motoneuron number was 481.4 ± 109.5 (2.20-fold greater than control values), and the motoneuron diameter distribution ranged from 7 to 43 μm. This study demonstrates that after regeneration of the facial nerve in adult rats, major changes occur in both the location and number of motoneurons that make up the stylohyoid subnucleus. The increase in the number of motoneurons, the location of the many cells in brain stem areas that normally do not contain motoneurons for facial muscles, and the appearance of contralateral motoneurons are unexplained. One possible mechanism is new cell proliferation, migration, and differentiation in the central nervous system. This, rather than activation of adjacent “dormant” neurons, could explain both the significant increase in the number of motoneurons in the stylohyoid subnucleus after facial nerve regeneration and the new somatotopic configuration of the subnucleus itself. Copyright © by the Congress of Neurological Surgeons.