QUANTITATIVE, MORPHOLOGICAL, AND SOMATOTOPIC NUCLEAR-CHANGES AFTER FACIAL-NERVE REGENERATION IN ADULT-RATS - A POSSIBLE CHALLENGE TO THE NONEW NEURONS DOGMA

THE ANATOMIC REORGANIZATION of the subnucleus that controls the styloh yoid muscle (the stylohyoid subnucleus) within the brain stem facial n ucleus was studied after regeneration of the facial nerve in adult rat s. Horseradish peroxidase was injected into the right stylohyoid muscl e 3 to 21 months after transection and repair of the right facial nerv e at the level of the stylomastoid foramen. Position, number, and soma diameter of retrogradely horseradish peroxidase-labeled motoneurons w ere established, as well as the rostro-caudal extension of the stylohy oid 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 stylo hyoid subnucleus had a more dispersed horizontal distribution pattern when compared with controls. More than 80% of the motoneurons were loc ated outside the borders of the normal stylohyoid subnucleus, either v entrally or, especially in the rostral sections, dorsally closer to th e floor of the fourth ventricle. The mean rostro-caudal length of the stylohyoid subnucleus was 2028.6 +/- 152.7 mu m. The mean motoneuron n umber was 481.4 +/- 109.5 (2.20-fold greater than control values), and the motoneuron diameter distribution ranged from 7 to 43 mu m. This s tudy demonstrates that after regeneration of the facial nerve in adult rats, major changes occur in both the location and number of motoneur ons that make up the stylohyoid subnucleus. The increase in the number of motoneurons, the location of the many cells in brain stem areas th at normally do not contain motoneurons for facial muscles, and the app earance of contralateral motoneurons are unexplained. One possible mec hanism is new cell proliferation, migration, and differentiation in th e central nervous system. This, rather than activation of adjacent ''d ormant'' neurons, could explain both the significant increase in the n umber of motoneurons in the stylohyoid subnucleus after facial nerve r egeneration and the new somatotopic configuration of the subnucleus it self.