Regenerative organization of the trigeminal ganglion following mental nerve section and repair in the adult rat

Sequential double‐fluorescence labeling techniques were employed to determine the regenerative somatotopic organization of first‐order mandibular neurons following mental nerve transection and surgical repair in the adult rat. Twenty‐four ganglia from 12 adult rats were examined microscopically in the following double‐labeling paradigm: (i) Fast Blue was injected directly into the mental nerves bilaterally; (ii) 7 days later the nerves were transected and immediately rejoined by microscopic suture techniques; (iii) Diamidino Yellow was then injected directly into the regenerated nerve, distal to the point of repair, 30, 60, and 90 days postrepair; and (iv) the animals were sacrificed 3 days later and the ganglia removed for fluorescent microscopic examination. Results were compared with 12 ganglia each of unrepaired/resected controls and sham surgery controls made in parallel. The organization of fluorescence‐labeled mandibular cells followed an orderly somatotopic distribution along the lateral dorsoventral axis of the trigeminal ganglion in all groups. The difference in mean total number of fluorescence‐labeled cells within and between groups was insignificant or minimal. There was no evidence of heteronymous (nonmandibular) or homonymous (mandibular) sprouting following neuronal regeneration. Regeneration, as determined by the presence of double‐labeled cells, was negligible if the transection injury was not repaired but significant 30 days following repair. Additionally, mandibular regeneration gradually improved, as shown by the significant increase of double‐labeling at 60 and 90 days postrepair. However, 90 days later, the percentage of regenerated cells had not reached sham control conditions. The results of these studies suggest that following nerve transection and immediate repair in the adult rat: (i) mental sensory neuronal perikarya regenerate from and maintain an organized somatotopic area within the mandibular division of the trigeminal ganglion; (ii) reorganization by collateral sprouts from nonmental sensory mandibular and/or nonmandibular trigeminal ganglion cells is not evident or is negligible in the adult rat; and (iii) regeneration of resected trigeminal sensory neurons is a gradual process which is enhanced by immediate surgical intevention.