Repair of the mandibular branch of the rat facial nerve through transmedian grafting in one or two stages. Morphological evaluation.

This study examined by electron microscopy the normal fibre composition of the mandibular branch (MB) of the rat facial nerve and the outcome of axon regeneration in the MB after transmedian grafting in one or two stages. The average normal MB contained 2,185 axons, 17% of which were unmyelinated. The myelinated axons had a unimodal diameter distribution (range 1.5-9.5 microns, mode 4.5 microns). After superior cervical ganglionectomy, the MB lost 1/3 of the C-fibres and 10% of the myelinated axons. In neonatally capsaicin-treated rats the occurrence of unmyelinated axons was reduced by about 50%. After repair in one or two stages the MB contained more myelinated and unmyelinated axons than normal. The myelinated axons showed a unimodal size distribution with a subnormal diameter range. Statistical comparisons showed that MBs from both experimental groups were significantly abnormal with respect to total axon number as well as numbers of unmyelinated and myelinated axons. In these respects the grafted MBs did not differ significantly from each other. However, the myelinated axons in MBs from one-stage cases showed larger mean and maximum diameters compared to MBs from two-stage cases. These data suggest that the normal MB of the rat contains myelinated and unmyelinated sympathetic axons and that about half the C-fibres in the normal MB come from capsaicin-sensitive sensory neurons. The comparison of the two reparative procedures used provides evidence in favor of the one-stage alternative.