Collateral and terminal sprouting of rat facial nerve axons into fresh or predegenerated nerve grafts after end-to-side neurorrhaphy
(English)Manuscript (preprint) (Other academic)
In spite of a continuous technical refinement the results of microsurgical facial nerve repair remain unsatisfactory. This situation prompted the present study, which evaluates axonal regeneration into a rat facial nerve branch after crossfacial end-to-side neurorrhaphy in adult rats. A fresh (group A, 12 rats) or predegenerated (group B, 12 rats) sural nerve graft (SNG) was sutured to a perineurial window in the mandibular branch (MB) of the right facial nerve. It was then tunneled under the chin and sutured to the distal stump of the cut left MB. Twelve weeks after grafting some rats were prepared for electron microscopy (EM). EM examination showed that distal to the window the right MB contained mainly myelinated axons with relatively thick sheaths. Aberrant small-diameter axons with thin sheaths were also observed in most specimens. The counts revealed a total average of about 1 800 axons (14 % unmyelinated) in both groups. In the SNG thin myelinated and unmyelinated axons predominated. The counts showed 900-1 000 axons (63-68% unmyelinated) with no significant difference between the groups. The left MB contained some thin myelinated axons and many unmyelinated axons. The number of axons was 1 200-1300 (81-83% unmyelinated). Again, the two groups did not differ. Other rats were subjected to retrograde tracing. The SNG-right MB coaptation was exposed and each nerve was cut off. Two or 3 days after application of fast blue (FB) to the right MB and fluoro-ruby (FR) to the SNG the animals were perfused with paraformaldehyde and the brain stem was transversely cryosectioned. Counts of labeled motoneurons were used to calculate the percentage of motoneurons with different tracer combinations. The results showed FBlabeling only (motoneurons with axons in the right MB distal to the coaptation) in 5-11% of the counted profiles, FR-labeling only (motoneurons with axons in the SNG) in 43-54% of the profiles, and a combined FB- and FR-labeling (motoneurons with axons in the right MB as well as in the SNG) in 41-46% of the profiles. The two groups did not differ statistically in this respect. We conclude that (i) coaptation of a SNG to the MB of the facial nerve in an end-toside fashion is followed by growth of myelinated and unmyelinated axons from the donor nerve into the graft; (ii) these axons originate from facial motor axons; (iii) some 50% of the traced facial neurons project into the SNG only, 45% project into the SNG and into the MB and 5% project into the MB only; (iv) predegeneration of the SNG does not enhance the number of axons in the graft.
Medical and Health Sciences
IdentifiersURN: urn:nbn:se:liu:diva-81520OAI: oai:DiVA.org:liu-81520DiVA: diva2:553101