Changes in facial nerve function, morphology and neurotrophic factor expression in response to three types of nerve injury.

Objective: To study the changes in facial nerve function, morphology and neurotrophic factor 3 expression in response to three types of nerve injury. Materials and methods: Changes in facial nerve function (in terms of blink reflex, vibrissae movement and position of nasal tip) were assessed in 45 rats in response to three types of nerve injury: partial section of the extratemporal segment (group one); partial section of the facial canal segment (group two); and complete transection of the facial canal segment (group three). All facial nerves were then dissected out from the sacrificed animals, on the first, seventh or 21st post-operative day, and the injured segment bisected for subsequent analysis. Morphological change and neurotrophic factor 3 expression in these facial nerve segments were evaluated by means of improved trichrome staining and immunohistochemical analysis, respectively. Results: Facial nerve function was more severely damaged and recovery was slower in group two compared with group one, although the facial nerve injury had been identical at the two sites involved. In group three (complete facial canal transection), scattered degenerative fibres were observed at the transection site on the seventh post-operative day. The number of degenerated fibres, assessed on the seventh post-operative day, was markedly increased in group two compared with group one. On the seventh post-operative day, axonal expression of neurotrophic factor 3 was strongly positive, whereas expression in Schwann cells was only weakly positive. Conclusions: The facial palsy of the group two rats was more severe than that of group one rats, although their facial nerve function had partly recovered on the 21st post-operative day. Fibre degeneration was not only dispersed throughout the injury site but also occurred throughout the length of the nerve. Neurotrophic factor 3 immunoreactivity increased in facial nerve fibres after partial transection.