Morphological changes of denervated and reinnervated rat facial muscle.

In 25 rats the facial nerve was resected on the right side and in another group of 30 rats the right facial nerve was transected and immediately repaired with an end-to-end anastomosis. Both groups were subdivided into groups of five rats. Size and histochemical profile of single muscle fibres were analysed by computer-assisted quantification on the basis of their myofibrillar ATPase (pH 4.3) and succinate dehydrogenase activities in serial cross-sections of the levator labii muscle at 7, 14, 21, 28 and 90 days after nerve resection and 7, 14, 21, 28 and 180 days after immediate anastomosis. Seven muscles of four normal rats were used as a control group. Four muscle fibre types could thus be functionally identified: (a) slow oxidative (SO); (b) fast oxidative glycolytic (FOG); (c) fast glycolytic (FG); and (d) succinate dehydrogenase intermediate (SDH-Int). The cross-section of the FOG fibres showed no changes following permanent denervation. while in comparison there was a significant reduction in the cross-section of FG and SDH-Int fibres. After immediate anastomosis no reduction in the cross-section of the FOG fibres was observed. In contrast the cross-section of FG and SDH-Int fibres showed a significant decrease following direct anastomosis and returned to normal levels at 90 days. Neither resection nor end-to-end anastomosis led to significant alterations in the incidence of FOG and FG muscle fibres over the period of evaluation. Changes in fibre size were transient and completely reversible 180 days after nerve repair, whereas fibre type composition was not. These findings may reflect a long-lasting impairment of the precise function of the levator labii muscle after immediate anastomosis.