Slow axonal regrowth but extreme hyperinnervation of target muscle after suture of the facial nerve in aged rats.

Unilateral transection and suture of the facial nerve was performed in 60 old rats (20 months of age). The time course of mimetic reinnervation was studied by counting all retrogradely labeled motoneurons in the facial nucleus after injection of HRP into the whiskerpad muscles for 14-112 days post operation. The comparison between these neuron counts and data for young rats yielded four conclusions. First, the qualitative equivalent of the phenomenon "misdirected reinnervation" in aged rats was the same as in young adults: HRP-labeled motoneurons were scattered throughout the facial nucleus lacking myotopic organization from 18 until 112 days post operation. Second, no age-related loss of motoneurons was detected. Third, the axonal regrowth was delayed in aged rats. Fourth, the postoperative hyperinnervation (the projection of more motoneurons into a muscle than under normal conditions, i.e., the quantitative aspect of misdirected reinnervation) was more than two times higher than in young rats. These data may provide reasonable explanations for the poor functional recovery after reconstructive surgery on the facial nerve in old patients.