Objectives/hypothesis: Anecdotal clinical findings suggest that denervated muscle may regain modest functional recovery via spontaneous collateral sprouts from intact adjacent nerve fibers. The current study evaluates the conditions needed for the denervated masseter muscle to induce axonal sprouting from the facial nerve. We hypothesize that epineurial injury is required to induce collateral sprouting toward a neighboring denervated muscle.
Study design: Twelve thy1-yellow fluorescent protein-16 (thy1-YFP-16) transgenic mice whose axons express yellow fluorescent protein were allocated into six groups, with four degrees of facial nerve injury (intact, crush, transection, removed segment) with or without masseter denervation.
Methods: Animals underwent serial in vivo imaging analyses under the fluorescent microscope weekly for 5 or 7 weeks and were subsequently perfused for analysis. Masseter muscle acetylcholine receptors (AChRs) were stained with Alexa Fluor 594 conjugated alpha-bungarotoxin, and whole mounts were imaged with confocal microscopy.
Results: In groups with intact or crushed facial nerves, no evidence of collateral sprouting was demonstrated. Mice with transected facial nerve branches or removed segments demonstrated sprouting from the proximal stump into the denervated masseter. Staining of the AChRs confirmed that new neuromuscular junctions were established between the facial nerve and the denervated masseter.
Conclusions: This study suggests that epineurial injury is required to stimulate axonal sprouting into adjacent denervated muscle. Nerves with compromised epineurium may be useful in promoting neo-neurotization after muscle denervation.