Reinnervation of the tibialis anterior following sciatic nerve crush injury: A confocal microscopic study in transgenic mice

Transgenic mice whose axons and Schwann cells express fluorescent chromophores enable new imaging techniques and augment concepts in developmental neurobiology. The utility of these tools in the study of traumatic nerve injury depends on employing nerve models that are amenable to microsurgical manipulation and gauging functional recovery. Motor recovery from sciatic nerve crush injury is studied here by evaluating motor endplates of the tibialis anterior muscle, which is innervated by the deep peroneal branch of the sciatic nerve. Following sciatic nerve crush, the deep surface of the tibialis anterior muscle is examined using whole mount confocal microscopy, and reinnervation is characterized by imaging fluorescent axons or Schwann cells (SCs). One week following sciatic crush injury, 100% of motor endplates are denervated with partial reinnervation at 2 weeks, hyperinnervation at 3 and 4 weeks, and restoration of a 1:1 axon to motor endplate relationship 6 weeks after injury. Walking track analysis reveals progressive recovery of sciatic nerve function by 6 weeks. SCs reveal reduced S100 expression within 2 weeks of denervation, correlating with regression to a more immature phenotype. Reinnervation of SCs restores S100 expression and a fully differentiated phenotype. Following denervation, there is altered morphology of circumscribed terminal Schwann cells demonstrating extensive process formation between adjacent motor endplates. The thin, uniformly innervated tibialis anterior muscle is well suited for studying motor reinnervation following sciatic nerve injury. Confocal microscopy may be performed coincident with other techniques of assessing nerve regeneration and functional recovery.