Schwann cell guidance of nerve growth between synaptic sites explains changes in the pattern of muscle innervation and remodeling of synaptic sites following peripheral nerve injuries.

Terminal Schwann cells (SCs) are nonmyelinating glia that are a prominent component of the neuromuscular junction (NMJ) where motor neurons form synapses onto muscle fibers. These cells play important roles not only in development and maintenance of the neuromuscular synapse but also restoring synaptic function after nerve damage. In response to muscle denervation, terminal SCs undergo dramatic changes in their gene expression patterns as well as in their morphology, such as extending elaborate processes into inter‐junctional space. These SC processes serve as a path to guide axon terminal sprouts from nearby innervated junctions, promoting rapid reinnervation of denervated fibers. We studied the role of terminal SCs in synapse reformation by using two different fluorescent proteins to simultaneously label motor axons and SCs; we examined these junctions repeatedly in living animals using a fluorescence microscope. Here, we show that alterations in the patterns of muscle innervation following recovery from nerve injury can be explained by SC guidance of regenerating axons. In turn, this guidance leads to remodeling of the NMJ itself. By guiding axon growth in inter‐synaptic, retrograde, then, anterograde way, Schwann cells play an important role for rapid reinnervation of denervated synaptic sites to maintain muscle functionality and prevent paralysis following nerve injury or death of motor neurons. This causes changes in muscle innervation pattern and remodeling of synaptic sites. [ABSTRACT FROM AUTHOR]