Brill et al. reveal that Schwann cells (SCs) keep nerve terminals fully covered by competing for space at neuromuscular junctions.
SCs wrap around nerve axons to help propagate action potentials, but they also cluster around axon terminals to maintain synapses and support neurotransmission. Terminal SCs are packed so tightly at nerve endings that it's hard to distinguish individual cells, so Brill et al. established techniques to label single SCs in vivo in order to investigate their organization and dynamics at young and adult synapses.
During development, SCs at immature synapses were highly dynamic, extending and retracting processes that intermingled with protrusions from neighboring cells. Later, however, mature SCs became much more static, occupying their own territories and showing no overlap with their neighbors.
This segregated pattern appears to be maintained by continuous spatial competition between the cells. When Brill et al. laser ablated a single SC at an adult neuromuscular junction, its neighbors rapidly swooped in to fill the vacated space, suggesting that contacts between SCs usually stop them from wandering into each other's territory. Contacts with the axon itself also help to position adult SCs, as they intermingled dynamically following axon fragmentation but resumed their segregated arrangement after the nerve re-grew.
Brill et al. also noticed that invading SCs rapidly gobbled up the debris left behind by ablated cells or fragmented axons. Author Thomas Misgeld and his team now want to investigate whether this phagocytic capacity allows SCs to assist in synaptic remodeling.