Abstract
Mature motoneurons respond to partial denervation of their target muscle by sprouting to reinnervate denervated fibers, thus maintaining muscle strength in the face of motoneuronal loss caused by injury or disease. Neonatal motoneurons, however, do not expand to innervate more muscle fibers. The present work seeks to understand this developmental change in motoneuron response to partial denervation. It has been suggested that neonatal motor units cannot increase in size because they are already at their maximum size (approximately five times larger than in adulthood). We ruled out this explanation by showing that after partial denervation on postnatal day 14 (P14), when motor units have decreased to their adult size, motoneurons still did not sprout to reinnervate as many fibers as in adulthood. Instead, we found evidence supporting an alternative explanation involving terminal Schwann cells. After partial denervation of neonatal (but not adult) muscles, terminal Schwann cells at denervated endplates undergo apoptosis. We found that terminal (but not nodal) sprouting was absent in partially denervated neonatal muscles. This finding suggests that terminal Schwann cells, previously reported to guide terminal sprouts to denervated endplates in adult muscles, are necessary for the formation and growth of terminal sprouts. Moreover, partial denervation on P14 severely weakened the remaining, uninjured synapses, suggesting that neonatal motoneurons may withdraw terminals after the denervation of nearby fibers. These findings have implications for the interpretation of previous studies on synapse elimination and offer insight into the failure of young motor units to expand after partial denervation.