Light and electron microscopic identification of a nerve sprout in muscle of normal adult frog.

Recent evidence from this laboratory indicates that axonal sprouting (and regression) occurs in neuromuscular junctions of normal adult frogs. In the present investigation, the appearance of a single nerve branch, which from light microscopy was assumed to be a sprout, was studied in ultrathin serial sections. In confirming the light microscopic evidence small synaptic contacts were found, which showed characteristics of new synapse formation. Unexpectedly, the Schwann cell surrounding the axon extended several microns distally from the axon tip. It appears that nerve sprouting (and regression) is a physiological event in adult frog muscles.