Light and electron microscopic identification of nerve terminal sprouting and retraction in normal adult frog muscle.

Abstract

A combined light microscopic and EM study was performed on neuromuscular junctions of normal adult frogs. In a previous investigation, signs of new synapse formation, as well as abandoned former synaptic sites, were observed in normal muscles. A detailed light microscopic and EM correlation to investigate those parts of junctions which, after staining for cholinesterase (ChE) and presynaptic axon terminals, were suspected either to be newly formed or sites abandoned by the presynaptic nerve and the Schwann cell. Thin presynaptic nerve branches, enclosed by Schwann cell sheaths along most of their length, formed synaptic contacts with the muscle fiber only at small circumscribed areas. In these regions post-synaptic secondary folds (invariably present at mature synapses) were either missing or were less well developed. At these small contacts, binding sites for fluorescein-labeled .alpha.-bungarotoxin were usually present. At other sites the ChE reaction product was present but an axon could not be detected in Ag-stained preparations. EM observation revealed post-synaptic secondary folds filled with ChE reaction product while the presynaptic axon and Schwann cell were missing. The sites with ChE remnants can thus be regarded as abandoned former synaptic contacts. No binding of fluorescein-labeled .alpha.-bungarotoxin could be detected at such sites. These findings confirm earlier suggestions that synaptic contacts in frog muscle are normally undergoing continual remodeling. The lack of binding sites for fluorescein-labeled .alpha.-bungarotoxin at abandoned synaptic sites suggests that a neural or Schwann cell factor is important for the maintainance of synaptic acetylcholine receptors.