The transient existence of 'en passant' nerve terminals in normal embryonic rat skeletal muscle

In a recent examination of labeled normal embryonic rat nerve terminals we have found evidence of synaptic specialisation at non-terminal regions of axons ('en passant' terminals). The specialisation typically consists of a vesicle-filled axonal swelling lying in close apposition to a zone of thickened sarcolemma, with the two structures separated by a basal lamina-filled cleft. Such 'en passant' terminals are never seen in normal mature rat muscle, where all synaptic sites form at the true termination of axonal branches. These terminals thus appear to be a transient feature of developing neuromuscular relations. Analogous structures may arise in mature muscle when axons undergo ultraterminal sprouting in response to paralysis or partial denervation. We discuss our finding in relation to the role such terminals may play in normal development and in partially denervated mature muscle.