An ultrastructural and cytochemical study of neuromuscular junctions in echinoderms.

The ultrastructure of neuromuscular junctions in various organs of a starfish and a holothurian was studied. All neurons were found to contain large (100-250 nm), dense-core vesicles. Cytochemical tests for acetylcholinesterase were negative for these neurons. The presence of proteinaceous neurosecretory material was contested by enzymatic digestion (pepsin) which did not attack the dense-core vesicles, as well as by incubation in phosphotungstic acid (PTA) which did not stain these structures. Staining with dichromate produced a positive reaction for 5-hydroxytryptamine. In the absence of synaptic modifications even after specific staining with PTA, the transmission of nervous impulses is effectuated through exocytosis of 5-hydroxytryptamine at nerve endings in the muscle tissue.