Ultrastructural observations on the submucous plexus of the large intestine of the rainbow trout (Salmo gairdneri, Rich.).

Ultrastructural studies on the large intestine of the Rainbow trout revealed the hitherto unreported presence of neurons within the submucous plexus. The neurons were closely associated with axons which expanded intermittently along their length to form varicosities. According to the type and proportion of vesicles within the varicosities, three groups were distinguished. Group I varicosities contained a predominant population of small electronlucent vesicles measuring 30 nm in diameter. Group II varicosities contained a few medium-sized granular vesicles measuring 60 nm in diameter mixed with large numbers of small electronlucent vesicles. Most of the Group III varicosities contained large numbers of large granular vesicles, 90 nm in diameter. Varicosities of all groups formed axosomatic synapses. The significance of the presence of neurons in the submucous plexus of the trout large intestine is discussed. Group I and II varicosity profiles were interpreted as representing cholinergic and adrenergic innervation respectively. The identification of the transmitter substance in Group III varicosities, however, has still to be resolved.