Fine structural studies on the myenteric plexus of the gastro-intestinal tract of the rainbow trout (Salmo gairdneri, Rich.).

The neuronal elements is most part of the myenteric plexus of the gastro-intestinal tract of the Rainbow trout consisted of nerve cell bodies and unmyelinated nerves successively invested by satellite and Schwann cells respectively, thin basal lamina, a layer of collagenous fibrils and interstitial cells, the processes of which were often held together by desmosomes. These were embedded in a loose vascular connective tissue. Encapsulated vagal nerve trunks formed a part of the myenteric plexus in the region of the stomach and they contained similar elements in the same order and, in addition, myelinated nerve fibres. Four subtypes of small neurons and two of large neurons were distinguished based on the presence and varying pattern of organization of cytoplasmic organelles. Five types of neuronal processes and several plasma membrane specializations were described and these were involved in the formation of axo-dendritic and axo-somatic synapses. Three main types of axon varicosities were also distinguished according to their vesicular contents. The significance of these findings is discussed and attempt is made to correlate them with existing experimental and histochemical information on teleost gut.