An Ultrastructural and Fluorescence Histochemical Study of the Myenteric Plexus of the Stomach of the Rainbow TroutSalmo gairdneri

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Abstract
The myenteric plexus of the rainbow troutSalmo gairdneriis enclosed within an incomplete Schwann‐like sheath which allows bundles of unmyelinated axons to pass into the adjacent smooth muscle layers. Neuronal and non‐neuronal constituents of the myenteric plexus are divided into smaller units by endoneurial collagen which in places condenses to form a perineurial covering. The myenteric plexus is avascular but arterioles and fenestrated capillaries are present close to the plexus in the intermuscular space. Small groups of neurones constitute the ganglia of the plexus but as yet few ultrastructural indications of differing neurone types have been observed. Within the neuropil of the ganglia five types of axon profile, characterised by their vesicle content, have been identified. One of these types was only recognisable following the administration of 5‐hydroxydopamine. Axo‐somatic and axo‐dendritic synaptic contacts were only made by Type 1 axons but these were uncommon. The presence of an adrenergic component of the myenteric plexus was confirmed ultrastructually following 6‐hydroxydopamine‐induced degeneration and also by Falck‐Hillarp fluorescence histochemistry which revealed an extensive distribution of adrenergic nerves in the plexus. The structural organisation of the plexus, the comparatively few ultrastructurally recognisable axon and neurone types and the sparsity of synaptic contacts all indicate that the teleost myenteric plexus is less complex than its mammalian counterpart.

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