Formaldehyde-induced fluorescence reveals numerous serotonin-containing cells within the primary epithelium of the fish gill. These cells are isolated or clustered and are supported by the epithelial basal lamina. They never reach the external medium and are found on the internal side of the primary lamellae, facing the respiratory water flow. With the electron microscope, these cells are found to contain dense-cored vesicles (DCV) of 80-100 nm. Nerve profiles are consistently found close to DCV cells. After having crossed the basal lamina, nerve fibers form endings on DCV-containing cells. These endings display both small clear vesicles and DCV and are in direct contact with DCV cells. Specific membrane alterations are suggestive of efferent synapses. These cells are considered neuroepithelial cells, similar to those found within the wall of lung airways in mammals and submammalian vertebrates. Structure and localization are suggestive of a tissue O2 sensor.

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The neuroepithelial cells (NECs) of the fish gill filament share several morphofunctional features with the cells of the neuroepithelial bodies in the lungs of air-breathing vertebrates. In the present study, a detailed indolamine-immunocytochemical analysis of the branchial neuroepithelial cells and nerves was undertaken in non-teleost and teleost species, with particular emphasis on the latter. In the rainbow trout, Oncorhynchus mykiss, the chemical degeneration of either catecholaminergic (by 5- and 6-hydroxydopamines) or indolaminergic (by 5,6-dihydroxy-tryptamine) innervations associated with the NECs was studied using electron microscopy. In teleosts, the NECs are located primarily on the distal half of the filament. In the trout particularly, these cells are innervated mainly by non-indolaminergic nerves taking up sympathetic neurotoxins. The proximal half of the filament contains isolated NECs innervated additionally by intrinsic indolaminergic neurons. Serotonin-like immunoreactivity of the NECs is evident in the granular vesicles packed within the basal soma and processes which surround non-vascular and vascular smooth muscles in the filament. Apical processes from the neuroepithelial cells occasionally contact the water on the surface of the filament epithelium. The secretory function of the NECs is discussed with reference to the probable involvement of serotonin in the modulation of fish gill function. In addition, their connections with both central and branchial nervous systems suggest a possible chemoreceptor role.

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Starved rats appeared to be in a phase of energy sparing in phase II, with depressed cellular events in the intestinal mucosa. In phase III, however, the preservation of functional cells and the early increase in crypt cell proliferation should prepare the mucosa to refeeding and could explain why jejunal repairs are complete after 3 days of refeeding following either phase II or phase III.