ABSTRACT
In the lungs of 13 species of Amphibia investigated so far, solitary neuroendocrine (NE) cells, as well as a group of these cells called “neuroepithelial bodies” (NEB), have been observed. They are located mainly in the ciliated epithelium covering the apical part of the 1st and Und order septa deeply protruding into the air space of the lung and may monitor gas composition. In two anuran species, Hymenochirus and Xenopus, the ciliated epithelium is absent and the NE cells do not occur, as in apodan amphibians. Substantial diversity in the structure of the NEBs has been observed. In anurans the NEBs are composed of 20–100 small NE cells, while in tailed amphibians they comprise 3–6 large cells. The NEBs are mostly of the “closed type” and the NE cells are covered by a thin layer of surrounding cells that are ciliated, goblet or pneumocytes. Only in two species, Bufo marinus and Ambystoma tigrinum, the NEBs are of “open type” and communicate with the air space via single cell equipped with microvilli and one atypical cilium with an 8+1 microtubule arrangement. NE cells possess characteristic dense-cored vesicles (DCVs) of various diameter, in which the serotonin and several neuropeptides have been demonstrated by immunohistochemical methods. The basal part of NEBs and some NE cells is innervated by sensory nerve terminals morphologically of afferent and efferent type. In ontogeny the NE cells in anuran amphibians appears after metamorphosis. In respiratory organs such as lungs and air bladders of the air-breathing fishes, solitary NE cells with or without innervation are found. As in other vertebrates, in the lungs of amphibians and air-breathing fishes the NE cells form an epithelial endocrine system that acts as endocrine or paracrine receptors.
