Neuroepithelial endocrine cells in the lung of Ambystoma mexicanum

Neuroepithelial endocrine (NEE) cells were for the first time identified in the lung of the entirely aquatic urodele, Ambystoma mexicanum, by using light and electron microscopy, histochemistry, and immunocytochemistry. In the basal part of the ciliated epithelium and, less often, in the respiratory portion of the lung, NEE cells were found to occur both solitarily and in small clusters. No typical neuroepithelial bodies could be found. Using the method of Fernandez Pascual, some NEE cells were found to be argyrophilic. Microspectrofluorimetric analysis of formaldehyde-induced fluorescence and immunocytochemistry revealed the presence of 5-hydroxytryptamine. With antibodies to neuron-specific enolase only a few NEE cells exhibited a faint immunostaining. Electron-microscopically, the NEE cells are provided with distinctive cytoplasmic membrane-bound dense granules of variable size, which gave a positive argentaffin reaction. The images of emiocytotic granule release are indicative of a secretory function. In the tracheal epithelium. NEE cells seem to occur only solitarily. They bear the same ultrastructural characteristics as the intrapulmonary NEE cells but here, the dense granules are larger and associated with numerous bundles of microfilaments. Intraepithelial nerve endings were observed near the airway lumen. Between nerve terminals and NEE cells, synaptic complexes with aggregations of clear-centered vesicles close to the presynaptic membrane thickenings were observed. In addition, some nerve endings from "reciprocal synapses" with NEE cells. A receptosecretory function for NEE cells in the lung of A. mexicanum is supposed.