A correlative light-, fluorescence- and electron-microscopic study of neuroepithelial bodies in the lung of the red-eared turtle, Pseudemys scripta elegans.

Neuroepithelial bodies (NEB) were identified for the first time in the respiratory tract of a reptile by the use of combined morphological and histochemical methods. In the red-eared turtle, Pseudemys scripta elegans, NEB were found within the trabecular epithelium of the respiratory tract, mainly in the branching regions of the trabeculae. An intracellular formaldehyde-induced fluorescent compound was identified as 5-hydroxytryptamine (5-HT) by means of microspectrofluorometry. Subsequent histochemical staining of the same fluorescent sections showed the 5-HT-containing cells to be argentaffin. In electron micrographs cell clusters characterized by the presence of distinctive cytoplasmic, membrane-bounded dense granules (+/- 100 nm) were observed, correlating with the distribution of the yellow-fluorescent epithelial cells. The granules of the NEB are positive when the argentaffin technique is performed directly on ultrathin sections. Cells of the NEB extend into the lumen of the airway via apical microvilli and a single modified cilium displaying a 9 X 2 + 0 or 8 X 2 + 2 microtubular pattern. Unmyelinated axons containing mostly small, clear vesicles were seen in close association with NEB cells, often forming synaptic junctions. Occasionally, axons containing a few small dense-cored vesicles were found. The relationship between NEB cells and capillaries, the images of emiocytotic granule release and the occurrence of synaptic contacts between axons and granule-containing cells are indicative of endocrine secretion. These features and the presence of intracytoplasmic granules containing 5-HT may justify the inclusion of NEB-cells of the turtle lung into the diffuse neuroendocrine system. Furthermore, structurally these cells appear to represent sensory elements capable of an intrapulmonary receptor-secretory function.