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Type III cells of the taste organs are widely considered to be chemoreceptors. The present study was performed on the frog taste disk and describes an axon-like process in type III cells, which often contains a bundle of densely-packed parallel microfilaments. These processes pass through the basal membrane of the gustatory epithelium, running into the lamina propria (transbasal membrane processes, tBMPs). In their intraepithelial tract, tBMPs contain dense-cored vesicles revealing their origin from type III cells. Type III cells showing both classic nonrigid processes (with vesicles and nerve contacts) and tBMPs are present. The connective tract of a tBMP usually contains dense-cored vesicles only in its proximal portion. In some cases, the connective tract of tBMPs is almost perpendicular to the basal lamina. In other cases, it runs parallel to and below the basal lamina. Some tBMPs contact nerve fibers running in the subepithelial connective tissue; the contact area is rather wide but evident synapse-like junctions were never detected. Contacts between tBMPs and nerve fibers innervating basal cells are also found. In conclusion, the data demonstrate the existence of epithelial cells resembling primitive neurons that display an apical dendrite and axon-like basal processes. Until now, it was not considered possible that epithelial receptor cells extend processes out of the epithelium.

A specific laryngeal sensory epithelium (SLSE), which includes arrays of solitary chemoreceptor cells, is described in the supraglottic region of the rat. Two plates of SLSE were found, one on each side of the larynx. The first plate was located in the ventrolateral wall of the larynx, and the second was located in the interarytenoidal region. In SLSE, immunoblotting showed the presence of alpha-gustducin and phospholipase C beta2 (PLCbeta2), which are two markers of chemoreceptor cells. At immunocytochemistry, laryngeal immunoreactivity for alpha-gustducin was localized mainly in solitary chemosensory cells. Double-label immunocytochemistry using confocal microscopy demonstrated that alpha-gustducin-expressing cells in large part colocalize type III IP3 receptor (IP3R3), another key molecule in bitter taste perception. However, some IP3R3-expressing cells do not colocalize alpha-gustducin. At ultrastructural immunocytochemistry, these cells showed packed apical microvilli, clear cytoplasmic vesicles, and cytoneural junctions. SLSE was characterized by high permeability to a tracer due to poorly developed junctional contacts between superficial cells. Junctions were short in length and showed little contact with the terminal web. Ultrastructural analysis showed deep pits among the superficial cells. In SLSE, high density of intraepithelial nerve fibers was found. The lamina propria of the SLSE appeared thicker than that in other supraglottic regions. It was characterized by the presence of a well-developed subepithelial nerve plexus. The immunocytochemical and ultrastructural data suggested that SLSE is a chemoreceptor located in an optimal position for detecting substances entering the larynx from the pharynx or the trachea., (Copyright 2004 Wiley-Liss, Inc.)