Ultrastructure of Merkel corpuscles in the tongue of the finch,Lonchura striata

Summary Merkel corpuscles in the lingual mucosa of the finch, Lonchura striata, were examined by means of the argyrophilic reaction and electron microscopy. These corpuscles are composed of 12 to 20 flattened Merkel cells and enclosed nerve terminals. The present study demonstrated for the first time argyrophilia in avian subepithelial Merkel cells with the use of Grimelius silver stain. Electron-microscopically, the Merkel cell was characterized by the presence of numerous densecore granules, approximately 80 to 140 nm in diameter, as well as specialized contacts with nerve terminals. The granules showed a tendency to accumulate in the cytoplasm in close association with both nerve terminals and basal lamina. This study also provided unequivocal evidence for exocytotic discharge of Merkel-cell granules at the plasma membrane facing not only the nerve terminals but also the basal lamina. The exocytotic figures toward the nerve terminals can be regarded as synaptic discharge of Merkel-cell granules, but the possibility also exists that the Merkel-cell granules may exert a trophic effect on the nerve terminals. The exocytotic release of Merkel-cell granules toward the basal lamina with no relation to nerve terminals may suggest an endocrine (paracrine) function for the Merkel cell. The avian subepithelial Merkel cells qualify as paraneurons, but their exact nature and function remain enigmatic as is the case of intraepithelial Merkel cells in other vertebrates.