Ultrastructural study of the dorsal lingual epithelium of the Asian snail-eating turtle, Malayemys subtrijuga.

The Asian snail-eating turtle, Malayemys subtrijuga, is classified phylogenetically as a member of the family Emydinae. Members of this family usually live in small rivers or ponds. However, this species is relatively well-adapted to terrestrial life. We describe here the light, scanning electron and transmission electron microscopic appearance of the dorsal lingual epithelium of the snail-eating turtle and we compare the results to those obtained from other freshwater turtles in an attempt to clarify the relationship between the histological and ultrastructural differences in the lingual epithelium and the living circumstances of the turtles. The tongue is triangular with a rounded apex when viewed dorsally but it appears flattened when viewed laterally. Under the scanning electron microscope, no lingual papillae were visible on the dorsal surface of the tongue. Instead, plicae were seen all over the dorsal surface. On the surface of the epithelium of the outermost side, dome-shaped bulges, each of which was coincident with an individual cell, were compactly distributed. At higher magnification, scanning electron microscopy revealed numerous microvilli and microridges on the surface of these cells, and the thickening of cell-margins was clearly seen. Light microscopy revealed that the mucosal epithelium of the tongue was of the non-keratinized, stratified squamous type. Under the transmission electron microscope, the cells of the basal and deep intermediate layers of the epithelium appeared irregularly elliptical in shape. The nucleus was large and also irregularly elliptical, lying in the central region of each epithelial cell. The cytoplasm of these cells contained mitochondria, free ribosomes, rough endoplasmic reticulum and bundles of tonofibrils. Cell membranes formed processes around individual cells. Desmosomes were intercalated between the processes of adjacent cells. In the shallow intermediate layer, the cells were also elliptical, and the elliptical nucleus was located in the central area of each cell. A large part of the cytoplasm was occupied by electron-dense, discoid granules. Filamentous structures filled the spaces between these granules. Small numbers of free ribosomes, mitochondria and rough endoplasmic reticulum were scattered in the cytoplasm. Cell membranes still formed processes around cells. Desmosomes were intercalated between the processes of adjacent cells. The cells of the surface layer were still elliptical, as were their nuclei. Most of the cytoplasm was filled with electron-dense, discoid granules. Fine filamentous structures were dispersed between these granules. Cell membranes formed processes around cells which were coincident with microvilli and microridges. Intercalated desmosomes were also seen. In some cells, many of the electron-dense, discoid granules were secreted into the oral cavity. In conclusion, the histology of the lingual epithelium of the snail-eating turtle is very similar to that of the freshwater turtle, reflecting similarities in the gross morphology of the tongues of these species, in spite of the differences in their life styles.