Villiform processes in the pharynx of the soft-shelled turtle, Trionyx sinensis japonicus, functioning as a respiratory and presumably salt uptaking organ in the water

Some species of soft-shelled turtle have been known to use a conspicuous ma ss of villiform processes of the pharyngeal mucosa as an aquatic respirator y organ when staying underwater for prolonged periods, such as hibernation. Using hibernating turtles, Trionyx sinensis japonicus, the present study e mployed scanning electron microscopy to demonstrate for the first time the detailed morphology and distribution of these villiform processes. Two type s of processes, complex and simple, could be identifited. Light microscope observation of the transverse sections of the villi demons trated a rich vascularization in the connective tissue of the villi, compri sing arterioles and venules running in the core and capillaries in the peri phery. Most of the capillaries were invaginated into the multilayered cuboi dal epithelium. Near the tip of the villi they became swollen, forming sinu soidal capillaries. Transmission electron microscopy clarified the fine structure of the blood- water barrier, which consisted of a non-fenestrated endothelium and an atte nuated epithelium that sandwiched a connective tissue with a discontinuous subendothelial and a continuous subepithelial basement lamina. The epitheli um consisted of secretory cells, mitochondria-rich cells, and basal cells. The mitochondria-rich cells contained a cytoplasmic area filled with tubulo vesicular elements. Based on their ultrastructural resemblance with the chl oride cells in the fish and tadpole, these cells are suggested to be involv ed in the uptake of Na+ and Cl- from fresh water for keeping ionic balance in the blood.