The digital pads of rhacophorid tree-frogs.

The digital pads of rhacophorid tree-frogs were studied by light and electron microscopy using a tannic acid-containing fixative. The digital pads are concave mucous epithelial structures surrounded by a soft raised epithelial border. The cells in the first epithelial layer are separated by deep intercellular fissures and the epithelial cell surface is densely covered with thousands of setaceous keratinized microvilli. These are estimated to be 0.1-0.5 microm in width and have flattened tips. A longitudinal section view of the pad's first epithelial cell layer shows a rugose pattern. Deep intercellular fissures in between the cells are formed by the enzymatic activity of invading mononuclear leukocytes in the interepithelial cell junctions. The 'rugose' surface epithelial cell layer is peeled off from the underlying second epithelial layer by the epithelial metabolism that occurs when the leukocytes invade the second interepithelial cell spaces. Thus, the second epithelial cell layer becomes the new 'rugose' epithelial cell layer. The ultrastructures of the frog digital pads are compared with those of other biological suction cups, such as those of octopuses and geckos. Further discussed are their interatomic or intermolecular mechanofunctional aspects, such as hanging upside down and moving easily over smooth surfaces with the aid of interatomic or intermolecular forces, the so-called 'van der Waals forces', without any energy expenditure.