Fine structure of the gill epithelium of the terrestrial mudskipper, Periophthalmodon schlosseri

In this paper, we describe the fine structure of the branchial epithelium o f the amphibious, air-breathing mudskipper Periophthalmodon schlosseri, and relate the observed structure to functions in gas exchange, and to the eli mination of sodium chloride and ammonia. Also, we describe the fine structu re of the opercular epithelium. The gill lamellar epithelium is thickened b y the presence of large mitochondria-rich (MR) cells. These MR cells are fu rther characterized by an extensive tubular system that is continuous with the basolateral plasma membrane and by a deep apical crypt often lined with microvilli. There are very few specialized MR accessory cells, which are a ssociated with NaCl excretion in marine teleosts. Instead, MR cells are com monly isolated from each other laterally by flattened cells rich in interme diate filaments. These filament-rich (FR) cells are interconnected by desmo somes and have unusual canaliculi. These branchial FR cells are unique to P . schlosseri and may have a structural role. Electron-dense pavement cells rich in vesicles and large vacuous mitochondria compose the superficial lay er of the epithelium. The unusual morphology of P. schlosseri's gill lamell ae may be related to the animal's ability to effectively eliminate ammonia during air exposure. The inner opercular lining and parts of the leading ed ge of the filament have intraepithelial capillaries, which provide a more s uitable gas exchange surface than the thickened lamellae with its restricte d interlamellar water spaces. The arrangement of respiratory and ion exchan ge epithelia is opposite to that found in all other fish in which the lamel lae typically function in gas exchange and the gill filament in ion regulat ion.