The structure of the gills and the epithelial mitochondria-rich (MR) cells of the carp, Cyprinus carpio, and goldfish, Carassius auratus, adapted to various hypotonic media was studied by scanning electron microscopy. The gills consist of arches, rakers, filaments, lamellae, and the septum. The epithelium of the filament could be identified as having an afferent region, interlamellar region, and efferent region. According to the morphology of the apical surfaces, branchial MR cells which are responsible for ionoregulation were found to exhibit different types. With apical membranes larger than 2 µm in maximum dimensions, type A MR cells displayed broad and shallow apical surfaces. Type B MR cells measuring 1 to 2 µm were small and had deep openings. Distribution and densities of MR cells differed in the cyprinid teleosts acclimated to various hypotonic media. In 5‰ salt water, type A MR cells were found to decrease significantly in size and number. In deionized water, proliferation of MR cells was found in lamellae. The results revealed different morphologies of MR cells in stenohaline freshwater teleosts which suggests a possible correlation between morphology and function of MR cells.
Carp, Goldfish, Gill, Mitochondria-rich cell, Hypotonic environments
