Variations in the arrangement of tonofilaments in the epidermis of teleost fish

Tonofilaments in epithelial cells of teleost skin can be aligned as bundles or skeins of appreciable bilk, or form a pattern of smaller bundles oriented in various directions, or there may be a condition where individual tonofilaments interlace. If sufficiently close together, interwoven tonofilaments can form a basket-like structure, a “capsule”, proximally in the cell. This arrangement, previously known in epithelial cells of Myxinoids, occurs in localised sites in various teleosts of diverse taxonomic position, for instance in clupeids and gadids. A less intimate interlacing of cortical tonofilaments can accompany a modification of the perinuclear cytoplasm previously described, by light microscopy, as “vesiculated”, as in the middle layers of the epidermis in Periophthalmus. In head epidermis of Sprattus, the outer layers of cells contain proximal capsules, but the middle layers consist of flattened cells with a restricted perinuclear cytoplasm, peripheral tonofilaments, and a second population of filaments of a larger calibre. One implication of these results is that the cytoskeleton can undergo profound modification as cells progress from the basal to the superficial layers of the epidermis.