ULTRASTRUCTURAL DISTRIBUTION OF CALCIUM IN CUTANEOUS ELECTRORECEPTOR ORGANS OF TELEOST FISH

The calcium distribution in the ampullary electroreceptor and the type B electroreceptor organ (gymnarchomast) of Gymnarchus niloticus (Gymn archidae) and in the tuberous organ of Apteronotus leptorhynchus (gymn otidae) was studied. Endogenous calcium appeared as electron-dense pre cipitates when the cutaneous organs were pre-fixed with phosphate-buff ered glutaraldehyde and postfixed with osmium tetroxide plus potassium bichromate. Calcium precipitates were localized in both intracellular compartments of sensory cells and afferent nerve fibers. In contrast to sensory cells, small amounts of calcium precipitates were found in the cytoplasm of accessory cells. In sensory cells, electron-dense dep osits were apparent mainly in synaptic vesicles near synaptic ribbons, inside vacuoles of the endoplasmic reticulum, and between the layers of the nuclear membrane. Very few deposits were found in mitochondria. Precipitates were also observed within the axons of afferent nerves a nd between the layers of the myelin sheath. The synaptic cleft was dev oid of calcium. Calcium deposits have a specific cellular distribution in electroreceptor organs of teleost fish.