tonofibril /ton′əfī″bril/ [Gk, tonos, stretching, fibrilla, small fiber] , a bundle of fine filaments found in the cytoplasm of epithelial cells. The individual strands, or tonofilaments, spread throughout the cytoplasm and extend into the intercellular bridge to converge at the desmosome. The system of fibers functions as a supportive element within the cytoskeleton. In keratinizing epithelium the strands are the main precursor of keratin. Also called epitheliofibril, tenofibril. See also keratohyalin, tonofilament.
