1. A thin fibril made up of tonofilaments.
2. One of a system of fibers found in the cytoplasm of epithelial cells.
3. A bundle of fine filaments (tonofilaments) in certain cells, especially epithelial cells, the individual strands of which transverse the cytoplasm in all directions and extend into the cell processes to converge and insert on the desmosomes; they are thought to have a supportive or cytoskeletal function and, in keratinizing epithelia, to be the principal precursor of cytokeratin.