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a filamentous formation in the epithelial cells of animals. While tonofibrils were previously thought to extend from one cell to another, electron-microscope investigations have disproved the notion that such formations are continuous. It has been shown that they converge in the region of the desmosomes, where they bend and reenter the core of the cell. The usual diameter of a tonofibril is approximately 60 angstroms. It is likely that tonofibrils ensure the mechanical stability of cells.