Wiktionary
tonofibril
n. (context biology English) A structural fibril of tonofilaments found in the cytoplasm of epithelial cells
Wikipedia
Tonofibril
Tonofibrils, discovered by Rudolf Heidenhain, and first described in detail by Louis-Antoine Ranvier, are cytoplasmic protein structures in epithelial tissues that converge at desmosomes and hemidesmosomes. Characterized by the fine fibrils in the epithelial cells.
Tonofilaments are also associated with desmosomes (macula adherens), anchoring them to the cytoskeleton.