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[3].
Tonofilaments are also associated with desmosomes (macula adherens), anchoring them to the cytoskeleton.
Composition
Tonofilaments are keratin intermediate filaments that makes up tonofibrils in the epithelial tissue. In epithelial cells, tonofilaments loop through desmosomes. Electron microscopy has advanced now to illustrate the Tonofilaments more clearly[2].
The protein filaggrin is believed to have an important role in holding them together as tonofibrils. This protein is known to interact with intermediate filaments, specifically keratins. It is synthesized as a giant Precursor protein, profilaggrin (>400 kDA in humans). When the filaggrin binds to keratin intermediate filaments, it causes aggregation in macrofibrils.