Tonofilaments are keratin intermediate filaments that make up tonofibrils in epithelial tissue. In epithelial cells, tonofilaments loop through desmosomes. Electron microscopy has advanced to now 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.[3]