Tonofibril

Tonofibrils are cytoplasmic protein structures in epithelial tissues that converge at desmosomes and hemidesmosomes.^[1] They consist of bundles of keratin intermediate filaments (tonofilaments) in epithelial cells that are anchored to the cytoskeleton.^[2][3] They were discovered by Rudolf Heidenhain, and first described in detail by Louis-Antoine Ranvier in 1897.^[4]

Composition

Tonofilaments are keratin intermediate filaments that make up tonofibrils in the epithelial tissue. They may also just be referred to as keratin intermediate filaments.^[5] In epithelial cells, tonofilaments loop through desmosomes. Electron microscopy has advanced now to illustrate the tonofilaments more clearly.^[1]

Transmission electron micrograph depicting tonofilaments, labeled tf, running longitudinally within the dorsal epidermal cells of the girdle of a chiton

The protein filaggrin is believed to be synthesized as a giant precursor protein, profilaggrin (>400 kDA in humans). When filaggrin binds to keratin intermediate filaments, the keratin aggregates into macrofibrils.^[3]