Tonofibrils are cytoplasmic protein structures in

epithelial Epithelium or epithelial tissue is a thin, continuous, protective layer of cells with little extracellular matrix. An example is the epidermis, the outermost layer of the skin. Epithelial ( mesothelial) tissues line the outer surfaces of man ...

tissues that converge at

desmosomes A desmosome (; "binding body"), also known as a macula adherens (plural: maculae adherentes) (Latin for ''adhering spot''), is a cell structure specialized for cell-to-cell adhesion. A type of junctional complex, they are localized spot-like ad ...

and

hemidesmosomes Hemidesmosomes are very small stud-like structures found in keratinocytes of the epidermis of skin that attach to the extracellular matrix. They are similar in form to desmosomes when visualized by electron microscopy; however, desmosomes attach ...

. They consist of fine

fibril Fibrils () are structural biological materials found in nearly all living organisms. Not to be confused with fibers or protein filament, filaments, fibrils tend to have diameters ranging from 10 to 100 nanometers (whereas fibers are micro to ...

s in epithelial cells that are anchored to the

cytoskeleton The cytoskeleton is a complex, dynamic network of interlinking protein filaments present in the cytoplasm of all cells, including those of bacteria and archaea. In eukaryotes, it extends from the cell nucleus to the cell membrane and is compos ...

. They were discovered by

Rudolf Heidenhain Rudolf Peter Heinrich Heidenhain (; 29 January 1834 – 13 October 1897) was a German physiologist born in Marienwerder, Province of Prussia (now Kwidzyn, Poland). His son, Martin Heidenhain, was a highly regarded anatomist. Academic career ...

, and first described in detail by

Louis-Antoine Ranvier Louis-Antoine Ranvier (2 October 1835 – 22 March 1922) was a French physician, pathologist, anatomist and histologist, who discovered the nodes of Ranvier, regularly spaced discontinuities of the myelin sheath, occurring at varying interval ...

in 1897.

Composition

Tonofilaments are

keratin Keratin () is one of a family of structural fibrous proteins also known as ''scleroproteins''. It is the key structural material making up Scale (anatomy), scales, hair, Nail (anatomy), nails, feathers, horn (anatomy), horns, claws, Hoof, hoove ...

intermediate filaments Intermediate filaments (IFs) are cytoskeletal structural components found in the cells of vertebrates, and many invertebrates. Homologues of the IF protein have been noted in an invertebrate, the cephalochordate ''Branchiostoma''. Intermedi ...

that make up tonofibrils in the epithelial tissue. In epithelial cells, tonofilaments loop through desmosomes. Electron microscopy has advanced now to illustrate the tonofilaments more clearly. Chiton epidermis TEM

The protein

filaggrin Filaggrin (filament aggregating protein) is a filament-associated protein that binds to keratin fibers in epithelial cells. Ten to twelve filaggrin units are post-translationally hydrolyzed from a large profilaggrin precursor protein during termin ...

is believed to be synthesized as a giant

precursor protein A protein precursor, also called a pro-protein or pro-peptide, is an inactive protein (or peptide) that can be turned into an active form by post-translational modification, such as breaking off a piece of the molecule or adding on another molecule ...

, profilaggrin (>400 kDA in humans). When filaggrin binds to keratin intermediate filaments, the keratin aggregates into macrofibrils.

References

External links

Diagram at ultrakohl.com
Keratins Cytoskeleton {{cell-biology-stub