Keratin is a family of structural fibrous proteins known as scleroproteins. It is a key structural material in scales, hair, nails, feathers, horns, claws, hooves, and the outer layer of skin in tetrapod vertebrates. Keratin protects epithelial cells from damage or stress and is extremely insoluble in water and organic solvents.
Keratins are polymers of type I and type II intermediate filaments found only in chordates (vertebrates, amphioxi, urochordates). There are two main types of keratin:
The human genome encodes 54 functional keratin genes, located in two clusters on chromosomes 12 and 17. These genes produce a variety of keratin proteins with distinct properties and functions.
Keratins are characterized by a unique amino acid composition and structure. Key features include:
Hair and other α-keratins consist of α-helically coiled single protein strands (with regular intra-chain H-bonding), further twisted into superhelical ropes that may be further coiled.
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Keratin filaments are intermediate filaments found in stratified squamous epithelial tissue. During epithelial differentiation, cells become cornified as keratin protein incorporates into longer keratin intermediate filaments. Eventually, the nucleus and cytoplasmic organelles disappear, metabolism ceases, and cells undergo programmed death as they become fully keratinized. Cells in the epidermis contain a structural matrix of keratin, which makes the outermost layer of the skin almost waterproof and, along with collagen and elastin, gives skin its strength. Rubbing and pressure cause thickening of the outer, cornified layer of the epidermis, forming protective calluses.
Keratins can be divided into 'hard' and 'soft' forms, or 'cytokeratins' and 'other keratins'.
Keratin's unique structure and chemical composition give it exceptional properties:
Keratin expression is helpful in determining epithelial origin in anaplastic cancers. Tumors that express keratin include carcinomas, thymomas, sarcomas, and trophoblastic neoplasms. The precise expression-pattern of keratin subtypes allows prediction of the origin of the primary tumor when assessing metastases.
Historically, the term ‘keratin’ stood for all proteins extracted from skin modifications, such as horns, claws, and hooves. It was later realized that this keratin is a mixture of keratins, keratin filament-associated proteins, and other proteins, such as enzymes. Keratins were then defined as certain filament-forming proteins with specific physicochemical properties and extracted from the cornified layer of the epidermis, whereas those filament-forming proteins extracted from the living layers of the epidermis were grouped as ‘prekeratins’ or ‘cytokeratins’.
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Currently, the term ‘keratin’ covers all intermediate filament-forming proteins with specific physicochemical properties and produced in any vertebrate epithelia. Similarly, the nomenclature of epithelia as cornified, keratinized, or non-keratinized is based historically on the notion that only the epidermis of skin modifications such as horns, claws, and hooves is cornified, that the non-modified epidermis is a keratinized stratified epithelium, and that all other stratified and non-stratified epithelia are non-keratinized epithelia.
Vertebrate tissues are traditionally divided into two major categories: (1) epithelial tissues of ectodermal or endodermal origin with little intercellular substances and (2) mesenchymal (connective) tissues of mesodermal origin with a substantial amount of extracellular substances. Epithelia line surfaces, form glands, and act as receptor cells in sensory organs. Epithelial tissues line internal and external surfaces, such as the external surface of the skin or the internal lining of the intestine.
Epithelial tissues are derivatives of all three germ layers: ectoderm, mesoderm, and endoderm. All stratified epithelia start as simple epithelia and stratify as well as differentiate during embryonic, fetal, and postnatal development. The structures of epithelia reflect their various functions, such as semipermeable or protective barriers.
Epithelia are distinguished as being simple, transitional, or stratified. In stratified epithelia, only the basal cells are attached to the basement membrane, and only the most superficial of the suprabasal cell layers form the surface of these epithelia. In the intermediate stratum of a stratified epithelium, the cells undergo various processes of differentiation, such as keratinization.
There are three types of filaments, each with specific properties, which interact with one another in the formation of the cytoskeleton of epithelial cells:
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Corneous, or horny, tissues have a long history of interest due to their economic, practical, and emotional value. Keratin-rich tissues are studied for their economic importance in the wool industry, for cosmetics, and for dermatology. The health of the hooves of farm and draft animals is of crucial economic importance to large animal producers and forms the basis of a longstanding interest in veterinary medicine concerning the structure and function of keratinized and cornified tissues.
Corneous tissues cover the surface of animals, representing the interface between an organism and its environment. Despite the great variety in appearance, structures as diverse as hairs, feathers, hooves, and baleen consist of a similar substance called ‘horn’ or ‘keratin’.
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