Keratins cortex

Appendageal structures commonly found within the skin are the hairs, hair follicles, associated sebaceous glands, apocrine and eccrine sweat glands, and arrector pili muscles. Hairs are formed by epidermal invaginations. These keratinized structures traverse the dermis and may extend into the hypodermis. The free part of the hair above the surface of the skin is the hair shaft, and the part deep within the dermis is the hair root, which forms an expanded knob-like structure called the hair bulb. This is composed of a matrix of epithelial cells in different stages of differentiation. Hair is composed of three concentric epithelial cell layers the outermost thin cuticle, a densely packed keratinized cortex, and a central medulla of cuboidal cells. The hair follicle consists of four major components (1) internal root sheath (internal root sheath cuticle, granular layer, pale epithelial layer) (2) external root sheath (several layers similar to the epidermis) (3) dermal papilla (connective tissue) and (4) hair matrix (comparable to the stratum basale of the epidermis). 

The essential genetic material ofthe original vegetative bacterium is retained in the core or protoplast around this lies the thick cortex which contains the murein or peptidoglycan already encountered as a cell wall component (see Fig. 1.2). The outer coats which are protein in composition are distinguished by their high cysteine content. In this respect they resemble keratin, the protein of hair and horn. 

Keratin IFs exhibit different motile behaviors compared with type III and IV IFs, and continue to move in the absence of MT in cells treated with MT inhibitors, such as nocodazole. It has been proposed that actin may be involved in their transport (Yoon et al., 2001). Time lapse imaging of keratin-GFP has revealed that keratin IF formation begins in the cell cortex in a region enriched in actin and required for both intact MT and actin, and that actin governs the organization and movement of keratin in Xenopus egg extracts (Weber and Bement, 2002). Less is known about specific mediators of these interactions, although the fact that myosin Va associates with NFs raises the interesting possibility that other such interactions may exist for keratins. 

Melanin granules are secreted by melanocytes in the hair papilla and distributed to keratin in the hair cortex and inner layers of the hair sheath during normal development. Melanogenesis is subject to hormonal control and has been the focus of intensive genetic studies. Two main forms of melanin exist in human skin—eumelanin and phaeomelanin, both of which are derived from tyrosine through the action of tyrosinase (a cupro-enzyme) and possibly other key enzymes (with nickel, chromium, iron, and manganese as cofactors). Tyrosine is converted to dihydroxyphenylalanine and, via a series of intermediate steps, to indole-5,6-quinone, which polymerizes to eumelanin. Phaeomelanins are produced by a similar mechanism but with the incorporation of sulfur (as cysteine) by a nonenzymatic step in the oxidation process. 

The average hair is composed of about 90% keratin, a fibrous protein, and about 10% water. Each hair consists of two or three layers. The outer layer is a sheath called the cuticle, which is thin and colorless. It is made up of overlapping scales in layers. Inside the cuticle are the long spindle-shaped cells of the cortex, which makes up the bulk of the hair. These cells also contain whatever pigments are present in the hair. The innermost layer, which is only present in large thick hair, is the medulla or pith. 

External hair of animals, generally called wool, was spun into yam and woven into fabrics. Like silk, wool is essentially protein it is composed of various amino acids, a majority of which are keratin. (Unfortunately, the keratin contains sulfur, which attracts certain insects that thrive on wool and contribute to the scarcity of historic woolen fabrics.) The outstanding morphological characteristic of wool fiber is its external scales that overlap in one direction toward the tip of the fiber. The scales can be chemically, mechanically, and temporally damaged and can disappear as the wool deteriorates. Outside of the scales is a membranous layer, the epicuticle inside them is the bulk of the wool fiber, the cortex, which consists of millions of double-pointed, needle-like cells neatly laid... 

Hair is composed of approximately 65 to 95% protein, 1 to 9% lipid, and small quantities of trace elements, polysaccharides, and water. - - The majority of hair protein is often referred to as keratin, which is a general term used to describe aggregates of protein with low or high sulfur content. These proteins are synthesized in the keratogenous zone of the hair follicle as matrix cells move upward from the hair bulb to form layers of the hair shaft. The cuticle, cortex, and medulla are comprised largely of keratin, although it is structurally different in each layer. Keratin in the exocuticle contains a high concentration of cysteine, which forms disulfide bonds which link the A-layer to the exocuticle, and this makes the cuticle... 

Keratin in the cortex comprises 85% or more of the mass of the hair shaft. Cortical keratin is composed of two types of structural proteins, matrix proteins and fibrous proteins. 2-1 Matrix proteins have a high sulfur content and contain polypeptides with a molecular weight of approximately 10 to 28 kDa. Fibrous proteins are embedded in matrix proteins and are characterized by a low sulfur content. They have a molecular weight of approximately 40 to 58 kDa. Also, matrix proteins have a nonhelical structure and are readily soluble at pH 4.5 in 0.5 M KCl, whereas fibrous proteins exhibit a helical structure and are insoluble in this same solution. 

The structure, organization, and ratio of matrix and fibrous proteins contribute to the physiochemical properties of keratinous tissues. For example, a primary difference between hair and nails is the arrangement of fibrous proteins and the concentration of matrix proteins present in each tissue. In cells destined to form the cortex of hair, fibrous proteins are oriented to form filaments which cluster to form fibrils. In the keratogenous zone, fibrils undergo lateral fusion to ultimately produce the cortex. The medulla also contains keratin which has been characterized as a collection of irregular fibrous proteins. Fibrous proteins form a trabecular framework comprising 95% of the medulla, and medullary proteins are less resistant to chemical degradation than proteins in the cortex. The cell membrane complex. 

Fibrous protein structure investigations applying X-ray diffraction and electron microscopy were reviewed by Blakely (31). Keratin fibers are made of three main structural components the cuticle, the cortex, and the medulla. The medulla is only present in coarse fibers. The cortex forms the bulk of the fiber. Various morphological models have been proposed to explain the mechanical properties of keratin fibers. It is generally agreed that the cortex consists of fibrils in which protein molecules exist in helical and nonhelical regions. 

The cortex forms the bulk of fine animal hairs and is derived from highly differentiated spindle-shaped cells that are densely packed with keratinous proteins. The long axes of the cortical cells are oriented parallel to the fiber length, and elongated cavities near the center of the cells are similarly oriented. These cavities are derived from the nuclei of the developing cells and contain debris usually referred to as nuclear remnants. Between cortical cells there is a layer 250-300 A in thickness which is similar to that found between cuticle cells this also is referred to sometimes as intercellular cement. Many nonkeratinous inclusions are found within the cortical cells and these are believed to be cytoplasmic debris. 

True hair is found only in mammals, and there is no such thing as a completely hairless mammal. Hair itself is dead, but is produced in hair follicles by specialized keratinocytes at the base of the hair. The outermost layer of hair is a cuticle, and most hairs have a cortex in which the dead keratinized cells are very densely packed, and an iimer medulla in which they are not as densely packed. The pigmentation in hair, like that of skin, comes from melanocytes. Hair exposure to some chemicals may produce hair discoloration, for example, green hair from copper in water or cosmetics, or blue hair in cobalt workers. 

A plausible explanation for these results is that inside the keratin structure water is molecularly dispersed and forms monomolecular layers around the various protein structural units, i.e., the micro-or protofibrils of the keratin. The low values obtained for Vw can be explained by assuming that when water molecules penetrate the hair structure, they fill, at least in part, pre-existing voids. The results also suggest that the cortex of the hair structure is more porous than the cuticle, since removal of the cuticle descaling of the hair) reduces the value of... 

The cortex is an amorphous and electron-dense layer composed of at least two sublayers which are abundant in keratin-like proteins and a highly insoluble protein termed cuticulin. The medial layer is a fluid-filled compartment that also contains fine collagenous fibers. In some species, the fluid contains hemoglobin. The composite basal layer contains crosslinked collagen fibers in two to three distinct sublayers which spiral around the nematode at an angle 75° to the longitudinal axis. 

A portion of the undermembrane of Figure 1-21 is also epicuticle. The cystine-rich proteins of the cuticle belong to the group of proteins called keratin-associated proteins. Although structurally different, keratin-associated proteins are also found in the matrix of the cortex. See the section that discusses keratin-associated proteins in Chapter 2. For more details of the intercellular structures, see Figure 1-23. Thus, the cuticle of human hair is a laminar structure similar to the cuticle of wool liber, and the different layers of the cuticle have been described for merino wool [64] and for human hair [58, 65, 66, 67], Figures 1-23 and 1-24 illustrate the... 

Evans et al. [25] have confirmed these conclusions of Wickett. In addition, the observation that Japanese hair is easy to perm and that fine Caucasian hair, less than 75 pm in diameter, is more difficult to perm was also confirmed. However, these scientists were unable to identify any common characteristics such as fiber diameter or cystine content that would account for this behavior. The fact that fine hair is more difficult to perm than thick hair may be due to the larger ratio of cuticle to cortex in fine hair and the fact that cortex plays a stronger role in waving than cuticle. This explanation is consistent with the experiments by Wortmann and Kure [2], demonstrating that the cuticle does inhibit the reduction reaction. In addition to pH, other important variables that influence the rate of reduction of keratin fibers by mercaptans are temperature, hair swelling, prior history of the hair, and structure of the mercaptan. 

The hair shaft is composed of three layers an outermost cuticle, a cortex of densely packed keratinized cells, and an innermost medulla of loose cuboidal or flattened cells. The cuticle is formed by a single layer of flat keratinized cells in which the free edges, which overlap like shingles on a roof, are directed toward the distal end of the shaft. The cortex consists of a layer of dense, compact, keratinized cells with their long axes parallel to the hair shaft. The medulla forms the center of the hair and is loosely filled with cuboidal or flattened cells. In the root, the medulla is solid, whereas in the shaft it contains air-filled spaces. The pattern of the surface of the cuticular cells, together with the cellular arrangement of the medulla, is characteristic for each species. 

Morphologically, the fibres are composed of the cortex and the cuticle. Each of the two components is formed of various other morphological components (Table 9.6.3). The cortex contains cortical cells and the cell membrane complex. The cortical cell is further composed of macro-fibrils and intermacro-fibrillar material. The macro-fibrils consist of micro-fibrils and intermicro-fibrillar matrix. In summary, the cortex is formed of micro-fibrils (intermediate filament, IF, or keratin proteins, KP) and keratin associated proteins (IFAP or KAP), which compose the intermicrofibrillar matrix containing cytoplasmatic and nuclear remnants. This ensemble is wrapped up in the cuticle, as an external sheath which also has its own architecture, being formed of four layers the epicuticle, the a-layer, the exocuticle and the endocuticle. 

It is generally thought that blood capillaries break in bulb of hair root and drugs are taken into the hair while matrix cells produced in hair papilla grow and are keratinized. The drugs are contained mainly in hair cortex and amount to 50-100 ng mg single... 

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