Structure keratin

The homy layer consists of about 10% extracellular components such as lipids, proteins, and mucopolysaccharides. Around 5% of the protein and lipids form the cell wall. The majority of the remainder is present in the highly organized cell contents, predominantly as keratin fibers, which are generally assigned an a-helical structure. They are embedded in a sulphur-rich amorphous matrix, enclosed by lipids that probably he perpendicular to the protein axis. Since the stratum comeum is able to take up considerably more water than the amount that corresponds to its volume, it is assumed that this absorbed fluid volume is mainly located in the region of these keratin structures. 

Under normal conditions, the transcellular route is not considered as the preferred way of dermal invasion, the reason being the very low permeability through the corneocytes and the obligation to partition several times from the more hydrophilic corneocytes into the lipid intercellular layers in the stratum corneum and vice versa. The transcellular pathway can gain in importance when a penetration enhancer is used, for example, urea, which increases the permeability of the corneocytes by altering the keratin structure. 

The beta-keratin structure is also foimd in the feathers and scales of birds and reptiles. 

Gregg, K., Wilton, S. D., Parry, D. A. D., and Rogers, G. E. (1984). A comparison of genomic coding sequences for feather and scale keratins Structural and evolutionary implications. EMBO J. 3, 175-178. 

Aprotic solvents such as dichloromethane and acetone have the advantage of not possessing extractive capabilities [42], being unable to penetrate into the keratin structure. Extraction tests with conventional techniques, for dichloromethane, showed that the analytes were not found in the extracts [51]. 

The keratin structure is destroyed through the use of proteolytic enzymes such as pronase and proteinase K. They are often used with chemical agents such as urea and thioglycolic to cleave the disulfide bonds and increase the dissolution rate of enzyme activity. The extracting procedures, using enzymatic digestion, last about 4-6 h and must be conducted at constant temperature and pH for providing maximum enzyme activity [155],... 

The hair treatment with concentrated solutions of sodium hydroxide allows the complete dissolution of the keratin structure by chemical hydrolysis of proteins in about 1 h. Under these conditions, some drugs such as amphetamines are volatile and thus there may be losses of the analyte. In more basic solutions occurs the complete hydrolysis of molecules such as cocaine, heroin, and 6-MAM [51]. Concentrated solutions of hydrochloric acid are also used it eliminates the problem of volatile basic compounds, but increases the time of dissolution. 

Norlen, L. and Al-Amoudi, A., Stratum corneum keratin structure, function, and formation the cubic rod-packing and membrane templating model, J. Invest. Dermatol., 123, 715, 2004. 

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). 

Keratin Structure. Keratin is a nonspecific term applied to various insoluble aggregations in hair, nail, skin, and mucosa. Rothberg (33) says of skin,. . what has been called and analyzed as keratin in the past is the total products of epidermal metabolism which are not returned to the metabolic pools but are instead excreted with the cornified epidermal cells. Much has been learned by selective extraction and analysis of these complex products, but the question often arises— what was analyzed Biologists have tried to determine keratin composition by observing synthesis, assembly, and diflFerentiation of the composite parts. 

In spite of the fact that stratum corneum cells are metabolically inert, changes in keratin structure and organization occur as each cell transits through the stratum corneum prior to desquamation (28). This suggests some asymmetry in physical and chemical properties through the thickness of the corneum. One demonstration of this is the swelling of fresh frozen transverse sections of corneum in dilute acid or base. The most mature surface cells swell considerably more slowly and to a lesser extent than the lower layers of the corneum (18). Such asymmetry is of particular importance in studying the diflFusion and mechanical properties of this membrane. 

Electron microscopic studies further substantiate the presence of highly-ordered macromolecular structures with the corneum cells (24, 31). The fibrous keratin structure has been described as low-density filaments, low in sulfur but embedded in a dense sulfur-rich amorphous interfilamentous matrix. The keratin fibrils organize into lipid-covered bundles preferentially oriented in the longitudinal plane of the acutely flattened cell (16). These filaments terminate at the periphery of the cell... 

Keratin Structure and Orientation. Acute flattening of the flbrous protein-fllled cells in the flnal stages of keratinization establishes a biaxial orientation. As would be expected, no birefrigence is observed normal to the plane of the comeum surface, but significant birefrigence is observed parallel to the plane of the corneum surface (I, 42). The x-ray diflFraction pattern of this isolated epidermal protein, when highly drawn, exhibits the classical alpha pattern (7, 43). 

The wide angle x-ray diffraction pattern of undeformed corneum exhibits diffuse halos at 4.6 A and 9.8 A common to proteins (Figure 4). The lack of the 5.1-A reflection characteristic of alpha-keratin structures in undeformed comeum suggests that the protein is considerably less oriented and perhaps of a lower alpha content than wool. This is supported by the fact that the 5.1-A reflection begins to appear in samples of comeum which were hydrated and stretched to 100% or more (Figure 6) and allowed to dry in the extended state. The increased orientation of the lipid reflections in the stretched sample demonstrates further their association with the orienting protein fibrils. 

TABLE lO-III Principal Developments in a-Keratin Structural Models... 

The jS form of keratin requires still additional models. And, continuing the order of decreasing certainty, these models are again less well corroborated by experimental observations than are the a-helix or a-keratin structures. Pauling and Corey (1598) presented the pleated sheets to explain /3-keratins. These sheets are made up of extended peptide chains H bonded essentially side by side. Two are shown in Fig. 10-7. 

J. B. Speakman. Nature 159, 338 (1947). Stress elongation keratin structure in wool. 

Why balance pH in skin products The outer layer of skin has a keratin structure just as hair does. Products aimed at making the skin look brighter and clearer have a higher pH. Their purpose is to... 

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... 

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