Keratins medulla

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. 

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). 

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... 

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. 

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. 

Keratin Subclasses As in many other organs, keratin subsets have been analyzed in the normal thymus and in tumors deriving from it. ° In particular, keratins 7, 13, and 18 appear to be expressed in this spectrum of tissues. Keratin 13 is restricted to epithelial cells of the thymic medulla but is shared by thymomas of all types. Keratins 7 and 18 likewise appear to be preferentially seen in neoplastic proliferations of this gland. 

Wools and other similar mammalian hairs are largely composed of keratin proteins. However, unlike the other natural proteinaceous fibre, silk, wool is cellular in nature the fibres consist of relatively hard, flattened, overlapping cuticle cells, which surround the central cortical cells in some fibres, these may in turn surround a hollow medulla (Figure 23). 

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. 

Medullary cells are resistant to attack by normal reagents used for solubilizing keratins [163]. Methods used for the isolation of medulla are based on its inherently inert nature. Thus, medullary isolates result as a residue after preferential dissolution of cuticle and cortical cells [160]. 

The cross-section and longitudinal views of wool are shown in Figure 3. Figure 4 is a schematic diagram of the structure of the fiber. Enclosed within an outer scaly layer of keratin (the cuticle) are three sets of cortical cells para, meso, and ortho. The cells may surround a hollow core, known as the medulla. 

Human hair is mainly composed of fibrous a-keratin proteins. Hair fibres are not continuous in their full length, but rather result from compact groups of cells within the fibre follicle, from which three farther basic morphological components of hair structure originate the multicellular cuticle sheath, the fibrous cortex and the medulla [1, 2],... 

Vacuolated cells may also be present along the axis of coarser a-keratin fibres, forming the medulla. These cells generally constitute only a small percentage of the mass of hair and are believed to contribute negligibly to the mechanical properties of human hair fibres. Physically, the medulla forms the empty space of the fibre [4, 7],... 

All of these fibers have a complex physical structure. For example, the outside of a wool fiber is covered with a layer of scales, with their free ends pointing toward the tip of the fiber. Inside is the cuticle, which surrounds an inner structure, called the cortex, which is made up of small spindles called cortical cells. Some of the coarser wool fibers have a hollow space in the center, called the medulla. The cortical cells are made up of keratin molecules, which are oriented into particular folds and crimps (54). 

The complex morphology of hair essentially consists of four components of different functionality (i) The cortex that gives the hair its mechanical properties consists of elongated, spin-shaped cells aligned in the direction of the fiber axis. The keratinized protein in the form of microfibrils resides in these cells, (ii) The medulla is located in the center of some thicker fibers and it consists of a loosely packed porous cellular structure (it does not contribute to the mechanical properties of the hair), (iii) Cell membrane complex which cements the various cells of the cuticula and the cortex and it consists of several layers, (iv) Cuticle, a multilayered structure which consists of flat cuticle cells and the most outer layer, i.e. the epicuticle (which is about 2.5 nm thick) is the most important part for deposition of surfactants and polymers in the shampoo formulation. This consists of 25 % lipids and 75 % protein, the latter having an ordered possibly p-pleated sheet structure with 12% cystine. The cystine groups are acylated by fatty acids which form the hydrophobic surface region. A schematic representation of the epicuticle is shown in Fig. 1.46. 

Surrounded by the inner root sheath, the fiber cells multiply and the constant stream of cell production pushes the fiber cells upward toward the skin surface. As the fiber cells move up the hair folhcle, they begin to differentiate into particular cell types cuticle and cortex. In addition to cuticle and cortex cells, the folhcle also produces a central strand of cells that are loosely organized, forming the medulla in the center of the hair fiber. Eventually, the tip of the fiber penetrates the superficial layers of the epidermis and the mature fiber emerges from the skin surface. In the mature fiber, cells are keratinized and hardened. 

---