Composition of Hair.
The overall chemical composition of hair is 45 % carbon, 28 % oxygen, 15 % nitrogen, 7 % hydrogen and 5 % sulphur.
The hair shaft is essentially composed of keratin. Hair keratin is hard, compact and strong. This fibrous protein is gradually formed inside cells from the germinal layer. These cuticle cells are characterised by the presence of amorphous keratin while the cortical cells have a structure of filaments surrounded by a keratinic substance that is richer in sulphur and contains amino acids. The keratin of these filaments forms a helix, with a distance between the turns of 0.51 nanometres and a structure maintained by hydrogen bonds. This protein plays a key role in the cohesion and physical properties of hair.
Hair also contains water (12 to 15 %) and traces of mineral elements (calcium, cadmium, chromium, copper, zinc, iron and silicon). These elements can be brought to the base of the hair follicle by blood circulation and then contribute to building the hair shaft. Besides this contribution, the environment can, for example through pollution, be the source of certain elements such as lead.
The hair also contains lipid components (3% of its composition). They are produced in the hair bulb from sterols, fatty acids and ceramides. Present essentially in the intercellular cement of the cortex and the cuticle, they provide the hair with a certain impermeability and ensure the cohesion of the hair fibre.
Other lipids come from the secretion of the sebaceous gland: sebum. Sebum is formed from mature sebaceous cells which have burst open and it mainly contains lipids (triglycerides, waxes, squalene, esterified cholesterol and free cholesterol). The most abundant triglycerides undergo partial hydrolysis by the bacteria that inhabit the scalp, Propionibacterium acnes and Propionibacterium granulosum. This hydrolysis liberates free fatty acids, di- and monoglycerides and glycerol. The fatty acids are characterised by carbon chains, even and uneven in number (from C11 to C19), with numerous unsaturated sections and branches. The waxes are esters of acids and long-chain alcohols. The waxes are apolar compounds which are metabolised by the flora and are little affected by oxidation.
In the sebaceous gland, acetate molecules condense to form mevalonic acid, an isoprenoid isomer and squalene, a linear hydrocarbon formed from 30 carbon atoms. The sterols are esterified by the fatty acids. The squalene/cholesterol ratio in the surface hydrolipidic film is a mirror of the biological activities of the sebaceous glands and the epidermis. The lipid mixture which forms this film on the surface of the skin lubricates the hair and thus preserves the suppleness and shine of the hair. Being hormone dependent, the sebum can be produced in excess and the hair becomes greasy and heavy. On the other hand, if too little is secreted, the hair becomes dry, dull and damaged.
Hair is an integrated system with a peculiar chemical and physical behavior. It is a complex structure of several morphological components that act as a unit. The hair shaft of mammals is divided into three main regions: Cuticle, cortex and medulla. The medulla is present in coarser hair like grey hair, thick hair and beard hair, and it is absent in fine hair of children. There is more medulla in the coarser hair of Asians than Caucasians. The medulla may be involved in the splitting of hairs since it provides an area of weakness as a pathway for the propagation of cracks along the axis of the fiber.
The cuticle is a chemically resistant region and consists of flap overlapping scales (keratinocytes) like shingles on the roof. The shape and orientation of the cuticle cells are responsible for the differential friction effect in hair. The cuticle is generally formed by 6–8 scales thick for Asians, slightly less in Caucasians and even less in African hair. A thinner cuticle layer makes African hair more prone to breakage. Each cuticle cell contains a thin proteinaceous membrane, the epicuticle, covered with a lipid layer that includes the 18-methyl eicosanoic acid (18-MEA) and free lipids. Beneath the cuticle cells membranes there are three layers, all containing heavily cross-linked protein, mostly cystine, the A-layer, the exocuticle or B-layer and the endocuticle. The first one contains the higher amount of cystine, and the third one contains the lowest. The 18-MEA is responsible for the hydrophobicity of the hair and its removal by alkaline chemical cosmetics procedures may damage hair by increasing hydrophilia.
The cell membrane complex (CMC) is intercellular matter. CMC consists of cell membranes and adhesive material (cement) binding the cell membranes between two cuticle cells, two cortical cells and cuticle-cortex cells. The most important layer of the CMC is called the beta-layer, and it is considered to be the intercellular cement and it is sandwiched by other layers from each cell. The CMC and the endocuticle are very vulnerable regions to the chemical treatments such as bleaching, dyeing and hair straightening/perm procedures. Also, the everyday grooming and shampooing friction may disrupt the CMC.
CMC fractures may be seen before the hair fiber is ruptured. The exposure to repeated rough washing, unprotected drying, friction actions, sunlight and alkaline chemical treatments lead to a decrease in the lipid content of the cell surface changing it from the state of hydrophobicity to a more hydrophilic, negatively charged surface.
The cortex constitutes the major part of the mass of the human hair, and it is formed by elongated, fusiform cells connected by a CMC and contains protein and melanin granules. The cortex cell also contains spindle-shaped fibrous structures called macrofibrils, each one consists of microfibrils that are highly organized fibrillar units and matrix. The matrix is formed by crystalline protein of high cystine content. The macrofibrils are arranged in a spiral formation. Inside the microfibrils there are subfilamentous units called protofilaments, each containing short sections of alpha-helical proteins in coiled coil formation polypeptide chains of proteins. The alpha-helix is held coiled by chemical forces such as: Ionic forces, hydrogen bonds, Van de Waal forces and disulfide bonds. The hair straightening process consists on breaking the forces that hold the coil, allowing it to be stretched. If the rupture of the chemical bonds is followed by curling the hair, it is called “perm,” meaning permanent curling. The process of reduction the hair involves hair swelling and very alkaline substances such as sodium or lithium hydroxide, guanidine, ammonium thioglycolate, pH higher than 9.0. All this can produce splits or cracks to the endocuticle and the CMC, but the major damage to hair after using hair reducing products is indeed due to misuse of the products and lack of care during combing hair in the reduced state. Hair damage caused by the use of chemical procedures can be minimized, avoided or repaired by the correct use of hair care products. Hair cosmetics may enhance hair hydrophobicity, strengthen the cuticle and minimize electrical charges and friction forces.
The hydrophobicity of the hair is possible thanks to the 18-MEA lipid layer. Removal of this covalently linked fatty acid renders the fiber hydrophilic. When wet, virgin hair can be stretched by 30% of their original length without damage; however, irreversible changes occur when hair is stretched between 30% and 70%. Stretching to 80% causes fracture. Hair is porous and damaged hair is intensely so. Water absorption causes the hair shaft swelling. Excessive or repeated chemical treatment, grooming habits, and environmental exposure produce changes in hair texture and if extreme can result in hair breakage. These changes can be seen microscopically as “weathering” of the hair shaft and contribute to tangling, and frizzing. Weathering is the progressive degeneration from the root to the tip of the hair. Normal weathering is due to daily grooming practices. When the hair is extremely weathered and chemically treated, there may be scaling of the cuticle layers, removal of the 18-MEA and cuticle crack. If the cuticle is removed, the exposure of the cortex and further cortex damage may lead to hair fiber fracture. The use of hair cosmetics may restore hair cuticle damage and prevent hair breakage by reducing friction and water pick up.