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 fibrilar 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 contains 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, hidrogene bonds, Van de Waal forces and disulfide bonds. 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 thioglicolate, 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.
18-MEA and Healthy Hair
Human hair is primarily protein, with lipids and melanin making up a majority of the remainder. The hair fiber is a shaft-like structure and consists of a cortex, a cuticle and a medulla. The cortex is a cylinder-like structure comprised of long, spindle-shaped cortical cells and constitutes the major portion of the fiber’s mass. The cuticle is a thin laminar-like structure comprised of layers of overlapping, flat scale-like cells. The cuticle constitutes the outer surface area of the hair fiber and surrounds the cortex, acting like a protective sheath. Because it is the visible area of the hair, the cuticle also gives hair its optical properties (e.g., shine, gloss).
Sandwiched in between the overlapping cuticle cells is a multi-component layer known as the cell membrane complex (CMC). Sometimes referred to as the epicuticle, the CMC cements the cuticle together and binds it to the cortex, thus becoming the top layer of one cuticle cell and the bottom layer of the one directly beneath it. Most of the evidence to date suggests that the CMC is comprised of a glycoprotein δ-layer pressed between two β-layers. The primary lipid, 18-methyl eicosanoic acid (18-MEA), is found in the upper β-layer, along with oleic and palmitic acid. This primary lipid is responsible for giving hair its natural hydrophobicity, as it coats the surface of abutting cuticle cells. When a cuticle is removed, 18-MEA becomes exposed and coats the newly exposed cuticle surface. This contributes to the hair’s wet and dry combing properties as the presence of, 18-MEA makes hair easier to comb.
Environmental factors and chemical treatment as well as everyday grooming processes can have an adverse effect on the integrity of 18-MEA. Although 18-MEA and the other hair surface lipids appear to be resistant to extraction with organic solvents, they can be removed readily by alkaline hydrolysis. This suggests that the use of high pH products, such as perms, hair dyes or hair relaxers, could deplete these important lipids from the hair surface. Studies on wool fibers have shown that exposure to natural, or artificial, weathering or UV irradiation increases the hydrophilicity and wettability of fibers.4 Additionally, exposure to UV irradiation has also been shown to deplete wool of its lipid content. Of the fatty acids, 18-MEA appears to be lost to the greatest extent; therefore, hair it is necessary to restore18-MEA to maintain healthy hair.