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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 corser 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.[3,4]
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.[3,4,5]
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.[3,4,5,6]
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.[4,5,6,7] 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.[2,5,6,8]