The daily grooming practices and the application of various products for cleansing, conditioning, and styling hair can cause changes in the structure of the hair. Therefore it is important to understand the structure of hair for hairstylists, consumers and hair chemists alike.
The structure of hair is very complex and there are certain differences in the structure of hair of various ethnicities such as Caucasian, Africans and Asians. Therefore, the study of the structure of human hair becomes a very interesting field and there is a plethora of literature available discussing the structure of human hair. This effort here is based upon collecting most of the information in a simple way so that beginner hair chemists, hairstylists and consumers can understand hair structure easily and hopefully treat hair in a serious manner in order to avoid damage.
The hair is a made up of water insoluble protein known as Keratin, which as a protein consists of many amino acids as its primary units. The physical shape of hair has mainly three parts, one being the cuticle, the second being the cortex, and the third is medulla. I will break down the three components over a series of blog posts beginning by looking at the cuticle.
The cuticles are the protective layers and make the outer surface of hair. There are 6 to 10 layers of cuticles (1) glued together with cell membrane complex (CMC)m (2) and in some cases, there are only two cuticle layers on the crimped side of the coily hair (3). The architecture of cuticle layers is shown in Figure 1
Figure 1: The architecture of Hair Cuticles
The diameter of this hair fiber is around 80 microns (µm). A micron is one millionth of a meter in length. This is medium texture hair. Fine hair has a diameter range of 50 to 60 microns, medium hair hair has a diameter of 61 to 80 microns and coarse thick hair has a diameter of 81 to 100 microns.
Each cuticle layer is about 60 microns (µm) long and 0.5 micron (µm) thick (4). The length of overlap area of one cuticle layer over the other cuticle layer is about 5.0 microns (5). The architecture of cuticle layers in terms of geometric arrangement of the cuticles of human hair is shown in Figure 1 (6) . The excessively curly hair has varying numbers of cuticle layers along the hair shaft. Only one to two cuticle layers are found on spots where hair is thin/flat along the minor axis and 6 to 10 layers along major axis where hair is thick (7). The major and minor axis of the cross section of hair is shown under the sub-heading of fiber ellipticity. The cuticles are also rich in amino acid known as cystine and some fatty acids. The outer most part of the cuticle of hair is composed of fatty acid called 18-MEA (18-methyleicosanoic Acid) also known as F-Layer (8). All of the 18-MEA is present in the upper most surface of the cuticle (9). The structure of the F-Layer is adopted from Robbins and presented in Figure 2 (10).
Figure 2: The structure of F-Layer (18-MEA) present on the outermost layer of the cuticle layers.
Cell Membrane Complex (CMC) of the Cuticles: Each cuticle layer is glued to the next cuticle layer with the help of the material called intercellular cement or cell membrane complex (CMC) and its intercellular spaces.
Types of CMC: There are three types of cell membrane complexes. The first type of CMC is between just cuticles. The second type of CMC is between inner most cuticle layer and the cortical cells. The third CMC is between the cortical cells themselves. All of these three CMCs are somewhat different in their compositions and are explained further (11).
The essential role of the CMC between the cuticles is the cohesion of the one cuticle layer to the other. The schematic presentation of the CMC of the cuticles is shown in Figure 3.
Figure 3: The Cell Membrane Complex between Cuticle – Cuticle, as per Rogers view (1959) and adopted from Fraser et al, 1972.
The outer Beta layers are 5 nano-meter (nm) thick and are comprised of lipids. The central Delta layer is 15 nano-meter (nm) thick.
The second type of CMC is the one which provides the cohesion between the inner most cuticle layer and the outer most cortical cells of the cortex. This cuticle-cortex CMC damages easily upon treatments with solvents, such as water or water containing detergents and harsh chemicals, as compared to cuticle-cuticle cortex but not as severely as cortical-cortical cell CMC (12).
The third type of CMC is the cell membrane complex that keeps the cortical cells glued together. The cortical-cortical cell CMC damages more easily with solvents compared to Cuticle-cuticle CMC and cuticle-Cortical cell CMC (13). All of these three CMCs are somewhat different in their behavior toward solvents (14).
The spaces within CMC are the preferred route of penetration and diffusion of many substances into the hair cortex. The structure of these spaces in the CMC is still unknown (15). The reactive chemicals such as alkalies (sodium hydroxides and guanidine hydroxides), reducing agents (ammonium thioglycolate), and oxidizing agents (hydrogen peroxide) attack the lipids of the CMC. The degradation of CMC between cuticles may cause cuticle layers to unravel, weaken faster and not play an effective role as the cortex protectant.
THE STRUCTURE OF THE CUTICLE
The structure of the cuticle itself is quite complex and is further subdivided in four parts, such as epicuticles, A - layer, exocuticles, and endocuticles as shown in Figure 4 (16).
Figure 4: Sub - layers of a cuticle layer. Source: Modified from Max Feughelman (1997), p. 3.
Epicuticle: The epicuticle is a very thin membrane that covers the cuticle as an outermost layer, as shown in Figure 4. It is about 5 to 7 nano-meter (nm) thick (17). It is very hydrophobic in nature. The F-Layer (18 – MEA) is not very resistant to alkalies, oxidizing agents, and proteolytic agents and on prolonged exposure, significant chemical changes take place in the epicuticle layer as well, thus rendering hair fibers more porous while increasing the interfiber friction. African descent fibers have less of 18 – MEA present at hair fiber surface compared to Caucasian untreated hair (18).
The A Layer: The ‘A’ layer exists below the epicuticle layer, as shown in Figure 4. The ‘A’ layer and exocuticle layer form about two-thirds of the scale structure of a cuticle. They are very rich in cystine contents. The ‘A’ layer contains about 35 % of cystine. The ‘A’ layer resists the attack from physical forces such as repetitive combing and brushing during grooming and blow drying; and chemical forces such as alkalies, reducing agents, oxidizing agents, and proteolytic enzymes that could otherwise be devastating to the integrity of the hair fiber without cuticles.
The exocuticle: The sub layer below the ‘A’ layer is called exocuticles and has about 15 % of cystine contents, as shown in Figure 4. The exocuticles do not have fibrillar structure. The ‘A’ layer is actually a part of the exocuticles that are divided into ‘A’ layer and ‘B’ layer, and this ‘B’ layer is usually termed as exocuticles (19).
The endocuticle layer lies next to the exocuticle layer and contains low cystine contents of about 3 %, as shown in Figure 4. It is the weakest component of the cuticle structure, mechanically. There is a big gap in the cystine contents between endocuticles (3%) and ‘A – Layer’ plus exocuticles (50%). Due to the very low presence of cystine contents in endocuticle layer, it has a very soft and deformable structure and swells considerably more in water than exocuticle layers. The pronounced projection of cuticles during wet state is because the endocuticle layer swells upon wetting with water. The entanglement of the hair fibers is greater in the wet state and it is due to the extraordinary swelling of endocuticles (20). The endocuticle layer can easily deteriorate from the proteolytic agents and other reactive chemicals such as alkalies. The special advantage of the endocuticle layer could be that it may offer some protection by providing a cushion underneath the tougher outer exocuticle layer from forces impacting the surface of the hair (21).
It is therefore, inferred that the role of the cuticles is to oppose the penetration of the reactive and non-reactive chemicals into the cortex of the hair (22). This protective role of the cuticles is a blessing for the long-term survival of the hair fibers. The cuticles of the hair fibers also oppose the bending of the hair fiber to almost 74%, of which 66% of this resistance is due to exocuticles and 8% attributed to endocuticles (23).
In my next post I will look at the cortex.
 Swift, J.A. 1999. Human hair cuticle: biologically conspired to the owner’s advantage. J Cosmet Sci. Vol 50: 23-47.
 Leon, N.H. 1972. Structural Aspects of keratin fibers. J. Soc. Cosmet. Chem. Vol 23. 432.
 Leslie, J.N., D.E. Rivett and D.J. Tucker. 2007. Wool and Related Mammalian Fibers. Editor Menachem Lewin. In: Handbook of Fiber Chemistry. 3rd Ed. CRC Press, Boca Raton, Fl., p. 341.
 Franbourg, A. and F. Leroy. 2005. Hair Structure, function, and physiochemical Properties. In Ed: Bouillon, C. and J. Wilkinson. The Science Of Hair Care, 2nd Ed., CRC Press Taylor & Francis Group, Boca Raton, Fl. p. 4.
 Swift, J.A. 1999. Human hair cuticle: Biologically conspired to the owner’s advantage. J. Cosmet. Sci., 50, p. 23.
 Ibid. p.24.
 Leon, N.H. 1972. Structural Aspects of keratin fibers. J. Soc. Cosmet. Chem. Vol 23. 435.
 DJ Evans, JD Leeder, JA Rippon, and DE Rivett. (1985). Separation and analysis of surface lipids of the wool fibre. In Proc. 7th Int. Wool Tex. Res. Conf. I, Tokyo, Japan, pp.135-142.
 LN Jones, et al. (1996). Hairs from the patients with maple syrup urine disease show a structural defect in the fiber cuticle, J. Invest. Dermatol. 106, 461-464.
 C Robbins. (2009). The cell membrane complex: Three related but different cellular cohesion components of mammalian hair fibers. J. Cosmet. Sci., 60, 437-465.
 C Robbins. (2009). The cell membrane complex:Three related but different cellular cohesion components of mammalian hair fibers. J. Cosmet. Sci., 60, 437-465.
 ibid, p. 446.
 JD Leeder, et al. (1985). Use of the transmission electron microscope to study dyeing an diffusion processes, Proc. 7th IWTRC, Tokyo, V, 99-108.
 C Robbins. (2009). The cell membrane complex:Three related but different cellular cohesion components of mammalian hair fibers. J. Cosmet. Sci., 60, p. 447.
 Franbourg, A. and F. Leroy. 2005. Hair Structure, function, and physiochemical Properties. In Ed: Bouillon, C. and J. Wilkinson. The Science Of Hair Care, 2nd Ed., CRC Press Taylor & Francis Group, Boca Raton, Fl. p. 15.
 Feughelman, M. 1997. Mechanical properties and structure of alpha – keratin fibres: Wool human hair and related fibres. Sydney: UNSW Press, p. 3.
 Negri, AP, Cornell HJ, Rivett DE.(1993). The modification of the surface diffusion barrier of wool. J Soc Dyers Col, 109, pp. 296-300
 Breakspear, S., Smith, J.R., Luengo, G. (2005). Effect of the covalently linked fatty acid 18 – MEA on the nanotribology of hair’s outermost surface. Journal of structural Biology, 149: 235-242.
 Feugheleman, M. (1997). Mechanical Properties and Structure of Alpha – Keratin Fibers Wool, Human Hair and Related Fibres. University of New South Wales Press, Sidney: Australia. p. 2.
 Swift, J.A. (1999). Human hair cuticle: Biologically conspired to the owner’s advantage. J. Cosmet. Sci., 50, p. 28.
 Wortmann, F.J., & Kure, N. (1994). Effects of the cuticle on the permanent wave set of human hair. J. oc. Cosmet., Chem., 45, p. 149.
 Swift, J.A. (2000). Letter to the Editor: The cuticle controls bending stiffness of hair. J. Cosmet. Sci., 51, p. 38.