The central core of the hair is called cortex, which is covered by the cuticle and it occupies 75 to 80% of the hair volume (24). It is mechanically the most important part of the hair and largely responsible for the elasticity and tensile strength of hair (25).
Cortical Cells: The cortex is primarily made up of cortical cells, which are proteinacious cells of elongated shape of regular and irregular cross-sections. The diameter of these cells is between 2 to 5 microns (µm). The length of the cortical cell is around 100 microns. These cells are spindle shaped, various sizes, and aligned along the main axis of the fibers. There are two types of cortical cells: para cortical cells and ortho cortical cells. The para cortical cells possess a uniform spindle shape while ortho cortical cells are non-uniform in shape, as shown in Figure 5.
Figure 5: Shape of para - Cortical and ortho –Cortical Cells
Each cortical cell is separated from the neighboring cortical cell and they are glued together by the cortortical-cortical cell membrane complex, which is approximately 25 nanometers (nm) thick (26).
The inner most cuticle layer and the outermost cortical cells are glued together with the cuticle-cortex CMC. The cortical cells are glued together with corticle-cortical CMC into a solid mass, which is shown in Figure 6.
Figure 6: The cortical cells are glued together with cortical-cortical cells CMC. The cuticle layers are shown to engulf the cortical cells.
Straight Caucasian Oriental/Mongolian and East Asian hair tends to have all para cortical cells that are more uniform in diameter and shape. Consequently the hair is straight in shape and relatively uniform in diameter with an ellipticity of 1.22. They contain more cystine and are more heat stable.
Figure 7: Straight Caucasian,Oriental /Mangoloid and East Asian hair with all para cortical cells
African descent hair consists of a significant amount of ortho cortical cells that are inherently irregular in shape and size. African descent hair has equal rows of para cortical cells and ortho cortical cells as shown in Figure 8 and that is why African descent hair is curly/coily in shape with a tremendous amount of diameter variation along the hair shaft. It is known to have very high ellipticity, which may range from 1.0 to 3.25 (27). The introduction of significantly more ortho cortical cells in African descent hair makes it more prone to thermal damage and chemical attack than its counter part Asian and European descent hair. On the para-cortical side, the overall thickness of the cuticles is higher than the ortho-cortical side, where the cuticle layers could be as low as one to two in number (28).
Figure 8: African-descent curly/coily hair
Mixed race hair has a slight to moderate amount of curliness and it is more uniform in diameter along the hair shaft, as compared to African descent hair. Even the mixed race hair with predominantly para cortical cells and a narrow row of ortho cortical cells is more resistant to chemical modifications, such as hair straightening, etc.
Figure 9: The wavy hair with a narrow row of ortho cortical cells on one side of the cortex.
THE INSIDE STRUCTURE OF CORTICAL CELLS
The cortical cells consist of macrofibrils that contain Intermediate Filaments (microfibril). The Intermediate filament consists of Protofibrils. And protofibrils consist of protofilaments. Each profilament consists of dimers that are two coiled protein chains.
Macrofibrils: The cortical cells consist of macrofibrils that are separated by a thin membrane in the cortical cell. These membranes also contain the pigment of the hair known as melanin, which is variably dispersed in the membrane. The macrofibrils are embedded in the cortical cells as units and are separated by inter-macrfibrilar material or matrix in the cortical cells (29) . The macrofibrils are rod like structures, with a length of few microns (µm) and a diameter of 0.1 to 0.4 microns (µm). These keratinized macrofibril units are oriented longitudinally within the cells, thereby, providing a very strong fiber-matrix composite, as shown in Figure 10. The macrofibril is a colony of many microfibrils also known as intermediate filaments (IFs).
Figure 10: A Cortical Cell has 10 to 12 macrobirils in it.
Microfibrils/Intermediate Filaments (IF): The macrofibril contains many microfibrils also know as intermediate filaments. As seen through electron microscope, the macrofibril consists of long uniform filaments known as Intermediate Fibrils (IF) that are oriented parallel to the axis of the hair fiber. Each IF is about 8 nm or 0.008 µm in diameter and is about 11 nm apart from another IF, in wet fibers (Figure 11). The IFs contain very organized helical material protein chains twisted together like a rope.
Figure 11: Intermediate Fillaments IFS (Microfibrils) present in each Macrofibril. There are about 40 to 50 IFs in a Macrofibril
The Matrix: Here, the microfibrils/Intermediate filaments are embedded in the matrix of macrofibrils and cortical cells. Its major role is to hold together microfibrils (Intermediate Filaments) in a stationary stable state with some freedom of movement also known as the elasticity of hair fibers. The degree of loss in tensile strength of hair fibers during chemical treatments, corresponds to the extent of damage to the matrix. If the microfibrils and the protein matrix are denatured partially or fully, the hair fiber looses its mechanical properties (tensile strength and elasticity) relatively. The cortex containing microfibrils/Intermediate filaments that are embedded in protein matrix is shown in Figure 12.
Figure 12: The matrix is the space between cortical cells, macrofibrils, and Intermediate Fillaments containing cysteine rich proteins.
Protofibrils: One microfibril/Intermediate filament consists of 8 protofibrils and each protofibril consists of four protofilaments. The structure of a protofibril is shown in Figure 13.
Figure 13: The eight protofibrils in an Intermediate Filament are shown in top section of this figure.
Protofilaments: The four protofilaments are present in each protofibril and they are the - helical formations to form a dimmer. Then two dimmers aggregate to form a tetramer. The tetramer is also known as protofilament. The diameter of a protofilament is 2.8 nm or 0.0028 mm. The structure of a protofilament is shown in Figures 14.
Figure 14: The Protofilaments in an Intermediate Filament, also known as tetramer.
Dimers: The protofilament (tetramer) is comprised of two dimers and the structure of a dimer is shown in Figure 15.
Figure 15: The structure of a dimer in a Protofilament.
A dimer has two protein chains twisted together in a special helical pattern as shown in Figure 15. There are 16 dimers in a microfibril/intermediate filament (IF). The combination of these elements produces a composite structure of hair fiber, as shown in Figure 16.
Figure 16: The structure of hair showing cuticles, cortical cells, macrofibrils, intermediate filaments, protofibrils, protofilaments, and dimers.
In my next post I will discuss the third component of the structure of hair which is the Modula.
 A Franbourg, and F Leroy. (2005). Hair Structure, function, and physiochemical Properties. In Ed: Bouillon, C., Wilkinson, J. The Science Of Hair Care, 2nd Ed., CRC Press Taylor & Francis Group, Boca Raton, Fl. p.7.
 M Feughelman. (1997. Mechanical properties and structure of alpha – keratin fibres: Wool human hair and related fibres. Sydney: UNSW Press, p. 3.
 Ibid, p. 3
 AN Syed, TN Ventura, and MN Syed. (2013). Hair ethnicity and ellipticity: A preliminary study. Cos & Toil, 128, (4), 250-259.
 ibid, p. 59, and Figure 12 p. 60.
 Leon, N.H. 1972. Structural Aspects of keratin fibers. J. Soc.