Curly Hair Structure

Curly hair is easy. Curly hair follicles grow parallel to the scalp. Straight hair grows at a right angle. Here’s what I have on structure. There are dozens of sublayers: but only three separate and distinct main layers: Medulla, Cortex and Cuticle.
The medulla is the innermost portion of the hair shaft. This section has between two and five rows of cells across. Usually, only thicker, coarse hair shafts contain a medulla. All male beard hair contains a medulla. It is quite common for fine and naturally blond hair to lack a medulla. As far as cosmetology is concerned, the medulla is an “empty” air space and is not involved in any salon services.
As much as 90 percent of the total weight of hair comes from the cortex. The cortex gives the hair its strength, flexibility, elasticity, and color. The cortex is made of rectangular-shaped cells that are tightly bonded together. These cells are filled with keratin. The natural color of the hair is due to the pigment in the cortex.
The elasticity of the hair is the result of the unique protein structure within the cortex. Wet setting, thermal styling, permanent waving, and chemical hair relaxing all take place within the cortex and would not be possible otherwise.
The cortex is surrounded by a single layer of overlapping transparent, scalelike cells of the cuticle. These scales overlap like shingles on a roof. A longitudinal section of hair shows that although the cuticle scales overlap, each individual cuticle scale is attached to the cortex. This single layer of cuticle scales makes up the cuticle layer. Although when viewed on end, the scales can be seen to overlap, hair has only one overlapping, cuticle layer. The number of overlapping layers varies depending on the length of the cuticle scales. The hard cuticle protects the more delicate cortex and medulla. Without the cuticle layer, the cortex would become frayed and fall apart. The cuticle also acts as a barrier to chemicals such as tints and permanent wave lotions. This barrier is not impenetrable. Factors such as high pH and temperature can loosen the barrier. Since this layer is transparent, it is the pigment in the cortex which determines hair color (
Hair is approximately 91 percent protein. Proteins are made of long chains of amino acids that are linked together end to end like pop beads. The chemical bond that links amino acids together is called a peptide bond or end bond. A long chain of amino acids linked by peptide bonds is called a polypeptide. Proteins are long, coiled, complex polypeptides made of amino acids. The shape of a coiled protein is called a helix.
All the protein structures of hair are made from these eighteen amino acids.
Cysteic acid Aspartic acid Threonine
Arginine Serine Glutamic acid
Proline Glycine Alanine
Valine Cystine Methionine
Isoleucine Leucine Tyrosine
Phenylalanine Lysine Histidine
The cortex is made of millions of polypeptide chains, which are cross-linked together by three types of side bonds: hydrogen bonds, salt bonds and disulfide bonds. These side bonds are essential to wet sets, thermal styling and permanent waving.
A hydrogen bond is a special type of ionic bond. Within the structure of hair, a hydrogen bond occurs when a hydrogen atom, from the acid portion of an amino acid, is attracted to an oxygen atom, in the acid portion of another amino acid. Hydrogen bonds are easily broken by water or heat and are responsible for wet sets and thermal styling. Although individual hydrogen bonds are weak, there are so many of them is the hair that they account for about one third of the hair’s total strength.
A salt bond is also an ionic bond. Within the structure of hair, a salt bond occurs when the negative charge of one amino acid is attracted to the positive charge of another amino acid. Salt bonds depend on pH and account for about one third of the hair’s total strength. Salt bonds are easily broken by strong alkaline or acidic solutions.
A disulfide bond is a covalent bond, which is different from the ionic bonding of a hydrogen or salt bond. A disulfide bond joins the sulfur atoms of two neighboring cysteine amino acids to create cystine. Although there are fewer disulfide bonds than hydrogen or salt bonds, disulfide bonds are stronger and account for about one-third of the hair’s total strength. Disulfide bonds are not broken by heat or water. Permanent waves and chemical hair relaxers work by creating chemical and physical changes in the hair’s disulfide bonds.
Individual protein chains are cross-linked by side bonds to create tiny, invisible, threadlike fibers. At least nine of these fibers twist around each other to make larger bundles called micro fibrils. Dozens of micro fibrils, in turn, twist together to create larger macro fibrils. Finally, six macro fibrils intertwine to form fibrils, the cells of the cortex.
Keratin is an important example of an insoluble, complex protein. Hair is made almost entirely of this important substance. Keratin is made from eighteen different amino acids, including cystine and cysteine (uncross-linked cystine).
Cystine is the most abundant amino acid in hair. Cystine makes up nearly 18 percent of hair. With so many possibilities for making cross-linked disulfide (sulfur) bonds, cystine gives hair much of its strength. It is also responsible for causing hair to hold a permanent curl or relaxer.
This highly organized structure is designed much like the high-strength cables used to support suspension bridges. The polypeptide chains in keratin are both physically and chemically bound together. Millions of these keratinized cells are securely bonded together in the cortex and covered with a protective cuticle shield. This is how nature has created hair, a superstrength structure with amazing physical characteristics and chemical resistance!

Leave a Reply