|Fig. 1: Undamaged hair|
Tonya McKay writes:
As a polymer scientist with a love for biological structures, I find hair and skin to be extremely fascinating systems. Human hair is an intricate composite structure comprised of keratin proteins, lipids, polysaccharides, water and pigment particles. All of the individual components are complex and perform very specific functions. Those of us with curly hair are concerned a lot about our hair’s texture and porosity (a popular buzz word of late). These two factors are primarily based upon the structure of the cuticle — the outer layer of our hair.
The scanning electron microscope image in Figure 1 shows highly magnified detail of the exterior surface of a strand of human hair. The external layer is called the cuticle, and is much like bark on a tree. Both the cuticle layer and tree bark are made up of many smaller, individual pieces (called scales when referring to the cuticle) that work together as one overall unit to perform a function. The job of the cuticle is to provide protection to the hair shaft from mechanical and thermal damage, while allowing moisture in and out as needed. The cuticle structure is an amazing work of nature, because it is strong, yet flexible, and is made up of many pieces, which allows it to act as a seal to protect the inner cortex of the hair, and yet also allows it to be permeable, or porous.
The center of the hair shaft is referred to as the cortex, and is a very complicated structure filled with many different substructures and clusters of structures made of keratin protein, lipids, and other matter. Water provides the means for the necessary hydrogen bonding between the keratin fibers to occur that is essential for the maintenance of hair strength, elasticity and integrity. Without moisture in the cortex, the hair becomes thin, frizzy, and much more prone to permanent damage and breakage. Thus, the cuticle layer performs a very important duty by protecting this delicate interior of the hair and helping it maintain the proper balance of moisture.
Structure of the Cuticle
The cuticle itself is a multi-laminate structure, like stacked sheets of paper, composed of fatty acids, proteins, and other cellular matter. Below is a description of each layer.
- Epicuticle — This surface layer of the cuticle is made up of lipids and proteins and is also found on the bottom of the stacks of layers.
- A-Layer — This layer is comprised of proteins very high (35%) in cystine, which enables the layer to be highly crosslinked. This layer gives toughness to the hair and also provides physical protection from heat and other potential threats.
- Exocuticle — This layer has approximately 15% cystine, so it is less strong and tough than the A-Layer, but provides similar protection.
- Endocuticle — This layer contains only 3% cystine, and so is only very lightly crosslinked. This means that this layer is the only cuticle layer to swell in the presence of water. This causes the entire cuticle to swell and lift away from the hair shaft, resulting in a ruffled cuticle that allows the passage of material both into and out of the hair.
- Cuticular Cell membrane Complex (CMC) — This layer is made up of polysaccharides and several lipids (fatty acids). This layer acts as the glue that holds the cuticle together and holds it to the hair shaft.
|Fig. 2: Chemically altered hair|
|Fig. 3: Damaged cuticle|
Syed, A.N., Askar, N.A., “Structure of Hair” Powerpoint presentation, www.dralisyed.com, July 3, 2008
Robbins, C. R., Chemical and Physical Behavior of Human Hair, Spring, 4th Editions, Dec. 14, 2001
Gray, John, “The World of Hair”, P&G Hair Care Research Center