The hair shaft is comprised of a hard, dead protein called keratin that forms in three distinct layers from the hair follicle. The medulla, cortex and cuticle.
Cut Away Of A Hair Shaft
Medulla
The medulla is the innermost layer or core of the hair shaft. It is composed of keratin and melanin. It is interspersed with tiny pockets of air. This layer provides the structure for the hair shaft and a small amount of color in some individuals. The medulla may or may not be present in all people.
Cortex
The cortex is the middle layer of the hair shaft. It is composed of keratin in many layers. The number of layers varies with each individual and thus the thickness of the cortex also varies with each individual. Melanin is also present in the cortex, but in a much greater concentration then in the medulla. This layer gives the hair shaft it’s strength, elasticity and color. It also gives the shaft it’s shape. Naturally occurring sulfur found in the cortex and the disulfide bonds between the sulfur atoms determines the shape of the keratin protein and if an individual will have curly or straight hair. Without the cortex, hair would not have the enormous elasticity that it does. A healthy hair shaft can stretch up to 30% of its length before breaking.
Cuticle
The cuticle is the outermost layer of the hair shaft and is comprised of flat, shingle like layers of keratin. The cuticle creates a protective layer around the cortex and medulla protecting them from the environment and providing a measure of water-proofing. The thickness or number of layers varies with each person and is the largest contributor to diameter of the hair shaft. If an individual has many cuticle layers, they will have thick hair. Only a few layers and the individual will have fine hair. A strong bond between the cuticle and the cortex is necessary for healthy hair.
