Hair is mostly made of a protein called keratin. Fingernails and the top layer of skin is also made of keratin protein.
Each strand of hair consists of three layers.
- An innermost layer or medulla.
- The middle layer is the cortex. The cortex provides strength, color, and the texture of hair.
- The outermost layer is the cuticle. The cuticle is thin and colorless which protects the cortex.
The center of the hair is called the cortex. It makes up 80 percent of hair. It’s made of small fibrils that twist together to make the longer fibers stronger.
The cuticle is made of dead cells that overlap each other in several layers. The condition of the cuticle plays a part in the appearance of the hair. If the dead cells lay closely together (closed cuticles) then the hair looks shiny and healthy, however, if they lift up (open cuticles), the hair appears dull, dry, and tangles easily.
Medulla cells contain air pockets that are found inside the hair shaft which form the medulla canal. Lipids, a fat substance, is passed through to the cortex or cuticle from the medulla cells. Layers of lipids are formed to bind moisture and protein to the hair shaft.
African American hair consists of 88% protein, 10-15% water, 5-10% pigments, minerals and lipids. The cortex and cuticle are formed from solid keratin fiber and the binding material is formed from amorphous keratin, which fills in the spaces inside the cortex and cuticle. The amorphous keratin holds the fibrous structure together and keeps it elastic
Below the surface of the skin is the hair root, which is enclosed within a hair follicle. At the base of the hair follicle is the dermal papilla. The dermal papilla is fed by the bloodstream which carries nourishment to produce new hair. The dermal papilla is very important to hair growth because it contains receptors for male hormones & androgens. Androgens regulate hair growth.
The Hair Growth Cycle
Hair follicles grow in repeated cycles. A cycle can be broken down into three phases.
- Anagen – Growth Phase
- Catagen – Transitional phase
- Telogen – Resting Phase
Each hair goes through the phases independent of the neighboring hairs.
Anagen Phase – Growth Phase
Almost 85% of all hairs are in the growing phase at any one time. The Anagen phase or growth phase can vary from three to seven years. Hair grows approximately 10cm per year and any individual hair is unlikely to grow more than one meter long.
Catagen Phase – transitional phase
At the end of the Anagen phase the hairs enter into a Catagen phase which lasts about one or two weeks, during the Catagen phase the hair follicle shrinks to about 1/6 of the normal length. The lower part is destroyed and the dermal papilla breaks away to rest below.
Telogen Phase – resting phase
The resting phase follows the catagen phase and normally lasts about 6 weeks. During this time the hair does not grow but stays attached to the follicle while the dermal papilla stays in a resting phase below. Approximately 10-15 percent of all hairs are in this phase at one time.
At the end of the Telogen phase the hair follicle re-enters the Anagen phase. The dermal papilla and the base of the follicle join together again and a new hair begins to form. If the old hair has not already been shed the new hair pushes the old one out and the growth cycle starts all over again.