How Does Hair Grow?
Information about Alopecia, Hair Loss, Hair Transplants, Hair Loss Treatments, Wigs, etc.
The Hair Growth Cycle
Hair growth is a cyclical process that is divided into three stages:
Anagen (Active Growth Phase)
The anagen phase occurs in the papilla. The hair follicle is attached to the papilla, and hairs grow out of the hair follicle. The cells in the papilla divide to produce new hair fibers, and the papilla is pushed into the dermal layer of the skin in order to nourish the hair follicle which holds the strand of hair. While the anagen phase can last up to 8 years, its length is determined by genetics. The length of the anagen phases also determines the length of our hair (if it is not cut). With a longer anagen phase comes longer hair, and with a shorter anagen phase comes shorter hair. At any given time, 85% of scalp hairs are in the anagen phase.
Catagen (Regressive Phase)
The catagen phase is the second phase of the hair growth cycle. In this phase, the hair follicle shrinks to 1/6 of its original length and the papilla detaches itself from the dermal layer and begins to rest. Consequently, hairs lose their blood supply. Because the blood supply does not reach the hairs, the hairs lack nourishment, contributing to hair loss. The catagen phase lasts approximately 2 to 3 weeks. Just 1% to 2% of hairs are in the catagen phase at a given time.
Telogen (Resting Phase)
After the catagen phase, the hair strand and follicle are at rest for between 1 and 4 months. At any time, 13% to 14% of hairs are in the telogen phase. When the anagen phase begins again, old hairs are pushed out by the new growing hairs. This process is known as shedding. Humans normally shed approximately 100 hairs a day. Shedding more than 100 hairs a day usually leads to hair loss.
What is Hair Made Of?
Human hair is made up of a protein called Keratin. Not only is our hair made of Keratin, but so are our fingernails and toenails. To create the structure of our hair, Keratin composes itself into intermediate filaments, which can be compared to a rope. The tightly wrapped structure of the Keratin gives our hair shaft its strength.
Parts Making Up Hair Roots
The hairs of the human scalp contain a highly structured root. The base of the root is called the bulb. The bulb holds cells which produce the hair shaft. Another important structure of the root is the sebaceous gland, which lubricates hair with oil. Lastly, we have the erector pili muscles, which cause hairs to stand up.
Parts Making Up Strands of Hair
Strands of hair are made up of three main parts: the medulla, cortex, and cuticle. The medulla is the innermost region of hair and is highly unstructured. In fact, the medulla is not always present and its purpose is unknown. The cortex, which is the middle layer of hair, is a highly structured area. It provides strands of hair with water and mechanical strength. Additionally, it holds melanin, a substance that gives your hair its color. The outermost layer of hair is the cuticle. The cuticle is covered in a one-molecule thick layer of lipids, causing hair to be water-repellent.
What Determines Hair Color and Texture?
First and foremost, genetics determines hair color and texture. But genetics determines these two characteristics in an indirect way. Regarding hair color, heredity determines the main type of melanin that the cortex will produce. If the cortex produces mainly eumelanin, chances are your hair will be black, brown, or dirty blond. If the cortex produces pheomelanin, your hair will be red. Blondes are blond due to a lack of melanin. Additionally, when the amount of melanin produced decreases hair becomes gray. While the type of melanin produced by the cortex determines hair color, the shape of the cortex determines hair texture. If the cortex has a round cross section, your hair will most likely be straight. If the cortex is oval shaped, your hair will most likely be curly or wavy.
The Growth Rate of Hair
Scalp hair grows at a rate of 1.25 cm (.5 inches) per month, or 15 cm (6 inches) per year. As we get older, the growth rate of hair decreases and can be as little as .25 cm (.1 inches) per month.
Amount of Hair
The human body, on average, contains 5 million hairs. Surprisingly, just 100,000 to 150,000 hairs are on the scalp. In Caucasians, hair color corresponds to the amount of hair on the scalp. On average, blondes have 140,000 hairs, brunettes have 105,000, and redheads have 90,000.
