Human Hair Growth Cycles

Hair Cycle Phase

Hair forms in a pouch-like structure below the skin called a hair follicle. What we see as hair is actually the hair shaft, which is the keratinized, hardened protein made up of three layers that grow from this follicle. Keratin is not a living structure; therefore our visible hair is actually dead. The three layers of the hair are the medulla, cortex, and cuticle. The cortex makes up the majority of the hair shaft and together with the medulla is responsible for giving hair pigment.

Unlike other mammals, human hair growth and shedding is random versus seasonal or cyclical. For example, humans do not produce a thicker a coat of hair for warmth in the winter, to shed in the summer. Humans have more hair follicles per square inch of skin than other higher primates, including chimpanzees and gorillas. Because most of this hair is fine and pale (called vellus hair), it usually isn't visible to the naked eye. Consider this: the forehead has more hair follicles than any other part of the body. The thicker, fully pigmented hair most people consider "real hair" is called terminal hair. This hair is found on scalp, eyebrows, legs, backs, underarms, and genital areas. This is the hair the Light Sheer diode laser treats.

Scalp hair grows approximately six inches per year. Everyone's hair grows differently, depending on age, weight, metabolism, hormones, ethnicity, medications, and other factors; however, all hair goes through three distinct growth phases. Throughout the year, hair follicles are in one of the three phases anagen, catagen, or telogen phase.

Anagen Phase:

The anagen phase is the active phase of hair growth. During this phase, cells in the hair root are rapidly dividing. New hairs are formed, pushing hairs that have stopped growing up and out of the follicle. Hair grows around one centimeter per twenty-eight days during the anagen phase, and scalp hair spends on average two to six years in the active phase. Those who have a difficult time growing their hair very long, have a shorter anagen phase compared with those who have very long hair. Hair on other parts of the body, such as the arms, legs, eyebrows, and eyelashes have a short anagen phase lasting only about thirty to forty-five days.

Catagen Phase:

Approximately three percent of all hair is in the catagen phase at a given time. During this phase, growth stops and forms what is called a club hair. This phase is referred to as the resting phase and lasts around two to three weeks.

Telogen Phase:

The last phase is the telogen or resting phase and lasts around one hundred days for scalp hair, longer for arm, leg, eyebrow, and eyelash hair. Around six to eight percent of all hairs are in the telogen phase at the same time. People shed anywhere from twenty to one hundred telogen hairs daily.

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