The Hair Growth Cycle in Good Health and Disease

by: Parsa Mohebi and William Rassman

A normal person loses about 100 hairs every day. “Why do we lose so many hairs every day” is a question that my patients frequently ask me. We re-grow about 100 new hairs a day which equals what we lose.

Every hair lives about 2 to 6 years. During this period hair goes through many physical and chemical insults. It is washed, dried, shampooed, conditioned, bleached, colored, and exposed to heat, cold and sun. Over time, some of these factors may affect the hair quality so constant recycling of the hair makes sense. There are three major phases in the hair cycle: Anagen, catagen and telogen.

Anagen (growth phase)

The anagen is the growth phase of hair. The hair follicle grows at an average rate of 0.35mm every day or almost 1 cm every month during this phase. Approximately 90-95 percent of scalp hairs are in anagen phase. The rate of hair growth may decrease with age and even the overall hair population may decrease as well. The duration of anagen phase dictates the final length of hair. Scalp hair continues to grow for 2 to 6 years. The growth phase is shorter in body and eyebrow hair (about one month), which explains why hair is shorter in these areas.

Catagen (involution phase)

Catagen starts when the anagen phase comes to an end. It usually lasts about 2-3 weeks. Catagen phase is the time cell division stops and the hair stops growing. During this phase, the hair follicle shrinks to about one sixth of its normal diameter. As the follicle root shrinks, dermal papilla (the active cell division center) breaks away and forms a club hair. Club hair rests below skin surface until the old hair falls out. Almost 1% of scalp hair is in catagen phase at any given time.

Telogen (Rest phase)

Telogen starts after catagen phase when most of normal activities of hair stop. Telogen phase for scalp hair last about 2-6 months and 5%-10% of hair is in this phase at any given moment. The number of hair follicles in telogen phase varies in different locations and it could be as high as 50-80% in some areas such as eyebrow, eyelash, and body hair. During this phase the club hair generally stays in place until next anagen phase, and falls out before the new hair is about to spout. In a normal non-balding person, about 100 telogen hairs are expected to shed each day. Some factors, such as mechanical traction, hormonal changes and stress can accelerate telogen hair fall.

Altered Hair Cycle

Normal hair growth cycle may be altered by physiologic changes or diseases. Telogen effluvium is a hair shedding state, which could be seen after childbirth, extreme dieting, anemia and sever blood loss. Telogen effluvium causes hair loss by disruption of the anagen phase and forcing a large number of hair follicles into telogen. This condition is almost always reversible and usually improves over a period of weeks to months. Telogen effluvium commonly occurs after childbirth when the level of pregnancy hormones drops. Anagen hair is pushed to telogen phase in large numbers.

Sever fever can increase hair loss through facilitating loss of club hairs in telogen phase. It usually happens as early as 2 months after sever fever. Stress can cause diffuse hair loss through different mechanisms. Stress related hair loss could be due to restriction of anagen phase, induction of telogen phase, or a combination of both. Hair shedding could be seen as early as 2 weeks after the stressful event and is usually reversible.

Many medications have been blamed for hair loss. Medications that are used for chemotherapy halt hair growth by suppressing anagen phase. Most other medications cause hair loss because of inducing a telogen effluvium.

The mechanism of male pattern baldness is through shortening of anagen cycles progressively in genetically susceptible follicles. The hair follicle gradually becomes miniaturized and during this process it gets replaced by thinner and lighter ones. Knowing hair growth cycle can help us understand why we lose hair every day, when this hair loss is not normal and what we can do about it.

Authors: Parsa Mohebi, M.D. and William Rassman, M.D.

Dr. Parsa Mohebi is a staff surgeon at the New Hair Institute. His goal for hair transplantation is to restore hair in a natural style using the most modern surgical approach. He did his internship in University of North Dakota followed by residency in surgery in University of New Mexico and York Hospital.

Dr. Mohebi performed research into on wound healing and hair growth at the Department of Surgical Sciences at Johns Hopkins Medical Center. The research project focused on growth factors and gene therapy for improvements in wound healing. Included in the research were projects that were applicable for use in hair restoration. He is enthusiastic at the potential for scarless surgery and in the techniques for revising surgical scars. Dr. Mohebi is an editor on baldingblog.com.

To get answers to your hair loss questions visit http://www.baldingblog.com or check http://www.newhair.com to get more info on hair restoratioin techniques.