All humans are born with a finite number of hair follicles. The diameters of the individual hairs in our follicles increase as we grow from infancy to adulthood; we never grow any more hair follicles. The human body contains approximately five million hair follicles. Prior to any kind of hair loss the scalp contains 100,000-150,000 hair follicles. Blondes have the greatest number of scalp follicles, followed by brunettes. People with red hair have the fewest number of scalp follicles. Scalp hair normally grows one-half inch per month.

The Normal Hair Growth Cycle

To understand why hair loss occurs, you must first understand the normal growth cycle. The hair follicle is an anatomical structure that evolved to produce and push out a hair shaft. Hair is made up of proteins called keratins. Human hair grows in a continuous cyclic pattern of growth and rest known as the "hair growth cycle," which consists of three phases: Anagen= growth phase; Catagen = degradation phase; Telogen= resting phase. Periods of growth lasting (anagen) between two and eight years are followed by a brief period, two to four weeks, in which the follicle is almost totally degraded (catagen). The resting phase (telogen) then begins and lasts two to four months. Shedding of the hair occurs only after the next growth cycle (anagen) begins and a new hair shaft then begins to emerge in its place. On average, 50-100 telogen hairs are shed every day. This is normal hair loss and accounts for the hair loss seen every day in the shower and with hair combing. These hairs will be replaced by new growth. No more than 10 percent of the follicles are in the resting phase (telogen) at any time. A variety of factors can affect the hair growth cycle and cause temporary or permanent hair loss (alopecia), including medication, radiation, chemotherapy, exposure to chemicals, hormonal and nutritional factors, thyroid disease, generalized or local skin disease, and stress.

Androgens (testosterone, dihydrotestosterone) are hormones, and are the most important control factors of human hair growth. Androgens must be present for the growth of axillary beard, axillary (underarm), and pubic hair. Growth of scalp hair is not androgen-dependent, but androgens are necessary for the development of male and female pattern hair loss.

Male Pattern Hair Loss (Androgenetic Alopecia)

It is estimated that 35 million men in the United States are affected by androgenetic alopecia. "Andro" refers to the androgens (testosterone, dihydrotestosterone) necessary to produce male-pattern hair loss (MPHL). "Genetic" refers to the inherited gene necessary for MPHL to occur. In men who develop MPHL, the hair loss may begin any time after puberty when blood levels of androgens rise. The first change is usually recession in the temporal areas, which is seen in 96 percent of mature Caucasian males, including those men not destined to progress to further hair loss. Dr. Hamilton and later Dr. O'tar Norwood have classified the patterns of MPHL. Although the density of hair in a given pattern of loss tends to diminish with age, there is no way to predict what pattern of hair loss a young man with early MPHL will eventually assume. In general, those who begin losing hair in the second decade are those in whom the hair loss will be the most severe. In some men, initial male-pattern hair loss may be delayed until the late third to fourth decade. It is generally recognized that men in their 20's have a 20 percent incidence of MPHL, in their 30's a 30 percent incidence of MPHL, in their 40's a 40 percent incidence of MPLH, etc. Using these numbers, one can see that a male in his 90's has a 90 percent chance of having some degree of MPHL.

Hamilton first noted that androgens (testosterone, dihydrotestosterone) are necessary for the development of MPHL. The amount of androgens present does not need to be greater than normal for MPHL to occur. If androgens are present in normal amounts and the gene for hair loss is present, male pattern hair loss will occur. Axillary (under arm) and pubic hair are dependent on testosterone for growth. But beard growth and male pattern hair loss are dependent on the testosterone derivative dihydrotestosterone (DHT). Testosterone is converted to DHT by the enzyme, 5¤ -reductase. Finasteride (Propecia®) acts by blocking this enzyme and, therefore, decreasing the amount of DHT. Receptors exist on cells that bind androgens. These receptors have the greatest affinity for DHT, followed by testosterone, estrogen, and progesterone. After binding to the receptor, DHT goes into the cell and interacts with the nucleus of the cell, altering the production of protein by the DNA in the nucleus of the cell. Ultimately, growth of the hair follicle ceases.

The hair growth cycle is affected in that the percentage of hairs in the growth phase (anagen) and the duration of the growth phase diminish, resulting in shorter hairs. More hairs are in the resting state (telogen) and these hairs are much more subject to loss with the daily trauma of combing and washing. With time, the hair shafts in MPHL become progressively miniaturized, smaller in diameter and length. In men with MPHL all the hairs in an affected area may eventually, but not necessarily affected by this miniaturization process. With time, miniaturization progresses to a point where the affected areas are covered with fine (vellus) hair. Pigment (color) production is also terminated with miniaturization, so the fine hair becomes lighter in color. The lighter colored, miniaturized hairs cause the area to first appear thin. Involved areas in men can completely lose all follicles over time.

MPHL is an inherited condition. There is a common myth that inheritance is only from the mother's side. This is not true; the gene can be inherited from either the mother's or father's side.

Female Pattern Hair Loss (Androgenetic Alopecia)

Female pattern hair loss (FPHL) differs from male pattern hair loss (MPHL) in a number of ways. It is more likely to be noticed later than in men, in the late twenties through early forties. It is likely to be seen at times of hormonal change, including the use of birth control pills, after childbirth, around the time of menopause, and after menopause. Recession at the temples is less likely than in men and women tend to maintain the position of their hairlines. In women, hair loss generally occurs as a diffuse thinning throughout the scalp. In men, hair loss is generally not diffuse, but rather a thinning in the crown. Ludwig has classified hair loss in women into three classes. The vast majority of women affected fall into the Ludwig I class.

In the United States it is estimated that 21 million women are affected by FPHL. The incidence in women has been reported to be as low as eight percent and as high as 87 percent. It does appear to be as common in women as in men. Hair loss in women becomes particularly notable in menopause.
Androgens are responsible for hair loss in women by the same mechanisms they cause hair loss in men. Women do produce small amounts of androgens by way of the ovaries and adrenal glands. Also, pre-hormones produced by the ovaries that are converted to androgens outside of the ovaries or adrenal glands. Women rarely experience total loss of hair in an area if the loss is due to FPHL. If they do, they should be evaluated for an underlying pathological (disease) condition that may be causing the hair loss. In women, the process of miniaturization of the hair follicle is more random, with some hair remaining unaffected. Normal thick hairs are mixed with finer, smaller diameter hairs. The end result is a visual decrease in density of hair rather than total loss of hair. The hair growth cycle is affected as in men. The growth phase (anagen) is shortened, resulting in shorter hairs, and the resting phase (telogen) is increased, which results in fewer hairs.

If the cause of hair loss is suspected to be abnormally elevated or decreased amounts of hormones, women should undergo laboratory tests to measure hormone levels.

OTHER CAUSES OF HAIR LOSS

Alopecia Areata

Alopecia areata (AA) is a recurrent disease that can cause hair loss in any hair-bearing area. The most common type of AA presents as round or oval patches of hair loss, most noticeably on the scalp or in the eyebrows. The hair usually grows back within 6 months to one year. Most patients will suffer episodes of hair loss in the same area in the future. Those who develop round or oval areas of hair loss can progress to loss of all scalp hair (alopecia totalis). The cause of AA is unknown, but at present is theorized to be an autoimmune disorder in which the body does not recognize the hair follicles and attacks them as if they were foreign "invaders." Stress and anxiety may be linked to this hair loss. The most common treatment is the use of steroids (cortisone is one form) either topically or by injection. The outcome of treatment is good when the AA process is present less than one year and poor, especially in adults, if the disease has been present for longer periods of time. Minoxidil (Rogaine®) can help to regrow hair. Surgical treatment for this disorder is not recommended.

Traction Alopecia

Traction alopecia is caused by chronic traction (pulling) on the hair follicle and is seen most commonly in African-American females who wear tight braiding or cornrow hair styles. It is generally present along the hairline. Men who attach hairpieces to their existing hair can experience this type of permanent hair loss if the hairpiece is attached in the same location over a long period of time.

Trichotillomania is a traction alopecia related to a compulsive disorder caused when patients pull on and pluck hairs, often creating bizarre patterns of hair loss. In long term case of trichotillomania, permanent hair loss can occur.

Scarring Alopecias

Diseases that cause permanent hair loss do so when scar tissue replaces destroyed normal tissue. They include:
1. Lupus Erythematosus- occurs more frequently in females than in males and is more common in adults than in children.
2. Scleroderma- hair loss tends to be slowly progressive.
3. Infectious Agents- Bacterial folliculitis, fungal infections, herpes zoster.

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