Though humans no longer make use of hair for protection, heat retention, or camouflage, it still remains a very important means by which individuals display and are recognized. Appropriate appearance and grooming are still very important in social organization and the human relationships.
The human body contains approximately five million hair follicles while the scalp (prior to any kind of hair loss) contains 100,000 - 150,000 hair follicles. Blondes have the greatest number of scalp follicles, followed by brunettes. Humans with red hair have the fewest number of scalp follicles. The normal growth rate of scalp hair is one-fourth to one-half inch per month.
THE NORMAL HAIR GROWTH CYCLE
It is important to understand the normal hair growth cycle to understand why hair loss occurs. The hair follicle is an anatomical structure which evolved to produce and extrude (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". Three phases of the cycle exist: Anagen= growth phase; Catagen=degradation phase; Telogen= resting phase. Periods of growth (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 begins to emerge. 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 regrow. Not 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 the most important control factors of human hair growth. Androgens must be present for the growth of 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. Hamilton and later Norwood have classified the patterns of MPHL (See Norwood-Hamilton Scale). 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. Beard growth and male pattern hair loss are dependent on dihydrotestosterone (DHT). Testosterone is converted to DHT by the enzyme, 5a -reductase. Finasteride (Propecia) acts by blocking this enzyme and 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 (see "The Normal 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. The hair shafts in MPHL become progressively miniaturized, smaller in diameter and length, with time. In men with MPHL all the hairs in an affected area may eventually (but not necessarily) become involved in the process and may with time cover the region with fine (vellus) hair. Pigment (color) production is also terminated with miniaturization so the fine hair becomes lighter in color. The lighter color, 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 and the gene can be inherited from either the mother or father’s side. There is a common myth that inheritance is only from the mother’s side. This is not true.
In summary, male pattern hair loss (Androgenetic Alopecia) is an inherited condition manifested when androgens are present in normal amounts. The gene can be inherited from the mother or father’s side. The onset, rate, and severity of hair loss are unpredictable. The severity increases with age and if the condition is present it will be progressive and relentless.
FEMALE PATTERN HAIR LOSS (Androgenetic Alopecia)
Female pattern hair loss (FPHL) differs from male pattern hair loss (MPHL) in the following 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, i.e., 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. Like in men, the entire top of the scalp is the area of risk. In women there is generally a diffuse thinning throughout the area as opposed to thinning in the crown of men. Ludwig has classified hair loss in women into three classes. (See Ludwig Classification) 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. The 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 prehormones are 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. In women, the process of miniaturization of the hair follicle is more random with some hair being 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 resulting in fewer hairs.
If the cause of hair loss is suspected to be abnormally elevated or decreased amounts of hormones the patient should undergo laboratory tests to measure hormone levels