By far the most common cause of hair loss in men is androgenetic alopecia, also referred to as “male pattern hair loss” or “common” baldness. It is due to the male hormone dihydrotestosterone (DHT) acting on genetically-susceptible scalp hair follicles that causes them to become progressively smaller and eventually disappear. This process is called “miniaturization.”
This sensitivity to DHT is characteristic of hair follicles that reside in the front, top, and crown of the scalp — rather than the back and sides — producing a characteristic and easily identifiable pattern. This pattern, described by Norwood in his widely used Norwood Classification, typically begins with recession of the hairline at the temples and thinning of the crown. It may progress to total baldness, leaving just a wreath of hair around the back and sides of the scalp.
DHT is formed by the action of the enzyme 5-alpha reductase on testosterone, the hormone that causes sex characteristics in men. DHT causes male hair loss by shortening the growth, or anagen, phase of the hair cycle, causing miniaturization (decreased size) of the follicles, and producing progressively shorter, finer hairs. Eventually these hairs totally disappear (see image below).
In the following patient, we see a close-up of the side of his scalp where the hair is not affected by DHT. We see mostly groups of full thickness hairs (called terminal hairs) and a few scattered fine, vellus hairs, normally seen in a donor area. The pointer (left) indicates the location on the scalp in the close-up view.
The hairs, while still present on the scalp, are so much finer in diameter than the patient’s original hair that they give the visual appearance of thinning.
Example 3 – Extensive Miniaturization
In the region that is balding below (circle in the center of the scalp), there is extensive miniaturization and only very little loss of actual hairs. However, due to the dramatic thinning of the hair diameter, there is an appearance of almost complete balding in the area despite the presence of fine hair.
What these examples show is that, particularly in the early stages, the appearance of balding is due to the progressive decrease in hair shaft size caused by the miniaturizing effects of DHT, rather than the actual loss of hair.
This is the reason why hair loss medications, such as finasteride (Propecia) or minoxidil (Rogaine), which can reverse the miniaturization process, work well in early hair loss and why, with early hair loss, a surgical procedure is usually not necessary. If medications fail to restore enough hair, or if a patient chooses not to use medical therapy, then a hair transplant may be considered. Certainly, if an area is completely bald, medications will not re-grow hair and surgery can be considered as a first option.
Androgenetic Hair Loss: Genes, Hormones, Age
The expression of Androgenetic hair loss is affected by three interdependent factors: genes, hormones, and age:
Genes
It is frequently stated that “hair loss comes from the mother’s side of the family.” The truth is that baldness can be inherited from either parent. However, factors on the x-chromosome have been shown to influence hair loss, making the inheritance from the maternal side of the family slightly more important than the paternal one.
Recent research suggests that the genetics in male pattern hair loss is more complex than originally thought. The identification of an androgen receptor gene (AR) on the x-chromosome helps to explain why the hair loss pattern of a man resembles his maternal grandfather more often than his father. However, this is clearly not the whole story since a direct inheritance of baldness from the father is observed as well. An autosomal (non-sex) linked gene would explain this type of transmission – but this gene has not yet been found.
By the way of background, a gene is a single bit of chemically encoded hereditary instruction that is located on a chromosome and actually represents a tiny segment of DNA. Chromosomes occur in pairs (humans have 23 pairs), and every individual gets one set of chromosomes from each parent. Hair loss in men is now felt to involve more than one gene. When several genes govern a trait, it is called polygenic.
Genes that are located on the X or Y-chromosomes are call sex-linked. Genes on the other 22 pairs of chromosomes are called autosomal. It is felt that the genes governing common baldness are both sex-linked and also autosomal (not sex linked). This means that the baldness trait can be inherited from the mother’s side of the family or the father’s side, but not necessarily with equal frequency.
The term, “dominant” means that only one gene of a pair is needed for the trait to show up in the individual. A “recessive” gene means that both genes need to be present in order for the trait to be expressed. The most important genes involved in balding from androgenetic alopecia are felt to be dominant.
Just because one has the genes for baldness, it doesn’t mean the trait will manifest itself. The ability of a gene to affect one’s characteristics, i.e. be visible in a particular individual, is called “expressivity”. Gene expression is related to a number of factors, the major ones being hormones and age, although stress and other factors can be reasons for hair loss in some individuals.
It is of interest that, although genes for some types of hair loss have been mapped, the genes responsible for male pattern baldness have yet to be fully identified. This suggests that any kind of genetic engineering to prevent common baldness is still many years away.
Hormones
Hormones are biochemical substances that are made in various glands throughout the body. These glands secrete their products directly into the bloodstream so that the chemical they make is spread throughout the body. These chemicals are very powerful so that only minute amounts of them have profound effects upon the body.
The major male sex hormone is called testosterone. Testosterone and other related hormones that have masculinizing effects are made primarily in the testicles; therefore, the hormonal levels that are seen in adults do not reached significant levels until the testicles develop and enlarge during puberty. In fact, these same hormones are the cause of many of the changes that occur in puberty; growth of phallus and scrotum, sperm production, development of a sex drive, change in the voice, Read about the discovery of the relationship between testosterone and hormonally-induced hair lossgrowth of axillary and pubic hair, development of an adult aroma in the sweat, increase in bone and muscle mass, and change in the basic body shape.
These same hormones that cause acne and beard growth can also signal the beginning of baldness. The presence of androgens; testosterone, and its related hormone DHT, cause some follicles to regress and die. In addition to the testicles, the adrenal glands located above each of our kidneys, produce androgenic hormones, and this would be similar in both sexes. In females, the ovaries are an additional source of hormones that can affect hair.
The hormone felt to be directly involved in androgenetic alopecia is actually dihydrotestosterone (DHT) rather than testosterone. DHT acts by binding to special receptor sites on the cells of the hair follicles to cause the specific changes associated with balding. DHT is formed by the action of the enzyme 5-a reductase on testosterone. In men, 5-a reductase activity is higher in the balding area. This helps to explain the reason for the patterned alopecia that males experience. The enzyme 5-a reductase actually comes in two forms, Type 1 and Type 2. The enzyme 5-alpha reductase Type II, the predominant form in hair follicles, is blocked by the hair loss medication finasteride (Propecia).
DHT decreases the length of the anagen (growing) cycle, and increases the telogen (resting) phase, so that with each new cycle the hair shaft becomes progressively smaller. In addition, DHT causes the bi-temporal reshaping of hairline seen as adolescents enter adulthood, as well as patterned baldness (androgenetic alopecia). DHT also causes prostate enlargement in older men and adolescent and adult acne.
It is interesting that testosterone effects axillary and pubic hair, whereas DHT effects beard growth, hair on torso and limbs, patterned baldness and the appearance of hair in the nose and ears (something that older men experience). Scalp hair growth, however, is not androgen dependent, only scalp hair loss depends on androgens.
Age
The presence of the necessary genes and hormones is not alone sufficient to cause baldness. Even after a person has reached puberty, susceptible hair follicles must continually be exposed to the hormone over a period of time for hair loss to occur. The age at which these effects finally manifest themselves varies from one individual to another and is related to a person’s genetic composition and to the levels of testosterone in the bloodstream.
There is another time factor that is poorly understood. Male hair loss does not occur all at once or in a steady, straight-line progression. Hair loss is characteristically cyclical. People who are losing their hair experience alternating periods of slow and rapid hair loss and even stability. Many of the reasons that hair loss rates speed up and speed down are unknown, but we do know that with age, a person’s total hair volume will usually decrease.
Even when there is no predisposition to genetic balding, as a patient ages, some hairs randomly begin to miniaturize (shrink in length and width) in each follicular unit. As a result, each group will contain both of full terminal hairs and miniaturized hairs (similar to the very fine hairs that occur on the rest of the body and are clinically insignificant) making the area look less full. Eventually, the miniaturized hairs are lost, and the actual follicular units are reduced in number. In all adult patients, the entire scalp undergoes this aging process so that even the donor zone is not truly permanent, but will gradually thin, to some degree, over time. Fortunately, in most people, the donor zone retains enough permanent hair that hair transplantation is a viable male hair restoration procedure even for a patient well into his 70s.
