Androgenetic alopecia
Encyclopedia
Androgenic alopecia is the most common cause of hair loss
and thinning in human
s. Variants appear in both men and women. Androgenic alopecia also occurs in chimpanzee
s, and orangutan
s. In humans, this condition is also commonly known as male pattern baldness. In classic pattern baldness, hair is lost in a well-defined pattern, beginning above both temples
. Hair also thins at the crown of the head. Often a rim of hair around the sides and rear of the head is left. This type of pattern is dubbed "Hippocratic balding" and may rarely progress to complete baldness. Women do not suffer classic male pattern baldness, instead the hair becomes thinner around the whole scalp, and the hairline does not recede. This is dubbed "female pattern baldness" and may occur in males. This variety of androgenic alopecia
in women rarely leads to total baldness.
s, particularly dihydrotestosterone
(DHT). Androgens are important for normal male sexual development before birth and during puberty. Androgens also have other important functions in both males and females, such as regulating hair growth and sex drive.
Male pattern baldness is caused by a genetic sensitivity of hair follicles to DHT. This hormone causes follicles to shrink or "miniaturize". In turn, this shortens their lifespan and prevents them from producing hair normally.
Recently the existing theories have been challenged on the ground that while the androgens in question are responsible for hair growth on the face and all over the body of men, hair loss only occurs at the top of the scalp. For example, it has been suggested that androgenic alopecia is a consequence of the anabolic effect of androgens such as hormonal changes leading to structural changes in skin and scalp which in turn cause hair loss.
Large studies in 2005 and 2007 stress the importance of the maternal line in the inheritance of male pattern baldness. German researchers name the androgen receptor gene as a necessary condition for balding. They conclude that a specific variant of the androgen receptor is needed for AGA to develop. This study has been confirmed by other researchers. This gene is recessive. Thus, a female would need two X chromosomes with the defect to show typical male pattern alopecia. Since androgens and their interaction with the androgen receptor are the cause of AGA, the androgen receptor gene plays an important part in its development.
Other research in 2007 suggests another gene on the X chromosome, that lies close to the androgen receptor gene, is an important gene in male pattern baldness. Researchers found the region Xq11-q12 on the X-chromosome to be strongly associated with AGA in males. They point at the EDA2R gene as the gene that is mostly associated with AGA.
Other genes involved with hair loss have been found, one of them being an allele on chromosome 3. The allele is located at 3q26. This allele is recessive.
There are also genes that are involved in hair loss, although not male pattern baldness per se. In particular, three genes have been shown to both affect hair texture and also cause baldness in some people. One of these is P2RY5. Mutations in this gene affects hair structure "woolly hair". Certain variants can lead to baldness.
In May 2009, researchers in Japan
identified a gene, SOX21
, a Y-linked gene that may be be responsible for some hair loss conditions in humans.
, higher levels of unbound/free testosterone, and higher levels of total free androgens including DHT.
5-alpha-reductase is responsible for converting free testosterone into DHT. The genes for 5-alpha-reductase are known. The enzymes are present predominantly in the scalp and prostate. Levels of 5-alpha-reductase are one factor in determining levels of DHT in the scalp. Drugs which interfere with 5-alpha-reductase (such as finasteride
, which inhibits the predominant type 2 isoform) have been approved by the FDA as treatments for hair loss.
Sex hormone binding globulin (SHBG), which is responsible for binding testosterone and preventing its bioavailability and conversion to DHT, is typically lower in individuals with high DHT. SHBG is downregulated by insulin
.
Increased levels of Insulin Growth Factor-1 (IGF-1) have been correlated to vertex balding.
High insulin levels seem the likely link between metabolic syndrome
and baldness. Low levels of SHBG in men and non-pregnant women are also correlated with glucose intolerance and diabetes risk, though this correlation disappears during pregnancy.
after World War II
. Pattern baldness (androgenic alopecia) was either rare or non-existent among hunter-gatherer
societies.
One study did show that free testosterone is lower 24 hours after intense aerobic exercise in men who already have high endurance but it was not investigated whether that level remains lowered beyond that point, or whether that lowering affects male pattern baldness in any way. It has been suggested that weight training may have a detrimental effect on hair by increasing testosterone levels; however, there is at least one study that indicates a decline in free testosterone as a result of weight training.
In a study by Muscarella and Cunnhingham, males and females viewed six male models with different levels of facial hair (beard and mustache or none) and cranial hair (full head of hair, receding and bald). Participants rated each combination on 32 adjectives related to social perceptions. Males with facial hair and those with bald or receding hair were rated as being older than those who were clean shaven or had a full head of hair. Beards and a full head of hair were seen as being more aggressive and less socially mature, and baldness was associated with more social maturity. A review of social perceptions of male pattern baldness has been provided by Henss (2001).
The assertion that male pattern baldness is intended to convey a social message is supported by the fact that pattern baldness is common in other primates and is often used to convey increased status and maturity. Gorillas have anatomically enlarged foreheads for this reason. This suggests that baldness may have happened to enhance the apparent size of the forehead and increase the area of the face to be displayed.
It should also be noted that most ancestral primates had a shorter life-span, so there was no significant baldness in the society. As the life expectancy continuously increases, baldness remains uneliminated.
Other hypotheses include genetic linkage to beneficial traits unrelated to hair loss and genetic drift
.
Differential diagnosis involves eliminating other causes of hair loss (such as poisoning) and comparing the pattern of hair loss to a typical male pattern baldness progression.
In most cases androgenic alopecia may be confirmed by trichoscopy
.
It can be hard to know the diagnosis for sure without a biopsy.
Regrowth of lost hair in the early stages of male pattern baldness can be fully restored with treatment. However, bald spots where hair follicles have been destroyed cannot be restored with non-invasive treatments and require a hair transplant to achieve coverage.
Baldness
Baldness implies partial or complete lack of hair and can be understood as part of the wider topic of "hair thinning". The degree and pattern of baldness can vary greatly, but its most common cause is male and female pattern baldness, also known as androgenic alopecia, alopecia androgenetica or...
and thinning in human
Human
Humans are the only living species in the Homo genus...
s. Variants appear in both men and women. Androgenic alopecia also occurs in chimpanzee
Chimpanzee
Chimpanzee, sometimes colloquially chimp, is the common name for the two extant species of ape in the genus Pan. The Congo River forms the boundary between the native habitat of the two species:...
s, and orangutan
Orangutan
Orangutans are the only exclusively Asian genus of extant great ape. The largest living arboreal animals, they have proportionally longer arms than the other, more terrestrial, great apes. They are among the most intelligent primates and use a variety of sophisticated tools, also making sleeping...
s. In humans, this condition is also commonly known as male pattern baldness. In classic pattern baldness, hair is lost in a well-defined pattern, beginning above both temples
Temple (anatomy)
Temple indicates the side of the head behind the eyes. The bone beneath is the temporal bone as well as part of the sphenoid bone.-Anatomy:Cladists classify land vertebrates based on the presence of an upper hole, a lower hole, both, or neither in the cover of dermal bone which formerly covered the...
. Hair also thins at the crown of the head. Often a rim of hair around the sides and rear of the head is left. This type of pattern is dubbed "Hippocratic balding" and may rarely progress to complete baldness. Women do not suffer classic male pattern baldness, instead the hair becomes thinner around the whole scalp, and the hairline does not recede. This is dubbed "female pattern baldness" and may occur in males. This variety of androgenic alopecia
Alopecia
Alopecia means loss of hair from the head or body. Alopecia can mean baldness, a term generally reserved for pattern alopecia or androgenic alopecia. Compulsive pulling of hair can also produce hair loss. Hairstyling routines such as tight ponytails or braids may induce Traction alopecia. Both...
in women rarely leads to total baldness.
Cause
A variety of genetic (and possibly environmental) factors apparently play a role in androgenic alopecia. Although researchers have long studied the factors that may contribute to this condition, many remain unknown. Minimally, pattern hair loss is related to hormones called androgenAndrogen
Androgen, also called androgenic hormone or testoid, is the generic term for any natural or synthetic compound, usually a steroid hormone, that stimulates or controls the development and maintenance of male characteristics in vertebrates by binding to androgen receptors...
s, particularly dihydrotestosterone
Dihydrotestosterone
Dihydrotestosterone is an androgen or male sex hormone. The enzyme 5α-reductase synthesises DHT in the prostate, testes, hair follicles, and adrenal glands...
(DHT). Androgens are important for normal male sexual development before birth and during puberty. Androgens also have other important functions in both males and females, such as regulating hair growth and sex drive.
Male pattern baldness is caused by a genetic sensitivity of hair follicles to DHT. This hormone causes follicles to shrink or "miniaturize". In turn, this shortens their lifespan and prevents them from producing hair normally.
Recently the existing theories have been challenged on the ground that while the androgens in question are responsible for hair growth on the face and all over the body of men, hair loss only occurs at the top of the scalp. For example, it has been suggested that androgenic alopecia is a consequence of the anabolic effect of androgens such as hormonal changes leading to structural changes in skin and scalp which in turn cause hair loss.
Hair loss and genetics
Much research concerns the genetic component of male pattern baldness, or androgenic alopecia (AGA). Research indicates that susceptibility to premature male pattern baldness is largely X-linked, which means it is linked to genes on an X-chromosome. Other genes that are not sex linked are also involved, however. Men whose fathers had experienced hairloss were 2.5 times more likely to experience hairloss themselves, regardless of the mother's side of the family.Large studies in 2005 and 2007 stress the importance of the maternal line in the inheritance of male pattern baldness. German researchers name the androgen receptor gene as a necessary condition for balding. They conclude that a specific variant of the androgen receptor is needed for AGA to develop. This study has been confirmed by other researchers. This gene is recessive. Thus, a female would need two X chromosomes with the defect to show typical male pattern alopecia. Since androgens and their interaction with the androgen receptor are the cause of AGA, the androgen receptor gene plays an important part in its development.
Other research in 2007 suggests another gene on the X chromosome, that lies close to the androgen receptor gene, is an important gene in male pattern baldness. Researchers found the region Xq11-q12 on the X-chromosome to be strongly associated with AGA in males. They point at the EDA2R gene as the gene that is mostly associated with AGA.
Other genes involved with hair loss have been found, one of them being an allele on chromosome 3. The allele is located at 3q26. This allele is recessive.
There are also genes that are involved in hair loss, although not male pattern baldness per se. In particular, three genes have been shown to both affect hair texture and also cause baldness in some people. One of these is P2RY5. Mutations in this gene affects hair structure "woolly hair". Certain variants can lead to baldness.
In May 2009, researchers in Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
identified a gene, SOX21
SOX21
Transcription factor SOX-21 is a protein that in humans is encoded by the SOX21 gene. It is a member of the Sox gene family of transcription factors.- Function :...
, a Y-linked gene that may be be responsible for some hair loss conditions in humans.
Hormone levels correlated with androgenic alopecia
Men with androgenic alopecia typically have higher levels of 5-alpha-reductase, lower levels of total testosteroneTestosterone
Testosterone is a steroid hormone from the androgen group and is found in mammals, reptiles, birds, and other vertebrates. In mammals, testosterone is primarily secreted in the testes of males and the ovaries of females, although small amounts are also secreted by the adrenal glands...
, higher levels of unbound/free testosterone, and higher levels of total free androgens including DHT.
5-alpha-reductase is responsible for converting free testosterone into DHT. The genes for 5-alpha-reductase are known. The enzymes are present predominantly in the scalp and prostate. Levels of 5-alpha-reductase are one factor in determining levels of DHT in the scalp. Drugs which interfere with 5-alpha-reductase (such as finasteride
Finasteride
Finasteride is a synthetic antiandrogen that inhibits type II 5-alpha reductase, the enzyme that converts testosterone to dihydrotestosterone...
, which inhibits the predominant type 2 isoform) have been approved by the FDA as treatments for hair loss.
Sex hormone binding globulin (SHBG), which is responsible for binding testosterone and preventing its bioavailability and conversion to DHT, is typically lower in individuals with high DHT. SHBG is downregulated by insulin
Insulin
Insulin is a hormone central to regulating carbohydrate and fat metabolism in the body. Insulin causes cells in the liver, muscle, and fat tissue to take up glucose from the blood, storing it as glycogen in the liver and muscle....
.
Increased levels of Insulin Growth Factor-1 (IGF-1) have been correlated to vertex balding.
High insulin levels seem the likely link between metabolic syndrome
Metabolic syndrome
Metabolic syndrome is a combination of medical disorders that, when occurring together, increase the risk of developing cardiovascular disease and diabetes. It affects one in five people in the United States and prevalence increases with age...
and baldness. Low levels of SHBG in men and non-pregnant women are also correlated with glucose intolerance and diabetes risk, though this correlation disappears during pregnancy.
Hair loss and lifestyle
While genetic factors seem to play the principal role in the development and progression of androgenic alopecia, lifestyle also plays a minor role as demonstrated by the vast increase in male and female pattern baldness in JapanJapan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
after World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
. Pattern baldness (androgenic alopecia) was either rare or non-existent among hunter-gatherer
Hunter-gatherer
A hunter-gatherer or forage society is one in which most or all food is obtained from wild plants and animals, in contrast to agricultural societies which rely mainly on domesticated species. Hunting and gathering was the ancestral subsistence mode of Homo, and all modern humans were...
societies.
One study did show that free testosterone is lower 24 hours after intense aerobic exercise in men who already have high endurance but it was not investigated whether that level remains lowered beyond that point, or whether that lowering affects male pattern baldness in any way. It has been suggested that weight training may have a detrimental effect on hair by increasing testosterone levels; however, there is at least one study that indicates a decline in free testosterone as a result of weight training.
Theories on male pattern baldness
One debatable theory, advanced by Muscarella and Cunningham, suggests baldness came about in males through sexual selection as an enhanced signal of aging and social maturity, whereby aggression and risk-taking decrease and nurturing behaviours increase. This may have conveyed a male with enhanced social status but reduced physical threat, which could enhance ability to secure reproductive partners and raise offspring to adulthood.In a study by Muscarella and Cunnhingham, males and females viewed six male models with different levels of facial hair (beard and mustache or none) and cranial hair (full head of hair, receding and bald). Participants rated each combination on 32 adjectives related to social perceptions. Males with facial hair and those with bald or receding hair were rated as being older than those who were clean shaven or had a full head of hair. Beards and a full head of hair were seen as being more aggressive and less socially mature, and baldness was associated with more social maturity. A review of social perceptions of male pattern baldness has been provided by Henss (2001).
The assertion that male pattern baldness is intended to convey a social message is supported by the fact that pattern baldness is common in other primates and is often used to convey increased status and maturity. Gorillas have anatomically enlarged foreheads for this reason. This suggests that baldness may have happened to enhance the apparent size of the forehead and increase the area of the face to be displayed.
It should also be noted that most ancestral primates had a shorter life-span, so there was no significant baldness in the society. As the life expectancy continuously increases, baldness remains uneliminated.
Other hypotheses include genetic linkage to beneficial traits unrelated to hair loss and genetic drift
Genetic drift
Genetic drift or allelic drift is the change in the frequency of a gene variant in a population due to random sampling.The alleles in the offspring are a sample of those in the parents, and chance has a role in determining whether a given individual survives and reproduces...
.
Diagnosis
The diagnosis of androgenic alopecia can be usually established based on clinical presentation in men. In women, the diagnosis usually requires more complex diagnostic evaluation.Differential diagnosis involves eliminating other causes of hair loss (such as poisoning) and comparing the pattern of hair loss to a typical male pattern baldness progression.
In most cases androgenic alopecia may be confirmed by trichoscopy
Trichoscopy
Trichoscopy is a method of hair and scalp evaluation and is used for diagnosing hair and scalp diseases. The method is based on dermoscopy and videodermoscopy. In trichoscopy hair and scalp structures may be visualized at many-fold magnification...
.
It can be hard to know the diagnosis for sure without a biopsy.
Treatments
While many people with male pattern baldness choose to accept the condition as they accepted their hair color or shape, there are baldness treatments which can manage and reduce or halt hair loss.Regrowth of lost hair in the early stages of male pattern baldness can be fully restored with treatment. However, bald spots where hair follicles have been destroyed cannot be restored with non-invasive treatments and require a hair transplant to achieve coverage.
See also
- DihydrotestosteroneDihydrotestosteroneDihydrotestosterone is an androgen or male sex hormone. The enzyme 5α-reductase synthesises DHT in the prostate, testes, hair follicles, and adrenal glands...
- Noncicatricial alopeciaNoncicatricial alopecia-Causes:Causes of noncicatricial alopecia include:*Alopecia areata*Anagen effluvium*Androgenetic alopecia*Dermatopathia pigmentosa reticularis*Telogen effluvium*Trichotillomania...
- Management of baldness
- Ludwig scaleLudwig scaleThe progression of female pattern baldness is generally classified on the Ludwig scale, which ranges from stages I to III.- External links :*...
- The scale which rates the severity of androgenic alopecia in females - Hamilton-Norwood scaleHamilton-Norwood scaleThe progression of male pattern baldness is generally classified on the Hamilton-Norwood scale, which ranges from stages I to VII.This measurement scale was first introduced by Dr. James Hamilton in the 1950s and later revised and updated by Dr. O'Tar Norwood in the 1970s....
- The scale which rates the severity of androgenic alopecia in males
External links
- NLM- Genetics Home Reference
- How Hair Replacement Works Covers oral medications, hair transplant surgery, and topical treatments.
- http://maxiarti.com/29196/male-pattern-baldness-genetics-what-can-you-do/ Male Pattern Baldness Genetics
- http://www.usatoday.com/money/companies/management/2008-03-13-bald-ceos_N.htm?loc=interstitialskip USA Today article examining the impact of baldness on career success.