Hair
From Free net encyclopedia
- Hair is also the name of a musical; see the stage production and the movie.
Image:Anderson Sophie Young Girl Fixing Her Hair.jpg Hair is a filamentous outgrowth skin found only in mammals. In some species it is absent at certain stages of life. It projects from the epidermis, though it grows from hair follicles deep in the dermis. So-called "hairs" (trichomes) are also found on plants. The projections on arthropods such as insects and spiders are actually insect bristles. The hair of non-human species is commonly referred to as fur. There are varieties of cats, dogs, and mice bred to have little or no visible hair.
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Structure
Chemical structure
Hair consists 90% of a biological polymer, α-keratin, and about 10% water, which modifies its mechanical properties. This α-helically coiled protein is further wound into supermolecular coiled-coil microfibrils, many of which are held together with a protein glue to form long macrofibrils, which are packed inside dead hair cells about 100 µm long by 3 µm across. Several of these associate to form one strand of hair, which is covered with tiny surface scales. The ends of individual keratin chains are high in the amino acids proline (an α-helix breaker) and cysteine. Adjacent keratin chains are held together by many disulfide bonds bridging their cysteines. These links are very robust; virtually intact hair has been recovered from ancient Egyptian tombs. Different parts of the hair have different cysteine levels, leading to harder or softer material.
Anatomical structure and physiology
Hair consists of an inner cortex, comprising spindle-shaped cells, and an outer sheath, called the cuticle. Within each cortical cell are the many fibrils, running parallel to the fibre axis, and between the fibrils is a softer material called the matrix. It grows from a hair follicle.
The cuticle is responsible for much of the mechanical strength of the hair fibre. It consists of scale-shaped layers. Human hair typically has 6-8 layers of cuticle. Wool has only one, and other animal hair may have many more layers. Hair responds to its environment, and to its mechanical and chemical history. For example, hair which is wetted, styled and then dried, acquires a temporary 'set', which can hold it in style. This style is lost when the hair gets wet again. For more permanent styling, chemical treatments (perms) break and re-form the disulphide links within the hair structure.
The diameter of a human hair ranges from about 18 µm to 180 µm. In people of European descent, blond hair and black hair are at the finer end of the scale, while red hair is the coarsest. The hair of people of Asian descent is typically coarser than the hair of other groups.
The cross-sectional shape of human hair is typically round in people of Asian descent, round to oval in European descent, and nearly flat in African peoples; it is that flatness which allows African hair to attain its frizzly form. In contrast, hair that has a round cross section will be straight. A strand of straight round cross-section hair that has been flattened, for example, with an edge of a coin, will curl up into a micro-afro.
The speed of growth is roughly 11 cm/yr = 0.3 mm/day = 3 nm/s. Cells at the base of the hair follicle divide and grow extremely rapidly.
With tensile strength of approximately 190 MPa[1], a single strand of human hair can hold approximately 100 g (3.5 oz) of weight, although this will vary greatly with thickness. Wet hair, however, is very fragile.
Functions
Image:Polar Bear 2004-11-15.jpg Hair serves a number of different functions. It provides insulation from the cold, and in some species from hot weather. It is generally pigmented, providing coloration, sometimes the same as the underlying skin. It often serves as camouflage, both for prey and predators. In some species the pigmentation changes with the seasons; e.g., becoming white during the snowy winter, and in cases even more rapidly than that with changes in background. Hair can also provide protection against abrasion, and head hair can buffer impacts to the skull. In some species, hair patterns can be a part of sexual dimorphism; e.g., the long manes of male lions.
Animal hair
The hair of non-human mammals is also known as pelage, fur or wool. Some of the various forms of hair that make up the pelage are guard hairs (such as the defensive spines of porcupines) and bristles (which make up the mane of lions). An interesting form of pelage has the polar bear, whose transparent and unpigmented hairs transport sunlight, particularly ultraviolet light, through the dense fur to the skin. Not all mammals have an intense fur, primarily many mammals living permanently within water as whales and dolphins.
Human hair
Lack of intense body hair
In contrast to most mammals, humans do not have an all-embracing, intense hair on their body. Several theories have been advanced to explain the bareness of human body hair. One suggests that nature selected humans for shorter and thinner body hair as part of a set of adaptations including bipedal locomotion and an upright posture. There are several problems with this savanna theory, not least of which is that cursorial hunting is used by (other) animals that do not show any thinning of hair, and that hair similar to chimpanzees and gorillas also shades the skin from radiant heat and protects it from hot winds, and thus another mechanism for heat loss is not required. Another problem is that bipedal locomotion apparently now predates hominids moving from a forest environment to a savanna environment. A more recent theory for human hair loss has to do with a possible period of bipedal wading in a salt marsh in the Danakil region of Ethiopia, possibly occurring in the hominid lineage between 5 and 7 million years ago. As a wading animal, it was more efficient to develop short body hair and a layer of subcutaneous fat for streamlining and insulation in the aquatic environment; the eccrine sweat glands developed later after the hominids left the water; see Aquatic ape hypothesis. One problem with this theory is that both chimpanzees and gorillas have the same density and distribution of the eccrine glands, but that they have not been developed for sweat production.
A third theory for the thin body hair on humans proposes that Fisherian runaway sexual selection played a role here (as well as in the selection of long head hair). Possibly this occurred in conjunction with neoteny, with the more juvenile appearing females being selected by males as more desirable; see types of hair and vellus hair. The human female body hair typically has more vellus hair (making the skin appear bare), while the male body typically has more terminal hair (especially on the chest, abdomen and back). Thus sexual selection can explain the sexual dimorphism in human body hair, with the results of selection being more evident (more extreme) in the female than in the male, a point which the other two theories cannot address without proposing substantially different behavior between males and females. Also, we see that artificially bare (shaved, etc.) legs, arms, etc. on women are seen as "sexy" even today, while body shaving is not nearly as common for men.
Types of hair
Image:Edward S. Curtis Collection People 043.jpg Humans have three different types of hair:
- Lanugo, the fine hair that covers nearly the entire body of embryos
- Vellus hair, the short, fine, "peach fuzz" body hair that grows in most places on the human body in both sexes
- Terminal hair, the fully developed hair, which is generally longer, coarser, thicker, and darker than vellus hair
Growth
The most noticeable part of human hair is the hair on the head, which can grow longer than on most mammals. Its absence is termed alopecia, commonly known as baldness. The other parts of the human body feature different types of hair. From childhood on vellus hair covers the entire human body regardless of sex or race except in the following locations: the lips, the nipples, the palms of hands, the soles of feet, certain external genital areas, the navel and other scar tissue. The density of the hairs (in hair follicles per square centimeter) vary from one person to another.
The rising level of male hormones (androgens) due to puberty causes a transformation process of vellus hair into terminal hair on several parts of the human body. The hair in these locations can be thus termed androgenic hair. The hair follicles respond to androgens, primarily testosterone and its derivatives. The rate of hair growth and the weight of the hairs increase. However, different areas respond with different sensitivities. As testosterone level increases, the sequence of appearance of androgenic hair reflects the gradations of androgen sensitivity. The pubic area is most sensitive, and heavier hair usually grows there first in response to androgens.
Areas on the human body that develop terminal hair growth due to rising androgens in both sexes, men and women, are the axillary hair and the pubic hair. In contrast to that, normally only men are growing androgenic hair in other areas. There is a sexual dimorphism in the amount and distribution of androgenic hair, with males having more terminal hair (particularly facial hair, chest hair, abdominal hair and hair on legs and arms) and females having more vellus hair, which is less visible. The genetic disposition determines the sex-dependant and individual rising of androgens and therefore the development of androgenic hair.
Increasing body hair on women following the male pattern can be referred to hirsutism. An excessive and abnormal hair growth on the body of males and females is defined as hypertrichosis. Considering an individual occurrence of body hair as abnormal does not implicitly depend on medical indications but also on cultural and social attitudes.
Individual hairs alternate periods of growth and dormancy. During the growth portion of the cycle, hair follicles are long and bulbous, and the hair advances outward at about a third of a millimeter per day. After three to six months, body hair growth stops (the pubic and armpit areas having the longest growth period). The follicle shrinks and the root of the hair rigidifies. Following a period of dormancy, another growth cycle starts, and eventually a new hair pushes the old one out of the follicle from beneath. Head hair, by comparison, grows for a long duration and to a great length before being shed. The rate of growth is approximately 1.25 centimeters, or about 0.5 inches, per month. Anthropologists speculate that the functional significance of long head hair may be adornment, a byproduct of secondary natural selection once other somatic hair had been lost. Another possibility is that long head hair is a result of Fisherian runaway sexual selection, where long lustrous hair is a visible marker for a healthy individual (with good nutrition, waist length hair—approximately 1 meter or 39 inches long—would take ~80 months or just under 7 years to grow), and this would explain why long head hair (in both sexes) is viewed as "sexy" even now.
Image:Old Hmong Man (Sapa Vietnam).jpg
Aging
Older people tend to develop grey hair because the pigment in the hair is lost and the hair becomes colorless. Grey hair is considered to be a characteristic of normal aging. The age at which this occurs varies from person to person, but in general nearly everyone 75 years or older has grey hair, and in general men tend to become grey at younger ages than women. People starting out with very pale blond hair usually develop white hair instead of grey hair when aging. Red hair usually doesn't turn grey with age; red hair usually turns a sandy color and then turns white after that. Some degree of scalp hair loss or thinning generally accompanies in both males and females, and it's estimated that half of all men are affected by male pattern baldness by the time they are 50. The tendency toward baldness is a trait shared by a number of other primate species, and is thought to have evolutionary roots.
It is commonly claimed that hair and nails will continue growing for several days after death. This is a myth; the appearance of growth is actually caused by the retraction of skin as the surrounding tissue dehydrates, making nails and hair more prominent.
Pathological impacts on hair
Drugs used in cancer chemotherapy frequently cause a temporary loss of hair, noticeable on the head and eyebrows, because they kill all rapidly dividing cells, not just the cancerous ones. Other diseases and traumas can cause temporary or permanent loss of hair, generally or in patches.
The hair shafts may also store certain poisons for years, even decades, after death. In the case of Col. Lafayette Baker, who died July 3, 1868, use of an atomic absorption spectrophotometer showed the man was killed by white arsenic. The prime suspect was Wally Pollack, Baker's brother-in-law. According to Dr. Ray A. Neff, Pollack had laced Baker's beer with it over a period of months, and a century or so later minute traces of arsenic showed up in the dead man's hair. Mrs. Baker's diary seems to confirm that it was indeed arsenic, as she writes of how she found some vials of it inside her brother's suitcoat one day.
Cultural attitudes
Head hair
Image:Hair.jpg The remarkable head hair of humans has gained an important significance in nearly any present society as well as any historical period throughout the world. The haircut played always and is still playing a significant cultural and social role.
Already in ancient Greece and ancient Rome men and women differed from each other through their diferrent haircut. The head hair of women was long and pulled back into a chignon. Many dyed their hair red with henna and sprinkled it with gold powder, often adorning it with fresh flowers. Men’s hair was short and even occasionally shaved. In Rome hairdressing became ever more popular and the upper classes were attended to by slaves or visited public barber shops. Image:Maasai tribe.jpg
The traditional hair styling in some parts of Africa gives also interesting examples how people coped with their head hair. The Maasai warriors tied the front hair into sections of tiny braids while the back hair was allowed to grow to waist length. Women and non-warriors however shaved their heads. Many tribes dyed the hair with red earth and grease; some stiffened it with animal dung.
Social and currently cultural conditions were always influencing the hair styling of its time. Before World War I men generally had longer hair and beards. The trench warfare between 1914 and 1918 exposed men to lice and flea infestations, which caused the order for hair to be cut short, establishing a norm that has persisted.
Body hair
The attitudes towards hair on the human body also vary between different cultures and times. In some cultures excessive chest hair of men is a symbol of virility and masculinity, other societies display a hairless body as a sign of youthfulness. Mostly women had to be hairless as a sign of their virginity.
In ancient Egypt the people regarded a completely smooth, hairless body as the standard of beauty. An upper class Egyptian woman took great pains to ensure that she did not have a single hair on her body, except for the top of her head. The ancient Greeks later adopted this smooth ideal. They also considered a hairless body to be representative of youth and beauty, what can be seen on Greek female sculptures, which do not display any pubic hair. The Islamic world also has a long tradition of hair removal from the Middle Ages until today.
In Western societies it became a public trend during the late twentieth century, particularly for women, to reduce or to remove their body hair. The bikini fashion as well as the sex image in advertising and movies are major reasons for this development. This trend, distributed by the media, has begun in the United States and is becoming ever more popular throughout other Western countries. It is increasingly affecting also men. Shaving or trimming male body hair is sometimes jokingly called "manscaping".
Hair as business factor
Hair care for humans is a major world industry with specialized tools, chemicals and techniques. The business with various products connecting the human hair has become an important industrial and financial factor in Western societies.
See also
- Xie Qiuping
- Facial hair
- Chest hair
- Abdominal hair
- Pubic hair
- Hypertrichosis
- Hirsutism
- Baldness
- Depilation
- Widow's peak
- Cowlick
- Social role of hair
- Blond
- Brunette
- Red hair
- Trichophilia
- Trichotillomania
- Trichology
- Hair care
References
- Template:Note"Uncovering the bald truth about hair loss." Springfield News-leader, May 10, 2005. "Half of men" estimate is made by the American Academy of Dermatology and specifically estimates prevalence in the U.S. population, though this should reflect prevalence in other populations.
External links
- Discussion about shaving and cultures
- Asian Hair This website discusses the differences in Asian hair.
- Hair disorders from the Cleveland Clinic
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