Hair
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Wikipedia-Article "Hair"
- Hair is also the name of a musical; see the stage production and the movie.
Hair is a filamentous outgrowth of the 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 follicles deep in the dermis. So-called "hairs" (trichomes) are also found on plants. The projections on insects and spiders are actually 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.
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.
In modern Western societies it is considered masculine for men to maintain the naturally thicker hair on their faces, arms, chests, backs, buttocks and legs, but the hair growing from the top of the head is generally kept relatively short. By contrast, it is considered feminine, for women to have little or no hair on their bodies, including pubic hair, but to let it grow long on the tops of their heads. 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. Hair care for humans is a major world industry with specialized tools, chemicals and techniques. In most societies, people style or adorn their hair for aesthetic reasons and often have it cut or removed by shaving or other means. In some, women usually shave their legs, armpits and sides of the pubic area, and shape their eyebrows.
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Human hair
Typically, humans have the longest hair on the top of the head, with shorter hair on the eyelids and eyebrows. The axillary (armpit) hair and pubic hair serves as lubrication during rubbing.
Sometimes, the term body hair is used, to distinguish it from hair on the head. 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 axillary 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 by-product of secondary natural selection once other somatic hair had been lost.
Hair grows from all areas of the skin on humans 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. Some people seem to have less body and facial hair than others, but in fact have shorter and finer body hair while the total number of follicles is relatively constant.
Several theories have been advanced to explain the unique features of human 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 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 possibly 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 proposes that sexual selection played a role, possibly in conjunction with neoteny, with the more juvenile appearing females being selected by males as more desirable; see types of hair and vellus hair. This would also explain the sexual dimorphism in human body hair. At this point the evidence is inconclusive as to the cause of the unique features of human hair.
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.
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 thinner end of the scale, while red hair is the thickest. The hair of people of Asian descent is typically thicker in diameter 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 = 300 nm/s. Cells at the base of the hair follicle divide and grow extremely rapidly. 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.
Hair is strong. A single strand can hold 100g (3.5oz) of weight. A head of hair could support 12 tonnes. It is equivalent in strength to aluminium or Kevlar. Wet hair, however, is very fragile.
Types of hair
On most adult humans there are two main types of hair: terminal hair, and vellus hair. A third type, lanugo hair, is present in the fetus, and some newborn babies. It can also be seen on the bodies of those who are extremely emaciated.
Terminal hair grows thick and long, and is what grows on the head, armpits and pubic area, as well as on the face, chest, arms and legs (better evident in men).
Vellus hair is a very soft and short hair that grows most places in the body in both sexes. In Caucasians it is often colourless, or blonde. It is best seen in women and children, as they have less terminal hair to obscure it.
Hair change with aging
Older people tend to develop gray hair (actually colorless) because the pigmentation in the hair is lost and the hair becomes colorless. The age at which this occurs varies from person to person, but in general nearly everyone 75 years or older has gray hair, and in general men tend to become gray at younger ages than women.
The older a person is, the more likely he or she is to have gray hair, and above 85 almost nobody has his or her original hair color. Gray hair is considered to be a characteristic of normal aging.
People starting out with very pale blond hair usually develop white hair instead of grey hair when aging.
Some degree of scalp hair loss or thinning generally accompanies aging in both males and females, and it's estimated that half of men are affected by male pattern baldness by the time they're 50.[1] The tendency toward baldness is a trait shared by a number of other primate species, and is thought to have evolutionary roots. (See evolutionary theories of baldness). There are also perhaps 50,000 bald women in the U.S.
Androgenic hair
The hair follicles on much of the body respond to androgens (primarily testosterone and its derivatives). The rate of hair growth increases and the heaviness of the hairs increases. However, different areas respond with different sensitivities. As testosterone level increases (normally at puberty), the sequence of appearance of sexual (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. The following regions also respond to androgens, in order of decreasing sensitivity: axillary and perianal areas, sideburns, above the upper lip, periareolar areas, chin and beard areas, center of chest, arms and legs, across the chest, shoulders, buttocks, back, and abdomen.
It is the hair in these areas that appears earlier or grows to excess in disorders of excess androgen (e.g., precocious puberty, late-onset congenital adrenal hyperplasia, and polycystic ovary syndrome).
Other information
Notable variations in physical appearance of the top and back of the head are:
- headgear
- hair color (original or artificial)
- hair type
- haircut, curls, dreadlocks, braids, ponytails, wigs, decorative hairpins, the way the hair is combed or otherwise arranged, or disarranged.
Hair spray, gel, etc. may be used for fixation of the arrangement and may also make it shiny.
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.
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.
External links
See also
- Facial hair
- Pubic hair
- Hirsutism
- Baldness
- Depilation
- Widow's peak
- Cowlick
- Social role of hair
- Blond
- Brunette
- Red hair
- Trichophilia
References
- ^ "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.