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hair, slender threadlike outgrowth from the skin of mammals. In some animals hair grows in dense profusion and is called fur or wool. Although all mammals show some indication of hair formation, dense hair is more common among species located in colder climates and has the obvious function of insulation against the cold. Other functions include camouflage and protection against dust and sand. The long, sensitive hairs, called tactile hairs, that are located around the mouth area of most mammals are extremely sensitive to touch. Each hair filament originates in a deep pouchlike depression of the epidermis, called a hair follicle, which penetrates into the dermis. The root of the hair extends down into the hair follicle and widens into an indented bulb at its base. Extending into the indentation is the papilla, the center of hair growth, which contains the capillaries and nerves that supply the hair. Newly dividing cells at the base of the hair multiply, forcing the cells above them upward. As the cells move upward, they gradually die and harden into the hair shaft. The hair shaft has two layers, the cuticle and the cortex. The cuticle (outer layer) consists of flat, colorless overlapping cells; below the cuticle is the cortex, containing pigment and a tough protein called keratin; it forms the bulk of the hair shaft. Coarse hair, such as that of the scalp, contains an additional inner core called the medulla. Hair is lubricated by sebaceous glands that are located in the hair follicle. Illness or stress may lessen the secretion of pigment, which normally gives color to hair, and cause the hair shaft to whiten. However, the normal process of whitening that comes with age is determined by heredity. In humans, scalp hairs are generally shed every two to four years, while body hairs are shed more frequently. Straight-textured hair, round in cross section, is common among Native Americans, Eskimos, and Mongolic peoples. Kinky or woolly hair, flat in cross section, prevails among the dark peoples of Africa, Australia, and elsewhere. Wavy or curly hair, common among Caucasians, is oval in cross section. The color of hair is determined by the amount of pigment and air spaces in the cortex and medulla. Hair color and texture are inherited characteristics.
Nonliving, specialized epidermal derivatives characteristic only of modern mammals. However, it is now thought that hair was present in at least some therapsid reptiles. It consists of keratinized cells, tightly cemented together, which arise from the matrix at the base of a follicle. A follicle is a tubular epidermal downgrowth that penetrates into the dermis and widens into a bulb (the hair root) at its deep end. The follicle, together with a lateral outgrowth called the sebaceous gland, forms the pilosebaceous system. Rapid cell production in the matrix, and differentiation in the regions immediately above, produces a hair shaft which protrudes from the follicle mouth at the skin surface. See Gland
Hairs are not permanent structures but are continually replaced throughout the life of a mammal. In some species, for example, the rat, hamster, mouse, chinchilla, and rabbit, the replacement pattern is undulant, and waves of follicular activity can be traced across the body. In other species, for example, humans, cats, and guinea pigs, each follicle appears to cycle independently of others in the immediate area.
a horny threadlike product of the skin, which forms the hairy covering characteristic of mammals. Besides providing protection against mechanical injuries, hair protects the body from heat loss, thus helping to maintain constant body temperature in mammals and to distribute them over the whole earth. In some animals with a greatly thickened epidermis (elephant, rhinoceros) or a strongly developed subcutaneous fat layer (whale), the hair covering is reduced.
Adult mammals have three distinct types of hair: cover or awn hair (fur), which is long and straight; bristles and needles, which are modified hairs; downy hair (underfur), which often lacks hair marrow and is usually randomly bent, of fine texture, and shorter than cover hair; and sensory hair, or vibrissae. Hair evidently arose from horny scales of the skin. During the course of embryonic development rudimentary hairs appear in the form of epidermal ingrowths directed obliquely into the connective tissue. (In human embryos this occurs during the third and fourth months of development.) The part of the hair which emerges freely on the skin surface is called the hair shaft; the part embedded in the skin is called the root. The root ends in a broadened part called the bulb, into the depression of which enters the papilla, containing blood vessels and nerves. The papilla is adjoined by the cambial cells of the bulb, whose reproduction activates hair growth.
There are three hair layers: (1) The marrow consists of large cells with a keratin-like substance; this layer contains bubbles of air. (2) A cortical substance represented by agglutinated cornified threadlike cells filled with partially connate threads (fibrillae) of keratin, oriented parallel to the hair axis; the fibrillae are in the form of densely packed smaller threads or filaments 50-100 angstroms (Å) in diameter, consisting in turn of protofilaments with a diameter of about 20 Å, which are formed of two or three spirally intertwined threadlike molecules of protein. The mechanical strength of a hair is determined primarily by the cortical substance, whose cells, like those of the marrow, can contain the pigment melanin; depending upon its quantity and degree of dispersion, this pigment gives hair its color, from the black to the white tones. (3) The cuticle is a layer of flat cornified cells lying like scales one upon the other and containing basically amorphous keratin.
The root part of the hair lies in the hair sac, which is formed by a continuation of the skin epithelium and consists of an internal and external root sheath with the surrounding connective tissue hair sac. The internal sheath, like the hair, is formed by the reproduction of the bulb cells, accompanies the initial growth of the hair, and is destroyed before reaching the level of the sebaceous gland ducts. The cells of the external sheath are themselves capable of reproduction. The sweat glands linked with hairs open above the place into which the sebaceous glands empty. Smooth muscle fibers are attached to the hair sac; when they are contracted the hairs assume a vertical position, which increases the thickness of the hair covering.
The speed of growth and the longevity of different hairs vary. In many animals a change of hair occurs periodically in the spring and autumn. In humans it usually occurs without a determined rhythm, although many people lose more hair in spring and autumn. When this happens, the bulb cells stop reproducing and cornify, the blood vessels of the papilla are obliterated, and the hair root separates from the papilla and moves up to the level of the sebaceous gland ducts. The internal sheath is destroyed, the distinctly changed lower extremity of the hair (the bulb or envelope) unites with the external sheath, and the hair eventually falls out. The remainder of the papilla likewise is displaced upward to the level of the sebaceous glands; at the place where it makes contact with the external sheath the cells of a new rudimentary hair begin to reproduce. In humans 50-90 days elapse between the time an old hair falls out and a new one appears. The graying of hairs, or their depigmentation, is an aging process, the onset of which is subject to individual variations.
REFERENCESMatveev, B. S. “O proiskhozhdenii cheshuichatogo pokrova i volos u mlekopitaiushchikh.” Zoologicheskii zhurnal, 1949, vol. 28, no. 1.
Barabash-Nikiforov, I. I., and A. N. Formozov. Teriologiia. Moscow, 1963.
Gistologiia. Edited by V. G. Eliseev. Moscow, 1963.
Rogers, G. E. “Electron Microscope Studies of Hair and Wool.” Annals of the New York Academy of Sciences, 1959, vol. 83, art. 3, p. 378.
Mercer, E. H. [et al.]. “A Suggested Nomenclature for Fine-Structural Components of Keratin and Keratin-like Products of Cells.” Nature, 1964, vol. 201, no. 4,917.
Straile, W. E. “Root Sheath-Dermal Papilla Relationship and the Control of Hair Growth.” In Biology of the Skin and Hair Growth. Sydney, 1965.
The hair root is located in the so-called hair sac, into which empties the duct of a sebaceous gland (the sebaceous-hair follicle); the follicle opens in a funnel shape onto the surface of the skin, and through this funnel the hair shaft emerges and skin oil is secreted. The outward appearance and growth of a hair depend upon the general condition of the organism, especially upon the condition of the nervous and endocrine systems. In a healthy adult a hair grows an average of 1 to 1.5 cm a month; in middle-aged people the growth of hair is slower. Hair gradually becomes thinner. Redheads have the thickest hairs; those of brunets are somewhat thinner, and those of blonds are the finest.
The longevity of a hair averages two to four years and depends upon the individual’s age, the condition of his nervous and endocrine systems, infections experienced by the individual, and the care of the hair. Temporary hair loss is observed during some infections (syphilis, typhus, influenza), as well as during treatment with a series of drugs (anticoagulants, for example); some drugs (resoxin, among others) can cause temporary graying of the hair. Hair growth is adversely affected by too frequent washing (especially in hard water and with alkaline soap). When the hair is dry, soap and water remove oil and so worsen the condition of the hair; when the hair is oily, frequent washing leads to increased secretion of skin oil, as the hygroscopicity of the hair with oil removed causes the hair to absorb skin oil, thus stimulating the sebaceous glands to even greater secretion of skin oil. It is recommended that the hair be washed no more frequently than once every seven to ten days with boiled water and toilet soap. Dyes and long periods of exposure to sun or frost with an uncovered head are also harmful to hair growth and cause hair to fall out.
Hair has great strength and can support a weight of up to 1.5-2 newtons (0.150-0.200 kg); it also has the property of hygroscopicity (it is used in instruments for determining the moisture in the air). Hair has great elasticity and can be bent; this principle is the basis for various hair styles. For a permanent wave, the hairs are treated with alkaline liquids (chemical wave) and heat, which are harmful to normal hair growth. Permanent waves are especially harmful for dry hair, for hair-loss conditions, during periods of recuperation from infectious diseases, and at the end of pregnancy. When growth stops, the hairs remain in place until removed by mechanical action such as washing, rough dressing, or brushing. During a number of illnesses hair replacement slows down or stops completely, and the hair begins to thin, leading to baldness.
There are three basic types of hair (with various subtypes): straight, wavy, and curly. Wavy and curly hair are characterized by a spiral twist of the shaft. Coarse straight hair is characteristic for peoples of Central, North, and East Asia and for American Indians; soft straight or wavy hair is found among Europeans and some peoples of South and Southeast Asia; and curly spiral hair is characteristic of the Negroid peoples of Africa and the population of New Guinea and Melanesia. Anthropologists also measure the amount of beard development; the lightest growth occurs among some groups of North Asia, and the heaviest growth is among Australians, Ainus, and the peoples of the Near East and Transcaucasia.
REFERENCERoginskii, Ia. Ia., and M. G. Levin. Antropologiia, 2nded. Moscow, 1963.
What does it mean when you dream about hair?
Hair has been a sacred and spiritual symbol throughout history. For example, it was the secret of the strength of Sampson in the biblical story. Hair carries an aura of sexual virility, seduction, and health. White hair denotes wisdom and age.