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.
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.
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.