Hair

Hair: Hair is of epidermal (the epidermis is the outer portion of the skin that lies over the deeper dermis) origin and consists of epidermal cells strengthened by keratin, a protein that also makes up much of fingernails, hooves, horns, etc. A hair is initiated when the growing layer of the epidermis, the stratum germinativum, invaginates into the dermis below, forming a hair follicle. (The living cells of the epidermis form the stratum germinativum which divides off outer cells which die, flatten, and end up filled largely by keratin; this outer layer, the stratum corneum, provides physical protection and water proofing.) A hollow hair bulb expands over a dermal papilla that supplies nutrients to the epidermal bulb. Cells divided from the bulb form the dead, keratinized cells of the root and shaft.

Hair generally consists of two or three layers. Externally, a cuticle is formed from a single layer of cells that frequently overlap as if they were scales; beneath the cuticle is a thick layer with (usually) pigment and air vacuoles. In thick hair, a thin central area of shrunken cells and large air vacuoles forms a medulla, with the layer beneath the cuticle being the cortex. Variations in the patterns formed by the cuticular cells, in the amount and distribution of the pigment and air vacuoles in the cortex, and in the presence, size, and amount of air in the medulla allow identification of hairs to some taxonomic level.

Normally a sebaceous gland opens into the follicle and supplies an oily material to the hair shaft. An arrector pili muscle (usually of smooth muscle) attaches to the hair follicle; contraction erects the hair for changes in insulating values or for display. The goose bumps some people get during a chill or a fright is a result of these muscles contracting.

Functions of hair include protection, insulation, display, communication, sensory input, and camouflage. The collective term for the hair of a mammal is its pelage; we also may talk of fur when the hair is relatively thick. In most mammals, the pelage is divided into guard hairs, which are relatively long hairs overlying the underfur, consisting of shorter, densely packed hairs.

Specialized guard hairs include spines (stiff and enlarged) and bristles (long, firm hairs with indeterminate growth). Vibrissae are specialized, long hairs with extensive inervation at the base; they serve as sensory structures and most commonly are found on the muzzle, although they may appear elsewhere.

Mammals commonly display countershading, with the underparts lighter in color than the dorsal parts. The lighter underparts help counteract the shadow thrown by the animal's body which otherwise would make the lower outline of the body stand out.

Since hair is a dead structure before it reaches the surface of the skin, there is no way to maintain a homeostatic situation—hair weathers, wears, and bleaches. To counteract the destruction of the pelage, hair is replaced. In a few mammals, this may be a continuous process (as in humans), but most show definite cycles of molting (replacement/shedding of hair, usually considered as a moderately rapid process separated from other molts by periods in which the hair is not molted). In many mammals, a molt spreads over the body in a definite pattern that has, in some cases, been used taxonomically. Many mammals undergo a spring and a fall molt, with the post-spring pelage sparser and/or shorter; frequently the seasonal pelages differ in coloration, also (with the most extreme being some arctic mammals which molt into a white pelage in fall).

An extensive unit could be undertaken on hair and pelage, but time beckons us onward to a consideration of another of the unique characteristics of modern mammals: mammary glands (and other epidermal glands).

Last Update: 6 Jan 2008