What is hair
Hair is a slender filament of keratinized cells that grows from an oblique tube in the skin called a hair follicle. Each hair is composed of columns of dead, keratinized epidermal cells bonded together by extracellular proteins. The hair shaft is the superficial portion of the hair, which projects above the surface of the skin. The hair root is the portion of the hair deep to the shaft that penetrates into the dermis, and sometimes into the subcutaneous layer.
Hairs project beyond the surface of the skin almost everywhere except the sides and soles of the feet, the palms of the hands, the sides of the fingers and toes, the lips, and portions of the external genitalia. There are about 5 million hairs on the human body, and 98 percent of them are on the general body surface, not the head. Hairs are nonliving structures that form in organs called hair follicles.
The density of hair does not differ much from one person to another or even between the sexes; indeed, it is virtually the same in humans, chimpanzees, and gorillas. Differences in apparent hairiness are due mainly to differences in texture and pigmentation.
Types of Hairs
Hairs first appear after about three months of embryonic development. These hairs, collectively known as lanugo, are extremely fine and unpigmented. Most lanugo hairs are shed before birth.
The two types of hairs in the adult skin are vellus hairs and terminal hairs:
- Vellus hairs are the fine “peach fuzz” hairs found over much of the body surface.
- Terminal hairs are heavy, more deeply pigmented, and sometimes curly. The hairs on your head, including your eyebrows and eyelashes, are terminal hairs. After puberty, it also forms the armpit and pubic hair, male facial hair, and some of the hair on the trunk and limbs.
Hair follicles may alter the structure of the hairs they produce in response to circulating hormones.
Figure 1. Hair structure
Figure 2. Hair follicle
Structure of Hair Follicle
The portion of a hair above the skin is called the shaft, and all that beneath the surface is the root. The root penetrates deeply into the dermis or hypodermis and ends with a dilation called the hair bulb. The only living cells of a hair are in and near the hair bulb. The hair bulb grows around a bud of vascular connective tissue called the dermal papilla, which provides the hair with its sole source of nutrition. Immediately above the papilla is a region of mitotically active cells, the hair matrix, which is the hair’s growth center. All cells higher up are dead.
Figure 3. Hair follicle and hair structure
In cross section, a hair reveals up to three layers. From the inside out, these are the medulla, cortex, and cuticle.
The medulla is a core of loosely arranged cells and air spaces. It is most prominent in thick hairs such as those of the eyebrows, but narrower in hairs of medium thickness and absent from the thinnest hairs of the scalp and elsewhere.
The cortex constitutes most of the bulk of a hair. It consists of several layers of elongated keratinized cells that appear cuboidal to flattened in cross sections.
The cuticle is composed of multiple layers of very thin, scaly cells that overlap each other like roof shingles with their free edges directed upward.
Cells lining the hair follicle are like shingles facing in the opposite direction. They interlock with the scales of the hair cuticle and resist pulling on the hair. When a hair is pulled out, this layer of follicle cells comes with it.
The hair follicle is a diagonal tube that contains the hair root. It has two principal layers: an epithelial root sheath and a connective tissue root sheath. The epithelial root sheath is an extension of the epidermis; it consists of stratified squamous epithelium and lies immediately adjacent to the hair root. Toward the deep end of the follicle, it widens to form a bulge, a source of stem cells for follicle growth. The connective tissue root sheath, which is derived from the dermis and composed of collagenous connective tissue, surrounds the epithelial sheath and is somewhat denser than the adjacent dermis.
Associated with the hair follicle are nerve and muscle fibers. Nerve fibers called hair receptors entwine each hair follicle and respond to hair movements. You can feel their effect by carefully moving a single hair with a pin or by lightly running your finger over the hairs of your forearm without touching the skin.
Each hair has a piloerector muscle—also known as a pilomotor muscle or arrector pili—a bundle of smooth muscle cells extending from dermal collagen fibers to the connective tissue root sheath of the follicle. In response to cold, fear, touch, or other stimuli, the sympathetic nervous system stimulates the piloerector to contract, making the hair stand on end and wrinkling the skin in such areas as the scrotum and areola. In humans, it pulls the follicles into a vertical position and causes “goose bumps,” but serves no useful purpose.
Figure 4. Hair structure
Hair follicles extend deep into the dermis, often projecting into the underlying subcutaneous layer. The epithelium at the follicle base surrounds a small hair papilla, a peg of connective tissue containing capillaries and nerves. The hair bulb consists of epithelial cells that surround the papilla.
Hair production involves a specialized keratinization process. The hair matrix is the epithelial layer involved in hair production. When the superficial basal cells divide, they produce daughter cells that are pushed toward the surface as part of the developing hair. Most hairs have an inner medulla and an outer cortex. The medulla contains relatively soft and flexible soft keratin. Matrix cells closer to the edge of the developing hair form the relatively hard cortex. The cortex contains
hard keratin, which gives hair its stiffness. A single layer of dead, keratinized cells at the outer surface of the hair overlap and form the cuticle that coats the hair.
The hair root anchors the hair into the skin. The root begins at the hair bulb and extends distally to the point where the internal organization of the hair is complete, about halfway to the skin surface. The hair shaft extends from this halfway point to the skin surface, where we see the exposed hair tip.
The size, shape, and color of the hair shaft are highly variable.
Growth and Replacement of Hair
A hair in the scalp grows for two to five years, at a rate of around 0.33 mm/day (about 1/64 inch). Variations in hair growth rate and the duration of the hair growth cycle account for individual differences in uncut hair length. A given hair goes through a hair cycle consisting of three developmental stages (see Figure 5):
At any given time, about 90% of the scalp follicles are in the anagen stage. In this stage, stem cells from the bulge in the follicle multiply and travel downward, pushing the dermal papilla deeper into the skin and forming the epithelial root sheath. Root sheath cells directly above the papilla form the hair matrix. Here, sheath cells transform into hair cells, which synthesize keratin and then die as they are pushed upward away from the papilla. The new hair grows up the follicle, often alongside an old club hair left from the previous cycle.
In the catagen stage, mitosis in the hair matrix ceases and sheath cells below the bulge die. The follicle shrinks and the dermal papilla draws up toward the bulge. The base of the hair keratinizes into a hard club and the hair, now known as a club hair, loses its anchorage. Club hairs are easily pulled out by brushing the hair, and the hard club can be felt at the hair’s end. When the papilla reaches the bulge, the hair goes into a resting period called the telogen stage. Eventually, anagen begins anew and the cycle repeats itself. A club hair may fall out during catagen or telogen, or as it is pushed out by the new hair in the next anagen phase.
You lose about 50 to 100 scalp hairs daily. In a young adult, scalp follicles typically spend 6 to 8 years in anagen, 2 to 3 weeks in catagen, and 1 to 3 months in telogen. Scalp hairs grow at a rate of about 1 mm per 3 days (10–18 cm/yr) in the anagen phase.
Hair grows fastest from adolescence until the 40s. After that, an increasing percentage of follicles are in the catagen and telogen phases rather than the growing anagen phase. Hair follicles also shrink and begin producing wispy vellus hairs instead of thicker terminal hairs. Thinning of the hair, or baldness, is called alopecia. It occurs to some degree in both sexes and may be worsened by disease, poor nutrition, fever, emotional stress, radiation, or chemotherapy. In the great majority of cases, however, it is simply a matter of aging.
Pattern baldness is the condition in which hair is lost unevenly across the scalp rather than thinning uniformly. It results from a combination of genetic and hormonal influences. The relevant gene has two alleles: one for uniform hair growth and a baldness allele for patchy hair growth. The baldness allele is dominant in males and is expressed only in the presence of the high level of testosterone characteristic of men. In men who are either heterozygous or homozygous for the baldness allele, testosterone causes terminal hair to be replaced by vellus hair, beginning on top of the head and later the sides. In women, the baldness allele is recessive. Homozygous dominant and heterozygous women show normal hair distribution; only homozygous recessive women are at risk of pattern baldness. Even then, they exhibit the trait only if their testosterone levels are abnormally high for a woman (for example, because of a tumor of the adrenal gland, a woman’s principal source of testosterone). Such characteristics in which an allele is dominant in one sex and recessive in the other are called sex-influenced traits.
Excessive or undesirable hairiness in areas that are not usually hairy, especially in women and children, is called hirsutism. It tends to run in families and usually results from either masculinizing ovarian tumors or hypersecretion of testosterone by the adrenal cortex. It is often associated with menopause.
Contrary to popular misconceptions, hair and nails do not continue to grow after a person dies, cutting hair does not make it grow faster or thicker, and emotional stress cannot make the hair turn white overnight.
Figure 5. Hair growth cycle
Genes determine hair color by directing the type and amount of pigment that epidermal melanocytes produce. Variations in hair color reflect differences in hair structure and in the pigment produced by melanocytes at the papilla. Hormonal or environmental factors may influence the condition of your hair. Whether your hair is black or brown depends on the density of melanin in your cortex. Dark hair has more of the brownish-black eumelanin, while blonde hair and red hair have more of the reddish-yellow pheomelanin. The white hair of a person with the inherited condition albinism lacks melanin altogether. A mixture of pigmented and unpigmented hairs appears gray. As pigment production decreases with age, hair color lightens toward gray. White hair results from the combination of a lack of pigment and the presence of air bubbles within the medulla of the hair shaft. Because the hair itself is dead and inert, changes in coloration are gradual; your hair can’t “turn white overnight,” as some horror stories suggest.
The texture of hair is related to differences in cross-sectional shape—straight hair is round, wavy hair is oval, and tightly curled hair is relatively flat.
Function of Hair
The 5 million hairs on the human body have important functions. Most hair of the human trunk and limbs is probably best interpreted as vestigial, with little present function. The roughly 100,000 hairs on the head protect the scalp from ultraviolet light and bumps to the head and insulate the skull. The hairs guarding the entrances to your nostrils and external auditory canals (ear canals) help block foreign particles and insects, and eyelashes perform a similar function for the surface of the eye.
A root hair plexus of sensory nerves surrounds the base of each hair follicle. As a result, you can feel the movement of even a single hair. This sensitivity gives an early-warning system that may help prevent injury. For example, stimulation of the hair receptors, however, alerts people to parasites crawling on the skin, such as fleas and ticks, and to remove them. Thus, you are less likely to become unknowingly infested with parasites.
A ribbon of smooth muscle, the arrector pili muscle, extends from the papillary layer of the dermis to the connective tissue sheath surrounding the hair follicle. When stimulated, the arrector pili muscle pulls on the follicle and raises the hair. Contraction may be due to emotional state, such as fear or rage, or to cold temperatures that produce characteristic “goose bumps.” In a furry mammal, this action thickens the insulating coat, rather like putting on an extra sweater. Although we do not receive any comparable insulating benefits, the reflex persists.