Hair provides no vital function for humans, but its psychological effect is nearly immeasurable. Luxurious scalp hair expresses femininity for women and masculinity for men. The lack of scalp hair or the presence of excessive facial or body hair is often as distressing to females as the loss of beard and body hair is to males. Male pattern baldness, although accepted in our society, is still distressing to most men, and they will often go to great lengths to preserve, restore, or regrow hair on their scalps. The hair responsible for secondary sexual characteristics, such as the beard or mustache in males and pubic and "axillary" hair in both males and females, serves no particular function. Axillary hair is a characteristic almost exclusive to humans. Most likely, such hair exists to disseminate glandular odor from the apocrine glands, which become functional when the hair develops. Hairs emerge in patterns on the skin's surface.1 Although axillary hair grows in rows, hair on the sacral region, on the umbilicus, and occasionally on areas of the abdomen grows in whirls.2,3 Hair on the vertex of the scalp can grow in rows or whirls. Mongoloids have straight hair because their hair follicles are straight and perpendicular to the surface of their skin. Blacks have spiral- shaped hair because their hair follicles are curved. Whites, on the other hand, may have hair that is straight, spiral, or wavy. All forms of hair are chemically indistinguishable and are dependent upon polygenic traits.4 Hair color is dependent on the number and types of melanosomes present in the cortex of the hair. Brown or black hair contains many melanized eumelanosomes, red hair contains pheomelanosomes, and the eumelanosomes in blond hair are incompletely melanized. Gray hair contains few melanocytes in the hair bulb, and its melanosomes are also incompletely melanized.

Morphologically, there are three types of hair: vellus, terminal, and intermediate. * Vellus hairs are short, fine, soft, usually nonpigmented, and unmedullated. * Terminal hairs are large, darkly pigmented, and medullated. Ninety percent of the hairs on the chest, trunk, shoulders, legs, and arms of men are terminal hairs, whereas only 4500 of hairs in the same regions on women are terminal.5 * Intermediate hairs occur on the scalp, and they demonstrate a morphology between those of terminal and vellus hairs. Intermediate hairs are medullated and contain a moderate amount of pigment, i.e., less than that found in terminal hairs.6

The balding process is a conversion of the follicles so that they produce vellus hairs rather than terminal hairs.7 Each type of hair undergoes repeated cycles of active growth and rest. The relative duration of each cycle varies with the age of the individual and the region of the body where the hair grows. The length of the cycle is often modified by a variety of physiologic and pathologic factors. The cyclic phase of the hair follicle is identified by an active growth period, known as anagen; an intermediate period, catagen; and a resting stage, telogen.

In the anagen phase, the follicle reaches its maximum length, and there is proliferation of the matrix cells, which produce the internal root sheath, the cortex and medulla of the hair shaft, and the cuticular layers of the hair shaft and inner sheath. Anagen hair generally has a thick shaft; and in given segments, its medulla is clearly visible. The proximal-most part of the bulb in anagen hair is deeply pigmented.

The bulb gradually tapers and becomes lighter in color at and beyond the keratogenous zone of the follicle. In "epilated" anagen hair the inner and outer root sheaths are intact and are wrapped around the bulb portion of the hair.

Catagen hair, in its involutional form, differs from telogen (clubbed) hair in two ways: (1) its keratinized (proximal) part is darker than that of clubbed hair and (2) its inner and outer root sheaths are better preserved. E-mail us!8

Telogen hair or clubbed hair is easily recognized because it generally contains a thin shaft, which is transparent near the root and devoid of a medulla and keratogenous zone. Epilated telogen hair may be wrapped in the remains of an epithelial sac, which is absent from nongrowing, spontaneously shed clubbed hair.

Of the 100,000 to 150,000 scalp hairs on a human adult (regardless of sex), 90% are in the growing, or anagen, phase (Table 1). The remaining 10% are in the resting (telogen) phase, which lasts for about 100 days. Approximately 50 to 100 clubbed hairs are shed each day. All of this, of course, differs among individuals.

On the scalp, human hair grows at a rate of 0.44 mm/day at the vertex and 0.39 mm/day at the temples,9 and scalp hair grows slightly faster in women than in men. The darkest hairs on the human body are usually the eyelashes. These hairs and the hairs that form the eyebrows are the first terminal hairs to appear.

Eyelashes grow

Table 1
Hair dynamics

Scalp hairs 100,000 to 150,000

• Anagen (90%), grows 4 to 8 years
• Telogen (10%), rests 2 to 4 months

Eyelashes. trunk, and extremities

• Anagen, grows 1 to 6 months
• Telogen, rests 2 to 4 months

Growth rate (mm/day)

• Scalp series 0.44
• Temple 0.39
• Body, heard 0.27

for approximately 30 days, undergo quiescence for 15 days, and remain dormant for about 100 days. Coarse body hairs and beards grow about 0.27 mm/day. Axillary hair is characteristically curled and twisted about its linear axis, and this hair varies from 1 to 60 mm in length.9 Hair in the pubic area typically grows in an inverted- triangle pattern in women; but in men, it grows in a rhomboid pattern with the apex of the rhomboid's long axis pointing toward the umbilicus.

The character of human hair is constantly changing from the prenatal period to old age; and under given physiologic conditions, the same hair follicle can successively form different types of hair. Lanugo, the first-generation hair appears during intrauterine life and is silky and glossy and contains no pigment or medulla.10 Near the end of pregnancy, lanugo is replaced by second- generation hair which is already pigmented and can grow to a maximum length of 2 cm.

Fine vellus hairs begin to change to terminal hairs before the onset of puberty; and with advancing age, the terminal hairs develop and thicken on all parts of the body. Eyelashes and eyebrows become fully formed before puberty. They grow steadily thicker during childhood but remain relatively unaltered throughout adulthood. Eyelashes have the widest diameter of body hairs and are the most highly pigmented of the terminal hairs.11 Maximum development of terminal hair in men occurs during the fourth decade. The formerly nonpigmented vellus hairs that occur in the ears develop into long, coarse terminal hairs particularly on the tragus, antitragus, and external auditory meatus. Long, coarse hairs also begin to grow on the lateral two thirds of the eyebrows.11 Despite differences among individuals, follicle development for all types of hair is virtually the same.

Hair grows from primary follicles, which are formed by the differentiation of cells in the embryonic epidermis, and further growth causes the hair to become embedded in the dermis (Figure 1). The hair papilla is formed from mesodermal cells. Completion of the terminal pilosebaceous unit requires (1) a vascular network, (2) nerve tissues to surround the follicle, and (3) the arrector pill muscle being inserted into the wall of the follicle.

Primitive hair germs, which are observed as a focal crowding of basal-cell nuclei in the fetal epidermis, are formed in the ninth week of intrauterine life and first appear in the regions of the upper lip, eyebrows, and chin. All further

figure 1. Embryonic development of follicle

primary follicle germs begin to develop over the surface of the body during the fourth month of gestation. As the fetus grows, new primary germs form among the existing ones, and secondary germs develop in such an orientation to the primary germs so as to form new follicles in groups of three. The mesenchymal cells that surround the new follicles are undifferentiated but begin to look like fibroblasts upon maturation of the follicles.12 Rapid, oblique proliferation of cells into the mesenchyme transforms the hair germs into hair pegs.

A solid column of epithelial cells, with radially arranged cells at its base, forms the matrix of the follicle. The free end of the peg becomes progressively clubbed and indented, and the concavity at the tip deepens to enclose the dermal papilla.

At this stage, two solid epithelial swellings begin to appear on the posterior side of the follicle. The one nearest the epidermis differentiates into the sebaceous gland. Just below this gland, the mesenchymal cells arrange themselves in a linear, slender band that is parallel to the posterior border of the follicle. These cells gradually extend downward and become attached to the bottom bulge and form the arrector pili muscle. Melanocytes, already present at the hair-germ stage (and at later stages), can be seen in the bulb and at all levels of the outer root sheath.13 In fetuses, Merkel's cells (unassociated with neurites) tend to concentrate around the outer root sheath, but they are not present around postnatal follicles.13

Hair follicles are not vascularized in the early stages of their development. When the follicles enlarge and contain hair; capillary networks develop nearby, and capillary loops are formed in the dermal papillae. As noon as an embryonic follicle attains its definitive length, mitotic activity in the cone of cells in the upper part of the bulb increases, and the differentiation of these cells produces hair A second concentric cone of cells surrounds the first and becomes the future internal root sheath. The inner cone produces the cortex and hair cuticle, but no medulla exists in fetal hair The cone of the internal root sheath extends upward and protects the tip of the hair as it grows into the hair canal. In the upper part of this canal, the internal root sheath breaks; and later the hair emerges on the surface of the skin. During its development, the follicle grows both downward into the dermis and upward into the epidermis. Its intra epidermal segment terminates at the infundibulum of the follicle. No race-related or fundamental sex differences occur in the number or distribution of primordial embryonic follicles.

The first coat of long, fine lanugo is shed in utero about one month before full- term birth. The second coat, which consists of short lanugo and appears on all areas except the scalp (where the hair is long and large), is shed during the first three to four months of life. Scalp hairs grow in groups of two to three or more, but each hair has its own follicular membrane. Active follicles are long and are embedded deep in the skin. In the pilosebaceous units - from the center of the face - the sebaceous gland is predominant, and rudimentary follicles (together with the hair) almost disappear among the mass of sebaceous tissue cells. Beard hairs sometimes grow pili multigemini from a double papilla, but each has separate epithelial follicular layers surrounded by a joint-connected tissue sheath.