H

hair The fibers that grow from the hair follicles.
Not far above the root of the hair follicle the cells
that form the hair fiber are dead, hardened into
their shape through compression within the folli-
cle as new cells emerging from the hair’s root push
them upward. A hair fiber is five or six cells in
thickness and varies in length, depending on its
location. Hair on the head can grow to several feet
in length, whereas the hair of the eyelashes is
generally no longer than about a quarter of an
inch long. The hair does not require nourishment
from the body, though the secretions of the seba-
ceous glands help moisturize the hair fibers to
keep them supple.
Genetic encoding determines the characteristics
of the hair, from how rapidly it grows to whether it
is curly or straight. Hair covers all SKINsurfaces
except the palms of the hands and the soles of the
feet, though is most prominent on the head and,
after PUBERTY, in the pubic region, on the legs, and
under the arms. Men typically have darker, coarser
body hair than women. Specialized hairs line the
auditory canals and the inside of the NOSE, func-
tioning to remove debris from these structures.
Like the skin, the hair provides clues to the
health of the body. Numerous conditions can
change the characteristics of the hair. Such
changes reflect circumstances that affect the hair
follicles in some way, from physical damage, such
as BURNS or scars that can destroy follicles, to
immune or disease processes that attack the folli-
cles and disrupt hair growth. Physical stress such
as the body experiences with major injury, illness,
or surgery can cause various changes in the hair,
from altered color and consistency to hair loss.
The hair’s characteristics also change with aging.
By midlife the hair typically starts to lose the

melanocytes that give it color. Sebum (the natural

oil that lubricates the hair follicle) production

slows, allowing the hair to become dry. Sun expo-

sure also can alter the hair, lightening its color or

extracting moisture to make it brittle. Hair-care

products can help restore moisture to the hair on

the head as well as to the skin of the scalp.

HEALTH CONDITIONS THAT MAY INVOLVE THE HAIR

ADVERSE REACTIONto a DRUG ALOPECIA

ALOPECIA AREATA ERYTHEMATOSUS(SLE)

DANDRUFF DISCOID LUPUS ERYTHEMATOSUS

FILLICULTIS (DLE)

FURUNCLE HIRSUTISM

HYPOTHYROIDISM INGROWN HAIR

KERATOSIS PILARIS LICHEN PLANUS

MENOPAUSE nutritional deficiencies

PREGNANCY PUBERTY

ringworm stress

SYSTEMIC LUPUS ERYTHEMATOSUS toxic exposure

(SLE) TRICHOTILLOMANIA

For further discussion of the hair within the

context of integumentary structure and function

please see the overview section, “The Integumen-

tary System.”

See also AGING, INTEGUMENTARY CHANGES THAT

OCCUR WITH; BEZOAR; MELANOCYTE;NAILS; SEBACEOUS

GLAND;SWEAT GLANDS.

hair replacement A surgical procedure, also

called hair transplantation, to relocate viable HAIR

follicles from sites on the scalp where they are

abundant to sites where there has been perma-

nent hair loss. The most common reason for hair

replacement is androgenic ALOPECIA(male pattern

hair loss). Hair replacement is nearly always a cos-

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