Deep at the base of the hair follicle within the
dermis (middle layer of the skin) is the hair root,
which forms the cells that will become the hair
fiber. At their formation, hair cells are pliable, liv-
ing, and colorless. The new cells that emerge from
the root divide, pushing the cells that preceded
them upward into the hair follicle. As the cells
rise, they move beyond the range of the blood
vessels that bring nourishment to the hair root
and so die.
By the time hair cells rise above the surface of
the skin, they are hard and compressed into an
elongated, two-layer fiber. The inner layer, the
cortex, is about three or four cells thick, composed
of melanocytes and keratinocytes. Melanocytes
form the melanin (pigment) that gives hair its
color and. Large, numerous melanocytes result in
black or brown hair; small, scattered melanocytes
result in red or blond hair. A reduction in the
number of melanocytes results in gray or white
hair. The keratinocytes produce the keratin that
gives hair its structure. A single layer of cells, the
cuticle, encases the cortex. The rounder the hair
fiber, the straighter the hair. Curly or kinky hair
fibers are flat.
A hair follicle on the scalp typically grows hair
for three to five years, then enters a period of dor-
mancy that lasts a few months after which time it
drops the hair and the growth cycle begins anew.
Some people can grow hair for longer, up to seven
or eight years, while others have a shorter growth
period. The amount of time hair grows influences
though does not regulate the hair’s length; a per-
son with a short growth period can have hair that
grows rapidly.
Between 10 and 20 percent of the body’s hair
follicles are dormant at any one time. Hair follicles
elsewhere on the body have shorter growth cycles
than those on the head. The hairs of the eyebrows
regenerate every 100 days or so, for example. An
individual’s genetic composition determines the
shape, color, and length of hair fibers. The cells of
the hair fibers within most of the hair follicle and
visible above the skin’s surface are dead and do
not require nourishment or have sensory capabil-
ity. Their keratin content and cuticle layer keeps
them attached to the body.
Fingernails and toenails Keratin also gives the
fingernails and toenails their shape and hardness.
The nails cover the top portion of the fingers and
toes. Though they may appear to have little more
function than to serve as decorative platforms, the
nails give the fingers (and to more limited extent,
the toes) the ability to grip, pry, and scratch. The
nails also give firmness and STRENGTHto the ends of
the fingers, and help protect the sensitive fingertips.Health and Disorders of the Integumentary System
The skin performs its myriad functions with
remarkable consistency. Because it replaces its
outer layer every three to four weeks, the skin
remains relatively healthy for most of the lifespan.
However, well over a thousand disorders, diseases,
and conditions can affect the skin, hair, and nails.
While a few can cause permanent tissue damage
or even death, most are fairly benign.Traditions in Medical History
For millennia the internal structures of the body
mystified and confounded healers and physicians.
The skin, however, was right there on the outside
of the body for all to see and touch. The skin’s
accessibility gave it a perceived, and erroneous,
simplicity as nothing more than the body’s cover-
ing. Not until the electron microscope made it
possible to explore the molecular structures of the
body did scientists begin to understand the true
complexity and intricate functions of the skin.
Today doctors recognize that the skin is a com-
plex and multifunctional organ. Its appearance
reveals much about the body’s internal functions
and a person’s health status. Numerous systemic
health conditions reveal or manifest themselves
through changes in the skin, hair, and nails. Dis-
coloration of the skin, for example, may suggest
SUNBURN(red), JAUNDICEand LIVERdisease (yellow),
or cardiovascular or pulmonary disease (cyanotic
blue). Variations on discoloration accompany
numerous rashes. The characteristics of skin erup-
tions are often unique to specific diseases, such as
the pustules of CHICKENPOX and the papules of
MEASLES. Disturbances of the fingernails and toe-
nails may similarly portend underlying health
conditions such as iron-deficiency ANEMIA
(koilonychia or spooning) or EMPHYSEMA(yellow
nail syndrome). And hair loss, always emotionally
distressing, may be a harbinger of undiagnosed
thyroid disease or toxic exposure.The Integumentary System 129