About Skin, Hair and Nails
Skin
is our largest organ. If the skin of a typical 150-pound
(68-kilogram) adult male were stretched out flat, it would cover
about 2 square yards (1.7 square meters) and weigh about 9 pounds
(4 kilograms). Our skin protects the network of muscles, bones,
nerves, blood vessels, and everything else inside our bodies. Our
eyelids have the thinnest skin, the soles of our feet the
thickest.
Hair
is actually a modified type of skin. Hair grows everywhere on the
human body except the palms of the hands, soles of the feet,
eyelids, and lips. Hair grows more quickly in summer than winter,
and more slowly at night than during the day.
Like hair,
nails
are a type of modified skin. Nails protect the sensitive tips of
fingers and toes. Human nails aren't necessary for living, but
they do provide support for the tips of the fingers and toes,
protect them from injury, and aid in picking up small objects.
Without them, we'd have a hard time scratching an itch or
untying a knot. Nails can be an indicator of a person's general
health, and illness often affects their growth.
Skin Basics
Skin is essential in many ways. It forms a barrier that prevents
harmful substances and microorganisms from entering the body. It
protects body tissues against injury. It also controls the loss of
life-sustaining fluids like blood and water, helps regulate body
temperature through perspiration, and protects from the sun's
damaging ultraviolet rays.
Without the nerve cells in skin, people couldn't feel
warmth, cold, or other sensations. For instance, goosebumps form
when the erector pili muscles contract to make hairs on the skin
stand up straight when someone is cold or frightened - the blood
vessels keep the body from losing heat by narrowing as much as
possible and keeping the warm blood away from the skin's
surface, offering insulation and protection.
Every square inch of skin contains thousands of cells and
hundreds of sweat glands, oil glands, nerve endings, and blood
vessels. Skin is made up of three layers: the
epidermis
,
dermis
, and the
subcutaneous tissue
.
The upper layer of our skin, the epidermis, is the tough,
protective outer layer. It's about as thick as a sheet of paper
over most parts of the body. The epidermis has four layers of cells
that are constantly flaking off and being renewed. In these four
layers are three special types of cells:
- Melanocytes
produce melanin, the pigment that gives skin its color. All
people have roughly the same number of melanocytes; those of
dark-skinned people produce more melanin. Exposure to sunlight
increases the production of melanin, which is why people get
suntanned or freckled.
- Keratinocytes
produce keratin, a type of protein that is a basic component of
hair, skin, nails, and helps create an intact barrier.
- Langerhans
cells help protect the body against infection.
Because the cells in the epidermis are completely replaced about
every 28 days, cuts and scrapes heal quickly.
Below the epidermis is the next layer of our skin, the dermis,
which is made up of blood vessels, nerve endings, and connective
tissue. The dermis nourishes the epidermis. Two types of fibers in
the dermis - collagen and elastin - help the skin stretch when we
bend and reposition itself when we straighten up. Collagen is
strong and hard to stretch, and elastin, as its name suggests, is
elastic. In older people, some of the elastin-containing fibers
degenerate, which is one reason why the skin looks wrinkled.
The dermis also contains a person's
sebaceous glands
. These glands, which surround and empty into hair follicles and
pores, produce the oil sebum that lubricates the skin and hair.
Sebaceous glands are found mostly in the skin on the face, upper
back, shoulders, and chest.
Most of the time, the sebaceous glands make the right amount of
sebum. As a person's body begins to mature and develop during
the teenage years, though, hormones stimulate the sebaceous glands
to make more sebum. When pores become clogged by too much sebum and
too many dead skin cells, this contributes to acne. Later in life,
these glands produce less sebum, which contributes to dry skin as
people age.
The bottom layer of our skin, the
subcutaneous tissue
, is made up of connective tissue, sweat glands, blood vessels, and
cells that store fat. This layer helps protect the body from blows
and other injuries and helps it hold in body heat.
There are two types of sweat glands. The
eccrine glands
are found everywhere, although they're mostly in the forehead,
palms, and soles of the feet. By producing sweat, these glands help
regulate body temperature, and waste products are excreted through
them.
The
apocrine glands
develop at puberty and are concentrated in the armpits and pubic
region. The sweat from the apocrine glands is thicker than that
produced by the eccrine glands. Although this sweat doesn't
smell, when it mixes with bacteria on the skin's surface, it
can cause body odor. A normal, healthy adult secretes about 1 pint
(about half a liter) of sweat daily, but this may be increased by
physical activity, fever, or a hot environment.
Hair Basics
The hair on our heads isn't just there for looks. It keeps
us warm by preserving heat. The hair in the nose, ears, and around
the eyes protects these sensitive areas from dust and other small
particles. Eyebrows and eyelashes protect eyes by decreasing the
amount of light and particles that go into them. The fine hair that
covers the body provides warmth and protects the skin. Hair also
cushions the body against injury.
Human hair consists of the hair shaft, which projects from the
skin's surface, and the root, a soft thickened bulb at the base
of the hair embedded in the skin. The root ends in the hair bulb,
which sits in a sac-like pit in the skin called the follicle, from
which the hair grows.
At the bottom of the follicle is the papilla, where hair growth
actually takes place. The papilla contains an artery that nourishes
the root of the hair. As cells multiply and produce keratin to
harden the structure, they're pushed up the follicle and
through the skin's surface as a shaft of hair. Each hair has
three layers: the
medulla
at the center, which is soft; the
cortex
, which surrounds the medulla and is the main part of the hair; and
the
cuticle
, the hard outer layer that protects the shaft.
Hair grows by forming new cells at the base of the root. These
cells multiply to form a rod of tissue in the skin. The rods of
cells move upward through the skin as new cells form beneath them.
As they move up, they're cut off from their supply of
nourishment and start to form a hard protein called keratin in a
process called
keratinization
. As this process occurs, the hair cells die. The dead cells and
keratin form the shaft of the hair.
Each hair grows about 1/4 inch (about 6 millimeters) every month
and keeps on growing for up to 6 years. The hair then falls out and
another grows in its place. The length of a person's hair
depends on the length of the growing phase of the follicle.
Follicles are active for 2 to 6 years; they rest for about 3 months
after that. A person becomes bald if the scalp follicles become
inactive and no longer produce new hair. Thick hair grows out of
large follicles; narrow follicles produce thin hair.
The color of a person's hair is determined by the amount and
distribution of melanin in the cortex of each hair (the same
melanin that's found in the epidermis). Hair also contains a
yellow-red pigment; people who have blonde or red hair have only a
small amount of melanin in their hair. Hair becomes gray when
people age because pigment no longer forms.
Nail Basics
Nails grow out of deep folds in the skin of the fingers and
toes. As epidermal cells below the nail root move up to the surface
of the skin, they increase in number, and those closest to the nail
root become flattened and pressed tightly together. Each cell is
transformed into a thin plate; these plates are piled in layers to
form the nail. As with hair, nails are formed by keratinization.
When the nail cells accumulate, the nail is pushed forward.
The skin below the nail is called the matrix. The larger part of
the nail, the nail plate, looks pink because of the network of tiny
blood vessels in the underlying dermis. The whitish crescent-shaped
area at the base of the nail is called the
lunula
.
Fingernails grow about three or four times as quickly as
toenails. Like hair, nails grow more rapidly in summer than in
winter. If a nail is torn off, it will regrow if the matrix
isn't severely injured. White spots on the nail are sometimes
due to temporary changes in growth rate.
Some of the things that can affect the skin, nails, and hair are
described below.
Dermatitis
The term dermatitis refers to any inflammation (swelling,
itching, and redness) possibly associated with the skin. There are
many types of dermatitis, including:
- Atopic dermatitis(eczema).
It's a common, hereditary dermatitis that causes an itchy
rash primarily on the face, trunk, arms, and legs. It commonly
develops in infancy, but can also appear in early childhood. It
may be associated with allergic diseases such as asthma and
seasonal, environmental, and food allergies.
- Contact dermatitis.
This occurs when the skin comes into contact with an irritating
substance or one that the person is allergic or sensitive to. The
best-known cause of contact dermatitis is poison ivy, but there
are many others, including chemicals found in laundry detergent,
cosmetics, and perfumes, and metals like nickel plating on
jewelry, belt buckles, and the back of a snap.
- Seborrheic dermatitis.
This oily rash, which appears on the scalp, face, chest, and
back, is related to an overproduction of sebum from the sebaceous
glands. This condition is common in infants and adolescents.
Bacterial Skin Infections
- Impetigo.
Impetigo is a bacterial infection that results in a
honey-colored, crusty rash, often on the face near the mouth and
nose.
- Cellulitis.
Cellulitis is an infection of the skin and subcutaneous tissue
that typically occurs when bacteria are introduced through a
puncture, bite, or other break in the skin. The area with
cellulitis is usually warm, tender, and has some redness.
- Streptococcal and
staphylococcal
infections.
These two kinds of bacteria are the main causes of cellulitis and
impetigo. Certain types of these bacteria are also responsible
for distinctive rashes on the skin, including the rashes
associated with
scarlet fever
and
toxic shock syndrome
.
Fungal Infections of the Skin and Nails
- Candidal dermatitis.
A warm, moist environment, such as that found in the folds of the
skin in the diaper area of infants, is perfect for growth of the
yeast
Candida
. Yeast infections of the skin in older children, teens, and
adults are less common.
- Tinea infection(ringworm).
Ringworm, which isn't a worm at all, is a fungus infection
that can affect the skin, nails, or scalp.
Tinea
fungi can infect the skin and related tissues of the body. The
medical name for ringworm of the scalp is tinea capitis; ringworm
of the body is called tinea corporis; and ringworm of the nails
is called tinea unguium. With tinea corporis, the fungi can cause
scaly, ring-like lesions anywhere on the body.
- Tinea pedis(athlete's foot).
This infection of the feet is caused by the same types of fungi
that cause ringworm. Athlete's foot is commonly found in
adolescents and is more likely to occur during warm weather.
Other Skin Problems
- Parasitic infestations.
Parasites (usually tiny insects or worms) can feed on or burrow
into the skin, often resulting in an itchy rash.
Scabies
and
lice
are examples of parasitic infestations. Both are contagious and
can be easily caught from other people.
- Viral infections.
Many viruses cause characteristic rashes on the skin, including
varicella, the virus that causes
chickenpox
and shingles;
herpes simplex
, which causes cold sores; human papillomavirus, the virus that
causes
warts
; and a host of others.
- Acne(acne vulgaris).
Acne is
most common in teens
. Some degree of acne is seen in 85% of adolescents, and nearly
all teens have the occasional pimple, blackhead, or
whitehead.
- Skin cancer.
Skin cancer is rare in children and teens, but good
sun protection
habits established during these years can help prevent skin
cancers such as
melanoma
(a serious form of skin cancer that can spread to other parts of
the body) later in life, especially among fair-skinned people who
sunburn easily.
In addition to these diseases and conditions, the skin can be
injured in a number of ways. Minor scrapes, cuts, and bruises heal
quickly on their own, but other injuries - severe cuts and burns,
for example - require medical treatment.
Disorders of the Scalp and Hair
- Tinea capitis
, a type of ringworm, is a fungal infection that forms a scaly,
ring-like lesion in the scalp. It's contagious and common
among school-age children.
- Alopecia
is an area of hair loss. Ringworm is a common cause of temporary
alopecia in children. Alopecia can also be caused by tight
braiding that pulls on the hair roots (called tension alopecia).
Alopecia areata (when hair falls out in round or oval patches on
the scalp) is a less common condition that can affect children
and teens.
Reviewed by:
Patrice Hyde, MD
Date reviewed: June 2006
Originally reviewed by:
Wayne Ho, MD
Note: All information is for educational purposes only. For specific medical advice,
diagnoses, and treatment, consult your doctor.
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