This most extensive
organ system has the skin and accessory structures, including hair,
nails, glands (sweat and sebaceous), and specialized nerve receptors
for stimuli (changes in internal or external environment) such as
touch, cold, heat, pain, and pressure. Its functions include protection
of internal structures, prevention of entry of disease-causing microorganisms,
temperature regulation, excretion through perspiration, pigmentary
protection against ultraviolet sunrays, and production of vitamin
D. The body stores about half its fat in the underlying hypodermis.
The skin is
the largest organ of the body, with a surface area of 18 square
feet. Its two main layers are the epidermis (outer layer) and dermis
(inner layer). The epidermis has several strata (layers) that contain
four cell types. Keratinocytes produce keratin, a protein that gives
skin its strength and flexibility and waterproofs the skin surface.
Melanocytes produce melanin, the dark pigment that gives skin its
color. Merkel's cells are probably involved with touch reception.
Langerhans' cells help the immune system by processing antigens
layer of the epidermis, the stratum basale, is a single layer of
cells resting on a basement membrane (layer between the dermis and
epidermis). The stratum basale cells divide continuously. As new
cells form, older ones are pushed toward the skin surface.
does not have a direct blood supply; all nutrients that feed these
cells come from the dermis. Only the deepest cells of the stratum
basale receive nourishment. The cells that are pushed away from
this layer die. When the cells reach the skin surface, they are
sloughed off in a process called desquamation.
The next layer,
the stratum spinosum, consists of spiny prickle cells that interlock
to support the skin. The stratum granulosum, the thin middle layer,
initiates keratinization (production of keratin). This process starts
the death of epithelial cells (the cell type that makes up skin).
keratinocytes are pushed toward the surface. These cells begin to
produce the keratin that eventually will dominate their contents.
When these cells reach the epidermis outer layer, they are little
more than keratin-filled sacs. Millions of these dead cells are
worn off daily, creating a new epidermis every 35 to 45 days.
lucidum protects against sun ultraviolet-ray damage. This thick
layer appears only in frequently used areas such as palms of the
hands and soles of the feet. Thick skin epidermis has all five strata.
Thin skin covers thinner epidermal areas such as eyelids. Thin skin
has three or four of the five strata; it never has stratum lucidum.
corneum, the fifth, outermost layer is thick with rows of dead cells.
These cells contain soft keratin, which keeps the skin elastic and
protects underlying cells from drying out.
called "true skin, " is the layer beneath the epidermis. Its major
parts are collagen (a protein that adds strength), reticular fibers
(thin protein fibers that add support), and elastic fibers (a protein
that adds flexibility). The dermis has two layers: the papillary
layer, which has loose connective tissue, and the reticular layer,
which has dense connective tissue. These layers are so closely associated
that they are difficult to differentiate.
layer lies directly beneath the epidermis and connects to it via
papillae (finger-like projections). Some papillae contain capillaries
that nourish the epidermis; others contain Meissner's corpuscles,
sensory touch receptors. A double row of papillae in finger pads
produces the ridged fingerprints on fingertips. Similar patterns
in the ridged fingerprints on fingertips are on palms of the hands
and soles of the feet. Fingerprints and footprints keep skin from
tearing and aid in gripping objects.
layer of the dermis contains criss-crossing collagen fibers that
form a strong elastic network. This network forms a pattern called
cleavage (Langer's) lines. Surgical incisions that are made parallel
to cleavage lines heal faster and with less scarring than those
made perpendicular. Parallel incisions disrupt collagen fibers less
and require less scar tissue (cells that aid in healing) to close
up a wound.
layer also contains Pacinian corpuscles, sensory receptors for deep
pressure. This layer contains sweat glands, lymph vessels, smooth
muscle, and hair follicles, described in the discussion on hair
follicles later in this overview.
(subcutaneous layer) lies beneath the dermis. Loose connective tissue
such as adipose tissue (fat) insulates the body, conserving heat.
It also contains blood vessels, lymph vessels, and the bases of
hair follicles and sweat glands. The fat distribution in this layer
gives the female form its characteristic curves.
(sweat) and sebaceous (oil) glands
associated structures such as sudoriferous (sweat) glands and sebaceous
(oil) glands. It also produces fingernails, hair, and sensory receptors
that enable humans to feel pressure, temperature, and pain.
of sudoriferous glands (sweat glands) are in most of the body: eccrine
glands are coiled ducts deep in the skin that connect to the surface;
apocrine glands are in armpits, areolae of nipples, and the genital
region. Eccrine glands secrete sweat, a mixture of 99 percent water
and 1 percent salts and fats. In warm conditions with low humidity,
perspiration (secretion of sweat) and evaporation cool the body.
which become active at puberty, are larger, deeper, and produce
thicker secretions than eccrine glands. The apocrine glands secretions
contain pheromones, substances that enable olfactory (sense of smell)
communication with other members of the species. This communication
provokes certain behavioral responses such as sexual arousal. Unlike
eccrine glands that respond to heat, apocrine glands respond to
stress and sexual activity by secreting sweat with a characteristic
odor. This odor differs from body odor that results from bacteria
decomposing skin secretions on the skin.
are modified apocrine glands in the external ear canal lining. They
secrete cerumen (earwax), a sticky substance that is thought to
repel foreign material.
in female breasts are modified apocrine glands. These glands are
adapted to secrete milk instead of sweat.
(oil glands) are all over the body except on the palms of hands
and soles of feet. The glands empty via ducts into the bases of
hair follicles and secrete sebum (a mixture of fats, waxes, and
hydrocarbons). Sebum keeps hair moist and prevents skin from drying.
Sebaceous glands are numerous on the face and scalp. During puberty,
increased sex hormone levels in the blood may produce excessive
sebum. This over secretion plugs the gland and hair follicle, producing
a skin disorder called acne.
Hair is composed
of cornified threads of cells that develop from the epidermis and
cover most of the body. Each hair has a medulla, cortex, and cuticle.
The medulla in the center contains soft keratin and air. The cortex,
the innermost thickest layer, has the pigment that gives hair color.
The cuticle, the outermost layer, has cells that overlap like scales.
Both the cuticle and cortex have hard keratin.
The hair root
in a hair follicle is embedded beneath the skin. The hair shaft
protrudes from the skin. Hair sheds and is replaced constantly during
growth and rest phases. Hair has a protective function: eyebrows
keep sweat from running into the eyes, nose and ear hairs filter
dust from the air, and scalp hairs protect against abrasion and
overexposure to sun rays.
extend into the dermis; the deep ends expanded parts are called
hair bulbs. A papilla (connective tissue protrusion that contains
capillaries) protrudes into the hair bulb and provides nutrients
for the growing hair. The hair follicle walls have an inner epithelial
root sheath and an outer dermal root sheath. The epithelial root
sheath has an inner and an outer layer that thins as it approaches
the hair bulb. It becomes the matrix, the actively growing part
of the hair bulb that produces the hair.
muscles are smooth muscle cells attached to hair follicles. When
they contract, they pull the hair into an upright position, causing
skin dimples (goose bumps). The nervous system regulates these muscles;
cold temperatures or fright can activate them.
begins in the third fetal month. By the fifth month, lanugo (thin
hair) covers the fetus. At 5 months, lanugo disappears from every
area except the scalp and eyebrows where coarser hair replaces it.
Vellus (a film of delicate hair) eventually covers the rest of the
body. Terminal hair is the early coarse scalp and eyebrow hair and
later armpit and genital hair that grow during puberty. No new hair
follicles develop after birth.
Like hair, nails
develop from the epidermis. These hard plates of keratinized cells
are at the ends of fingers and toes. Nails appear pink because their
translucency reveals the vascular tissue beneath. They aid in grasping
objects, scratching, and protecting fingers and toes.
of the nail are the lunula, body, root, and free edge. The lunula
is the white half-moon shaped part at the nail base. Both the body
and free edge region that overhangs the end of the finger or toe
are visible. The nail rests on the thick layer of epithelial skin
called the nail bed. The root is hidden under skin folds. Under
the root lies the matrix (thick layer of skin). Eponychium (thin
layer of epithelium) covers the nail during development; in the
adult, it remains at the nail base only and is called the cuticle.
The hyponychium is the epithelium of the nail bed.
Skin color results
from the presence of melanin, carotene (yellow to orange pigment),
and underlying blood reflected through skin. Melanin keeps excessive
ultraviolet rays from burning the skin. Exposure to sunlight causes
the skin to produce more melanin, causing suntan, a temporary change
in skin color. Melanin-rich cells continually move toward the surface,
where they are sloughed. Too much sun is dangerous to skin; it increases
the risk of cancer by affecting the genetic material of cells.
Variety of skin
color is caused mainly by the number and distribution of melanocytes.
Darker skin has more melanin that is produced by more melanocytes.
However, the different skin colors among individuals and races do
not reflect different numbers of melanocytes; instead, they show
different kinds and amounts of melanin production by melanocytes.
Oriental skin has a greater amount of carotene in the stratum corneum,
producing a yellowish tinge. Albinism is a condition where skin
does not produce melanin.