The structural organization of the epidermis varies from
region to region. It is thickest on the palms of the hands and
the soles of the feet, where it may be 0.8–1.4 mm thick. In
most areas, only four layers are distinguishable. They are the
(stratum germinativum, or basal cell layer), the
deepest layer; the
a fully keratinized outermost layer.
An additional layer, the
(between the stra-
tum granulosum and the stratum corneum) is in the thickened
skin of the palms and soles. The cells of these layers change
shape as they are pushed toward the surface
(f g. 6.3)
In body regions other than the palms and soles, the epi-
dermis is usually thin, averaging 0.07–0.12 mm. The stratum
lucidum may be missing where the epidermis is thin.
describes the characteristics of each layer of the epidermis.
In healthy skin, production of epidermal cells closely
balances loss of dead cells from the stratum corneum. As a
result, skin does not completely wear away. The rate of cell
division increases where the skin is rubbed or pressed regu-
larly, causing the growth of thickened areas called
on the palms and soles and keratinized conical masses on
the toes called
In psoriasis, a chronic skin disease, cells in the epidermis divide seven
times more frequently than normal. Excess cells accumulate, form-
ing bright red patches covered with silvery scales, which are kerati-
nized cells. Medications used to treat cancer, such as methotrexate,
are used to treat severe cases of psoriasis. Immune suppressing
medications, such as topical corticosteroids, are used for treatment of
chronic psoriasis. Five million people in the United States and 2% of
people worldwide have psoriasis.
The epidermis has important protective functions. It
shields the moist underlying tissues against excess water loss,
mechanical injury, and the effects of harmful chemicals. When
intact, the epidermis also keeps out disease-causing micro-
The adipose tissue of the subcutaneous layer insulates, help-
ing to conserve body heat and impeding the entrance of heat
from the outside. The subcutaneous layer also contains the
major blood vessels that supply the skin. Branches of these
vessels form a network (rete cutaneum) between the dermis
and the subcutaneous layer. They, in turn, give off smaller
vessels that supply the dermis above and the underlying adi-
Drugs that cannot be taken orally are delivered through the skin in
several ways: into the skin (intradermal injections), beneath the skin
(subcutaneous injections), and into muscles (intramuscular injec-
tions). (Subcutaneous and intramuscular injections are also called
hypodermic injections.) Another, less painful route is with an adhe-
sive transdermal patch. The drug is in a small reservoir in the patch.
It leaves through a permeable membrane at a known rate, di±
through the epidermis, and enters blood vessels in the dermis. Drugs
that alleviate chest pain, prevent motion sickness, lower blood pres-
sure, and help people stop smoking are delivered with transdermal
patches. A new type of transdermal patch may extend the approach
to drugs that can be taken by mouth because they can use lower
doses with fewer adverse e±
ects. This technique uses “microneedles”
to painlessly punch tiny holes in the stratum corneum.
List the general functions of the skin.
Name the tissue in the outer layer of the skin.
Name the tissues in the inner layer of the skin.
Name the tissues in the subcutaneous layer beneath the skin.
What are the functions of the subcutaneous layer?
The epidermis is composed entirely of stratiF
epithelium, and therefore it lacks blood vessels. However,
the deepest layer of epidermal cells, called the
is close to the dermis and is nourished by dermal
blood vessels, which enables the cells to divide and grow.
As new cells enlarge, they push the older epidermal cells
away from the dermis toward the surface of the skin. The
farther the cells are moved, the poorer their nutrient supply
becomes, and in time, they die.
The cell membranes of older skin cells (keratinocytes)
thicken and develop many desmosomes that fasten them to
each other (see chapter 5, p. 144 and the opening vignette
to this chapter, p. 171). At the same time, the cells begin
to harden, in a process called
shun). Strands of tough, F brous, waterproof keratin pro-
teins are synthesized and stored in the cell. As a result, many
layers of tough, tightly packed dead cells accumulate in the
epidermis, forming an outermost layer called the
These dead cells are eventually shed. Rubbing the
skin briskly with a towel sheds dead cells.
The epidermis receives its nutrients from blood vessels in the dermis,
so interference with blood ²
ow may kill epidermal cells. For example,
when a person lies in one position for a prolonged period, the weight
of the body pressing against the bed blocks the skin’s blood supply.
If cells die, the tissues begin to break down (necrosis), and a pressure
ulcer (also called a decubitus ulcer or bedsore) may appear.
Pressure ulcers usually form in the skin overlying bony projec-
tions, such as on the hip, heel, elbow, or shoulder. Frequently changing
body position or massaging the skin to stimulate blood ²
ow in regions
associated with bony prominences can prevent pressure ulcers. For a
paralyzed person who cannot feel pressure or respond to it by shift-
ing position, caregivers must turn the body often to prevent pressure
ulcers. Beds, wheelchairs, and other specialized equipment can period-
ically shift the patient, lowering the risk of developing pressure ulcers.