Encyclopedia of Health and Medicine

skin 195
The palms of the hands and the soles of the feet
are the only skin surfaces that do not have sebaceous
glands.
The oily consistency of sebum gives the skin a
highly water-resistant coating. The lubricating
qualities of sebum keep the keratinocytes, the cells
that make up the epidermis, supple and flexible.
Without adequate lubrication the skin becomes
dry and the keratinocytes scale and flake, presenting
not only an undesirable cosmetic appearance
but also compromising the skin’s resistance against
pathogenic (disease-causing) microorganisms.
Sebum also helps regulate the skin’s natural flora,
the collective of BACTERIA, yeasts, and other microscopic
organisms that inhabit the epidermis. These
microorganisms draw nutrients from the lipids in
the sebum.
For further discussion of the sebaceous glands
within the context of integumentary structure and
function, please see the overview section, “The
Integumentary System.”
See also ACNE; DANDRUFF; KERATINOCYTE; SWEAT
GLANDS.
seborrheic keratosis A condition in which noncancerous
(benign) growths arise on the SKIN. The
lesions resemble warts though appear pasted on
rather than attached to the skin. The lesions are
most commonly brown or black, though may be
yellow, gray, tan, or other colors. Seborrheic keratosis
becomes increasingly common in people age
40 and older. Most people develop multiple
lesions. The lesions cause no symptoms beyond
their presence, unless frequent irritation causes
them to itch, hurt, or bleed.
Seborrheic keratosis requires medical assessment
only to ascertain that the lesions are not
cancerous, which typically is apparent on the basis
of their appearance and history. The doctor should
biopsy any lesions that are questionable. There is
no medical reason to remove the lesions once
diagnosed, however, as they do not turn malignant.
People sometimes want lesions removed
that are cosmetically undesirable or in locations
where they receive frequent irritation such as
from clothing. Cryotherapy (freezing), curettage
and electrodesiccation (scraping and burning), and
shave excision (cutting out; requires no sutures)
are the most common methods of removal.
Though removed lesions do not recur, others may
grow nearby.
See also ACROCHORDON; LESION; NEVUS; SKIN SELF-
EXAMINATION; WART.
skin The body’s largest organ, making up the
body’s covering and about 15 percent of the total
body weight The skin’s three layers—epidermis,
dermis, and subcutaneous layer—help the body
maintain its structure; protect against INFECTION;
and regulate fluids, electrolytes, and temperature.
Numerous health conditions, localized and systemic,
can affect the skin and its functions.
The subcutaneous layer, innermost to the body,
contains primarily adipose tissue more familiarly
called body fat. The dermis, the middle layer, provides
the structure of the skin. It contains connective
tissue, the SEBACEOUS GLANDS, and an abundant
supply of nerves and blood vessels. The dermis
nourishes the epidermis above it and attaches to
the subcutaneous layer beneath it, holding the
skin in place. HAIR follicles and SWEAT GLANDS
extend from the epidermis into the dermis and a
bit into the subcutaneous layer.
The primary cells of the skin, melanocytes and
keratinocytes, originate in the base, or basal, level
of the epidermis. Keratinocytes migrate outward
to form the upper epidermis, gradually flattening
and hardening. The epidermis varies in thickness
and other characteristics, accommodating the
needs of different body surfaces. The epidermis of
the palms of the hands and the soles of the feet is
thick and tough, for example, while that of the
eyelids is soft and only two or three cells in depth.
The skin is also the body’s organ of tactile sensory
perception, or touch. Millions of NERVE endings
in the skin continually sense environmental
factors such as pressure, temperature, moisture.
Other specialized nerve cells, called nociceptors,
perceive itching and PAIN. Sweat evaporation on
the skin’s surface is the body’s primary cooling
mechanism, as well as a secondary mechanism for
electrolyte regulation and balance.
For further discussion of the skin within the
context of integumentary structure and function,
please see the overview section “The Integumentary
System.”
See also KERATINOCYTE; MELANOCYTE; NAILS; SEBA-
CEOUS GLAND.