keloid An overgrowth of collagen after a wound
has finished HEALING. A keloid typically forms as
folds or bunches of tissue. Keloids are fibrous,
spongy in consistency, and often dark red. They
form most often on the earlobes, upper chest, and
shoulders though can develop anywhere on the
body. Keloids are more common in people who
have dark skin, and in people under age 50.
Though keloids do not present any health prob-
lems, they can become irritated from rubbing on
clothing. A corticosteroid medication injected into
the keloid often halts its growth and causes the
existing excess tissue to recede. The dermatologist
can surgically remove large keloids or keloids that
recur.
See also ACROCHORDON; CORTICOSTEROID MEDICA-
TIONS; SCAR.
keratinocyte The cell type that makes up most of
the epidermis, also called a squamous cell.
Keratinocytes originate in the first of the four layers
of the epidermis, the stratum basale. Here they
either replicate to generate new keratinocytes or
migrate upward. Migratory keratinocytes acquire
melanin from melanocytes. The keratinocytes carry
this pigment to the outer layers of the SKIN, where it
appears as the skin’s normal color or causes the skin
to darken (as in a tan). At each of the epidermis’s
layers the keratinocytes become more compressed.
Their internal structures break down, and the ker-
atin they contain causes them to harden.
At the stratum corneum, the outer layer of the
epidermis, the keratinocytes overlap tightly, look-
ing somewhat like irregular shingles when viewed
under the microscope. At the culmination of this
journey, which takes about four weeks, the ker-
atinocytes die and slough from the skin’s surface.
The fingernails and toenails are much more tightly
compressed and hardened keratinocytes. They do
not shed as does the stratum corneum but instead
grow forward over the front of the fingers and
toes at the rate of about one eighth inch every
four to five weeks.
Hyperkeratosis is a state in which the ker-
atinocytes migrate through the epidermis far more
rapidly than normal, sometimes cutting the jour-
ney to 10 days. This accelerated journey causes
more keratinocytes than the body can shed to
accumulate in the HAIR follicles and sebaceous
structures, causing numerous hyperkeratosis-
related conditions from ACNEand atopicDERMATITIS
to PSORIASISand SEBORRHEIC KERATOSIS. Squamous
cell carcinoma, a common type of SKIN CANCER,
arises from keratinocytes.
For further discussion of keratinocytes within
the context of integumentary structure and func-
tion please see the overview section “The Integu-
mentary System.”
See alsoICHTYOSIS; MELANOCYTE; NAILS.keratoacanthoma A form of squamous cell car-
cinoma (SKIN CANCER) that appears suddenly and
grows rapidly, though has a low rate of METASTASIS
(spreading). Like other forms of skin CANCER, kera-
toacanthoma is the consequence of extensive sun
exposure that manifests decades later. Researchers
have identified a number of chromosomal abnor-
malities that appear connected with keratoacan-
thoma, suggesting a strong genetic component or
familial predisposition (tendency of the cancer to
run in families).
Most keratoacanthomas develop in people over
age 50, though may occur at younger ages in peo-
ple who are taking IMMUNOSUPPRESSIVE THERAPY
(such as following ORGAN TRANSPLANTATION) or who
are IMMUNOCOMPROMISED. A keratoacanthoma
lesion typically develops on skin surfaces that
receive or have received significant sun exposure
and may initially appear to be a FURUNCLE(boil) or
a cyst. The lesion often appears to heal, though
seems to take a long time to do so (up to a year).
Though it appears that keratoacanthoma even-
tually resolves (heals) on its own, the risk that the
lesion could instead be squamous cell carcinoma
or that the keratoacanthoma could metastasize
causes dermatologists to recommend immediate
removal with microscopic examination to confirm
the diagnosis. Keratoacanthomas tend to recur.
The dermatologist may recommend surgical
removal of the lesion or inject it with a
chemotherapeutic agent, either of which generally
is adequate treatment.
See also ACTINIC KERATOSIS; CANCER TREATMENT
OPTIONS AND DECISIONS; SKIN SELF-EXAMINATION.keratosis pilaris A very common condition in
which the keratocytes produce excessive keratin,
clogging the HAIR follicles and forming small170 The Integumentary System