- Beau’s lines: indentations in the nail surface
that extend across the width of all the nails at
about the same position on each - clubbing: end of the finger or toe becomes
enlarged and the angle between the nail fold
and the nail plate exceeds 180 degrees - discoloration: may be widespread throughout
the nail or occur in spots or streaks - koilonychia: nails soften and the edges rise,
leaving a large spoonlike indentation in the
center of the nail - leukonychia: white spots or streaks in the nail
- onycholysis: separation of the nail from the nail
bed - PETECHIAE: red or dark spots beneath the nail
- stippling: the formation of small pits in the sur-
face of the nail
For further discussion of the nails within the
context of integumentary structure and function
please see the overview section “The Integumen-
tary System.”
See also INGROWN NAIL; KERATINOCYTE; ONYCHOMY-
COSIS.
neurodermatitis See LICHEN SIMPLEX CHRONICUS.
nevus A discolored LESIONon the SKIN. Nevi are
very common and may take various shapes and
colors. Most nevi contain primarily melanocytes
and may differ in texture from the surrounding
skin. A nevus may be smooth, distinguishable
only by its color, or rough and segmented. Some
nevi contain coarse HAIR. The most common form
of nevus is a mole, a small lesion that can be
smooth or raised and is usually darker in color
than the surrounding skin. A nevus may be con-
genital (present at birth) or acquired (develops at
any point in the lifespan after birth). Nearly
everyone has some nevi by early adulthood.
Giant congenital nevus, a rare presentation of
congenital nevus, may cover a large area of the
skin’s surface. In another uncommon genetic dis-
order, neurocutaneous melanosis, nevi develop
within the structures of the BRAINand SPINAL CORD.
The presence of melanocytes in some NERVOUS SYS-
TEMtissue is normal and functional. Melanocytes
populate the substantia nigra, for example, a
structure of the midbrain. The pigmented cells of
substantia nigra produce dopamine, a NEUROTRANS-
MITTER essential for the brain’s coordination of
MUSCLEfunction throughout the body. However,
melanocytes in other parts of the nervous system
can generate overgrowths—nevi—just as they do
on the skin. Such nevi cause pressure as they
grow, resulting in neurologic symptoms.
Acquired nevi begin to appear during child-
hood in most people, with the most intense activ-
ity occurring in middle adulthood (ages 30 to 50).
Though sun exposure plays a role in their devel-
opment, genetic encoding seems to regulate char-
acteristics such as size, color, shape, and numbers.
In the typical structure of the dermis, the layer of
skin where melanocytes reside in greatest concen-
tration, melanocytes are frequent but are not in
contact with each other. Researchers believe this
distribution pattern results from “contact inhibi-
tion” genetic encoding. A nevus can form when
the contact inhibition lapses, allowing the
melanocytes to drift into contact with one
another. Sometimes nevi appear after extensive
injury to the skin, such as occurs with conditions
that cause widespread blistering. This suggests that
such injuries disrupt contact inhibition in some
way, though the mechanisms through which this
occurs remain unknown.A nevus should have regular borders,
consistent coloration, and a symmetri-
cal (balanced) appearance. A doctor
should evaluate a nevus that has or
develops irregular borders, variable col-
oration, or an asymmetrical appearance
as these characteristics may indicate a
nevus that is becoming cancerous.Though nevi are themselves benign (non-
cancerous), they can become cancerous over time
and particularly with repeated, unprotected sun
exposure. Congenital nevi in particular carry an
increased risk for malignant melanoma, a serious
type of SKIN CANCER. Most often, the only health
concerns with nevi are the increased risk for
malignancy and cosmetic appearance. The derma-
tologist may choose to remove nevi that receive
frequent irritation, such as those that form in178 The Integumentary System