senses light touch. Still other receptors (free nerve endings)
respond to temperature changes or to factors that can dam-
age tissues and extend into the epidermis. Sensory recep-
tors are discussed in chapter 12 (p. 440). The dermis also
contains accessory structures including blood vessels, hair
follicles, sebaceous glands, and sweat glands.
To create a tattoo, very f
ne needles inject inks into the dermis. The
color is permanent, because dermis cells are not shed, as are cells oF
the epidermis. To remove a tattoo, a laser shatters the ink molecules,
and the immune system removes the resulting debris. BeFore laser
removal became available in the late 1980s, unwanted tattoos were
scraped, Frozen, or cut away—all painFul procedures.
What types oF tissues make up the dermis?
What are the Functions oF these tissues?
OF THE SKIN
Accessory structures of the skin originate from the epider-
mis and include nails, hair follicles, and skin glands. As long
as accessory structures remain intact, severely burned or
injured dermis can regenerate.
are protective coverings on the ends of the F
and toes. Each nail consists of a
that overlies a
surface of skin called the
cells continuous with the epithelium of the skin produce the
nail bed. The whitish, thickened, half-moon-shaped region
(lunula) at the base of a nail plate is the most active grow-
ing region. The epithelial cells here divide, and the newly
formed cells become keratinized. This gives rise to tiny,
keratinized scales that become part of the nail plate, push-
ing it forward over the nail bed. In time, the plate extends
beyond the end of the nail bed and with normal use gradu-
ally wears away
Nail appearance mirrors health. Bluish nail beds may reFlect a cir-
culation problem. A white nail bed or oval depressions in a nail can
indicate anemia. A pigmented spot under a nail that isn’t caused by
an injury may be a melanoma. Horizontal Furrows may result From a
period oF serious illness or indicate malnutrition. Certain disorders
oF the lungs, heart, or liver may cause extreme curvature oF the nails.
Red streaks in noninjured nails may be traced to rheumatoid arthritis,
ulcers, or hypertension.
The boundary between the epidermis and dermis is usually
uneven. This is because the epidermis has ridges projecting
inward and the dermis has conical
into the spaces between the ridges (see F gs. 6.2 and 6.3).
Dermal papillae increase the surface area where epidermal
cells receive oxygen and nutrients from dermal capillaries.
±ingerprints form from these undulations of the skin
at the distal end of the palmar surface of a finger. The
undulations increase friction at the fingertips for grasp-
ing. ±ingerprints may be used for purposes of identiF cation
because they are individually unique. The pattern of a F
gerprint is genetically determined, and the prints form dur-
ing fetal existence. However, during a certain time early in
development, fetal movements can change the print pattern.
No two fetuses move exactly alike, so even the F ngerprints
of identical twins are not exactly the same.
The dermis binds the epidermis to the underlying tissues.
It is largely composed of dense irregular connective tissue that
includes tough collagenous F bers and elastic F bers in a gel-
like ground substance. Networks of these F bers give the skin
toughness and elasticity. On the average, the dermis is 1.0–2.0
mm thick; however, it may be as thin as 0.5 mm or less on the
eyelids or as thick as 3.0 mm on the soles of the feet.
The dermis also contains muscle F bers. Some regions,
such as the skin that encloses the testes (scrotum), contain
many smooth muscle cells that can wrinkle the skin when
they contract. Other smooth muscles in the dermis are
associated with accessory organs such as hair follicles and
glands. Many skeletal muscle F bers are anchored to the der-
mis in the skin of the face. They help produce the voluntary
movements associated with facial expressions.
Nerve cell processes are scattered throughout the der-
mis. Motor processes carry impulses to dermal muscles and
glands, and sensory processes carry impulses away from
specialized sensory receptors (see F
One type of dermal sensory receptor, lamellated
(Pacinian) corpuscles, is stimulated by heavy pressure,
whereas another type, tactile (Meissner’s) corpuscles,
Some newborns develop the yellowish skin oF jaundice shortly aFter
birth. A blood incompatibility or an immature liver can cause jaun-
dice. An observant British hospital nurse discovered a treatment For
newborn jaundice in 1958. She liked to take her tiny charges out in
the sun, and she noticed that a child whose skin had a yellow pallor
developed normal pigmentation when he lay in sunlight. However,
the part oF the child’s body covered by a diaper and thereFore not
exposed to the sun remained yellow. ±urther investigation showed
that sunlight enables the body to break down bilirubin, the liver sub-
stance that accumulates in the skin. Today, newborns who develop
persistently yellowish skin may have to lie under artif
cial “bili lights”
For a Few days, clad only in protective goggles.