New skin cells form in the stratum basale and are slow-
ly pushed to the surface. As the cells move from one epi-
dermal layer to the next, they accumulate more and more
keratin through a process called keratinization (ker'-a-
tin-i-ZA-shun). Eventually, the keratinized cells slough off
and are replaced by underlying cells. The cycle from cell
division to sloughing off at the surface takes about 4 weeks
in an average epidermis of 0.1 mm (0.004 in.) thickness.
When an excessive amount of keratinized cells shed from
the skin of the scalp, it is called
dandruff.
The
dermis
is the second, deeper part of the skin. It
is composed mainly of dense irregular connective tissue
containing collagen and elastic fibers. The superficial part
of the dermis, also known as the
papillary region,
makes up
about one-fifth of the thickness of the total layer (see Figure
4.1b). It contains fine elastic fibers. Its surface area is great-
ly increased by small, fingerlike projections called dermal
papillae (pa-PIL-e;
papula
= nipples; singular is papilla).
These nipple-shaped structures project into the undersur-
face of the epidermis and can contain the following:
Blood capillaries
(capillary loops)
Nerve endings
(sensory receptors):
Corpuscles of touch
or
Meissner corpuscles
—Associated
with touch
Free nerve endings
—Associated with
sensations
of
warmth, coolness, pain, tickling, and itching
The deeper part of the dermis, also known as the
re-
ticular region,
which is attached to the subcutaneous layer,
contains bundles of collagen and some coarse elastic fibers
interspersed with adipose cells, hair follicles, nerves, oil
glands, and sweat glands. This combination of collagen and
elastic fibers provides the skin with strength,
extensibility
(ability to stretch), and
elasticity
(ability to return to original
shape after stretching). The extensibility of skin can read-
ily be seen in pregnancy and obesity. The skin has its lim-
its, however; extreme stretching may produce small tears
in the dermis, forming
striae
(STRl-e;
stria
= streaks), or
stretch marks, that are visible as red or silvery white streaks
on the skin surface.
You can see the difference between the epidermis and
dermis when you get a blister. The top part of the blister is
the epidermis, while the bottom part beneath the fluid is
the papillary region of the dermis.
Skin Color Is Caused by Pigments
A wide variety of skin colors are produced by three pig-
ments:
melanin,
hemoglobin,
and
carotene.
Differing
amounts of melanin (MEL-a-nin) and the shade of melanin
cause the skin to present in a wide range of colors from pale
yellow to reddish brown to black. Melanin absorbs damag-
ing ultraviolet (UV) light to protect the underlying tissues
of the skin. Melanin-producing cells, called melanocytes,
are present in the skin (stratum basale of the epidermis)
and mucous membranes all over the body. However, they
are most plentiful in the epidermis of the penis, nipples of
the breasts, areas just around the nipples (areolae), face,
and limbs. Because the number of melanocytes is about the
same in all people, differences in skin color are due mainly
to the amount and shade of pigment that the melanocytes
produce and transfer to keratinocytes. Consequently, the
epidermis has a variation of pigmentation with skin color
ranges from yellow to red to tan to black:
Dark-skinned
individuals
have
large
amounts
of
melanin in the epidermis. The more melanin that is
present, the darker the skin.
Light-skinned individuals have little melanin in the
epidermis. Thus, the epidermis appears translucent,
and skin color ranges from pink to red, depending on the
oxygen content of the blood moving through capillaries
in the dermis. The red color is due to hemoglobin, the
oxygen-carrying pigment in red blood cells.
Albinism (AL-bin-izm;
albin-
= white) is an inherited
trait that causes individuals to not produce melanin.
People affected by albinism are called albinos
(al-
Bl-nos). Because most albinos do not have melanin
in their hair, eyes, and skin, they need to take extra
precautions when exposed to the sun.
Melanocytes may not be evenly scattered throughout the
skin, causing uneven melanin distribution:
Freckles
—With
freckles,
melanin
accumulates
in
patches.
Age (liver) spots
—With age spots, melanin accumulates
with age, forming flat blemishes that look like freckles
but range in color from light brown to black.
Nevus or mole
—A nevus (NE-vus)
is a round, flat,
or raised area that represents a benign localized
overgrowth of melanocytes and usually develops in
childhood or adolescence.
Vitiligo
—Vitiligo (vit-i-Ll-go) is a condition in which
melanocytes are partially or completely lost from
areas of the skin, producing irregular white spots.
One possible cause is a malfunction of the immune
system in which antibodies attack and destroy the
melanocytes.
Melanin sits superficial to the nucleus in the keratinocyte,
blocking ionizing radiation from reaching and damaging the
DNA. Melanin levels in the skin may change with exposure
The Integumentary System Is Composed of Skin, Glands, Hair, Nails, and Nerve Endings
95
previous page 130 Craig Freudenrich, Gerard J  Tortora   Visualizing Anatomy and Physiology   2011 read online next page 132 Craig Freudenrich, Gerard J  Tortora   Visualizing Anatomy and Physiology   2011 read online Home Toggle text on/off