Renewal and protection: the epidermisThe pri-
mary cells of the skin’s surface layer, the epidermis,
are melanocytes, which produce the pigment
melanin, and keratinocytes, which produce the
fibrous protein keratin. Both types of cells arise
from the base level of the epidermis, the stratum
germinativum (“birth”), also called the basal level
or the Malpighian level. As the Latin name implies,
this level germinates, or originates, cells. Melan-
ocytes remain in the stratum germinativum for all
of their existence. Keratinocytes migrate upward to
the stratum corneum (“horny”), the cornified or
hardened surface level of the epidermis.
Melanocytes produce melanin, the pigment
that gives color to the skin, hair, and eyes (iris).
This color has a purpose: melanin is the skin’s pri-
mary protection against the sun’s ultraviolet rays.
It absorbs ultraviolet lightwaves, preventing them
from causing damage to cell structure and func-
tion. Melanocytes store the enzyme tyrosinase,
which other cells in the body produce, and
acquire the amino acid tyrosine from the circulat-
ing blood. The tyrosinase catalyzes a series of
chemical actions that convert tyrosine to two
tones of pigment: eumelanin (black-brown pig-
ment) and pheomelanin (yellow-brown pigment).
The melanocytes package these pigments into
granules collectively called melanin.
Melanocytes also pigment cells in other organs.
When the neural crest differentiates early in
embryonic development, some melanocytes
migrate with the cells that form the structures of
the brain. A dense population of melanocytes set-
tles in a structure in the midbrain, the substantia
nigra (a name that means “black substance”). The
melanocytes pigment specialized cells in the sub-
stantia nigra that produce DOPAMINE, a brain NEURO-
TRANSMITTERessential to neuromuscular function.
When melanocytes in the skin slow melanin pro-
duction, the result may be depigmentation disor-
ders such as VITILIGOor white hair. Though often
distressing, these consequences are not serious
threats to health and life. When melanocytes in the
substantia nigra stop producing melanin, however,
the substantia nigra stops producing dopamine, and
the body stops moving, a degenerative condition
called PARKINSON’S DISEASE.
Keratinocytes produce two proteins: keratin, a
fibrous substance, and cytokine, an IMMUNE
RESPONSEmediator. After they mature in the stra-
tum germinativum, keratinocytes pick up melanin
granules and begin their migration to the surface.
The three and a half week journey to the stratus
corneum is a final rite of passage that literally
squeezes the life from the keratinocytes. During
this passage, the keratinocytes undergo denu-
clearization, a process of nucleus deterioration
that gradually diminishes cell function. The
upswell of continuous movement that carries the
keratinocytes along compresses their remaining
cell structure. By the time the keratinocytes break
through to the surface, they are flat, brittle, and
lifeless husks of keratin. They layer tightly against
one another, forming the tough covering the skin
presents to the outside world. The friction of inter-
acting with the external environment brushes
them loose and they fall away, a process called
exfoliation. Keratinocytes that funnel through the
hair follicles become hair shafts.WHY A WOUND THAT FAILS TO HEAL
MAY SUGGEST SKIN CANCER
The epidermis completes a total turnover of cells every
24 to 28 days, meaning that this outer layer of the SKIN
is fresh and new about once a month. Any skin wound
that takes longer than three or four weeks to heal rep-
resents an area of cells that is growing at its own pace,
distinct from the other skin cells—in short, a CANCER.Structure, sustenance, and sense: the dermis
The skin’s middle layer, the dermis, nurtures and
supports the skin. Primarily fibrinogens create a
collagen matrix that supports an abundant and
effusive network of blood vessels, nerves, and
glands (sweat and sebaceous). These structures are
elemental to the body’s heat regulation mecha-
nisms. The dermis also contains the Langerhans
cells, the gatekeepers of the SKIN-ASSOCIATED LYM-
PHOID TISSUE(SALT) that are the front line of the
body’s immune response.
The tiny blood vessels—arterioles and
venules—that web through the dermis cause the
skin to flush when the body is too warm. This
function of heat regulation uses the BLOODvessels
for conduction heat dispersal, dilating them to
bring a flood of blood to the cooler temperatures
near the surface. The vessels contract when body
temperature returns to an acceptable range, send-The Integumentary System 127