Page 194 - Fundamentals of anatomy physiology

Stratum Granulosum
Superficial to the stratum spinosum is the stratum granulosum,
or “grainy layer.” It consists of three to five layers of keratinocytes
derived from the stratum spinosum. By the time cells are pushed
into this layer, most have stopped dividing and have started mak-
ing large amounts of the proteins keratin (KER-a-tin; keros, horn)
and keratohyalin (ker-a-to. -HI.-a-lin). Keratin is a tough, fibrous
protein. It is the basic structural component of hair and nails in
humans. p. 146 As keratin fibers accumulate, the cells grow
thinner and flatter, and their plasma membranes thicken and
become less permeable. Keratohyalin forms dense cytoplasmic
granules that promote dehydration of the cell as well as aggrega-
tion and cross-linking of the keratin fibers. The nuclei and other
organelles then disintegrate, and the cells die. Further dehydra-
tion creates a tightly interlocked layer of cells that consists of kera-
tin fibers surrounded by keratohyalin.
Stratum Lucidum
In the thick skin of the palms and soles, a glassy stratum
lucidum (“clear layer”) covers the stratum granulosum. The
cells in the stratum lucidum are flattened, densely packed,
largely without organelles, and filled with keratin.
Stratum Corneum
At the exposed surface of both thick skin and thin skin is the stra-
tum corneum (KOR-ne.-um; corneus, horny). It normally contains
15 to 30 layers of keratinized cells. Keratinization, or cornification,
is the formation of protective, superficial layers of cells filled with
keratin. This process takes place on all exposed skin surfaces except
the anterior surfaces of the eyes. The dead cells in each layer of the
stratum corneum remain tightly interconnected by desmosomes.
The connections are so secure that keratinized cells are generally
shed in large groups or sheets rather than individually.
It takes 7 to 10 days for a cell to move from the stratum ba-
sale to the stratum corneum. The dead cells generally remain
in the exposed stratum corneum for an additional two weeks
before they are shed or washed away. This arrangement places
the deeper portions of the epithelium and underlying tissues
beneath a protective barrier of dead, durable, and expendable
cells. Normally, the surface of the stratum corneum is relatively
dry, so it is unsuitable for the growth of many microorganisms.
Lipid secretions from sebaceous glands coat the surface, help-
ing to maintain this barrier against water loss or entry.
The stratum corneum is water resistant, but not waterproof.
Water from interstitial fluids slowly penetrates to the surface
and evaporates into the surrounding air. You lose about 500 mL
(about 1 pt) of water in this way each day. The process is called
insensible perspiration, because you are unable to see or feel
(sense) the water loss. In contrast, you are usually very aware of
the sensible perspiration produced by active sweat glands.
Chapter 5 The Integumentary System 181
Damage to the epidermis can increase the rate of insen-
sible perspiration. If the damage breaks connections between
superficial and deeper layers of the epidermis, fluid accumu-
lates in pockets, or blisters, within the epidermis. (Blisters
also form between the epidermis and dermis if the basement
membrane is damaged.) If damage to the stratum corneum
makes it less effective as a water barrier, the rate of insensible
perspiration skyrockets, and a potentially dangerous fluid
loss occurs. This loss is a serious consequence of severe burns
and a complication in the condition known as xerosis (exces-
sively dry skin).
5Why do wrinkles form in the skin on the fingers and toes
after a long soak in the bath? Previously, it was common to ex-
plain this effect as the osmotic movement of water across the
epithelium. That is, the fingertips absorb water and swell, making
the skin wrinkle. However, such a process failed to explain why
wrinkling did not occur if there was nerve damage in the fingers.
It turns out that the fingertips do not swell in water, but they actu-
ally shrink when they wrinkle, an effect caused by the constriction
of underlying blood vessels. The effect is due to the autonomic
nervous system, which functions mostly outside our awareness
and controls blood vessel diameter. Is there any benefit to having
wrinkled fingertips? Recent studies have shown that the wrinkles
improve the ability to grip wet or submerged objects, much like
the treads of a tire improve traction on a wet road.
Checkpoint
1. Identify the layers of the epidermis.
2. Dandruff is caused by excessive shedding of cells from
the outer layer of skin in the scalp. So dandruff is
composed of cells from which epidermal layer?
3. A splinter that penetrates to the third layer of the
epidermis of the palm is lodged in which layer?
4. Why does taking a bath cause wrinkly fingertips
and toes?
5. Some criminals sand the tips of their fingers so as not
to leave recognizable fingerprints. Would this practice
permanently remove fingerprints? Why or why not?
See the blue Answers tab at the back of the book.
5-2 Factors influencing skin color
are epidermal pigmentation and dermal
circulation
Learning Outcome Explain what accounts for individual differences in
skin color, and discuss the response of melanocytes to sunlight exposure.
One of the most obvious distinguishing characteristics among
humans is skin color. What gives skin its various colors? In this
section we examine how pigments in the epidermis and blood
flow in the dermis influence skin color. We also discuss clinical
implications of changes in skin color.