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The Epidermis
The outermost or epidermal layer of the skin is com-posed
of stratified, squamous, keratinized (no nu-cleus) epithelial
cells. These cells are held together by highly convoluted,
interlocking cellular links called desmosomes (DEZ-
meh-somz). These desmosomes are responsible for the
unique flexibility, entirety, and whole continuous structure
of the skin. The epi-dermis is thickest where it receives the
most abrasion and weight—on the palms of the hands and
the soles of the feet. It is much thinner over the ventral
surface of the trunk.
The epidermis, which is not vascularized, rests on a
basement membrane of connective tissue. The low-ermost
cells on this membrane divide by mitosis, so new cells push
older cells up toward the surface. As they move up, they
change shape and chemical com-position because they lose
most of their water and eventually die. This process is
called keratinization (kair-ah-tin-ih-ZAY-shun) because
the cells become filled with keratin (KAIR-ah-tin), a
protein material. These dead, outermost cells are constantly
being shed. This outermost layer forms an effective barrier
to sub-stances that would penetrate the skin, and this layer
is very resistant to abrasion.
The process of keratinization produces distinctive
layers of the epidermis called strata (plural) or stratum
(STRAT-um) [singular] (Figure 6-2). There are five layers
from outermost to deep. They are the stratum corneum
(STRAT-um COR-nee-um), commonly called the horny or
leathery layer; the stratum lucidum (STRAT-um LOO-sid-
um), commonly called the clear layer; the stratum
granulosum (STRAT-um gran-you-LOH-sum), com-
monly called the granular layer; and the stratum spino-sum
(STRAT-um spye-NOH-sum), commonly called the spiny
or prickly layer. The innermost layer and the most
important is the stratum germinativum (STRAT- um jer-
mih-NAY-tih-vum) or the regenerative layer, also called
the stratum basale.
The Stratum Corneum
The stratum corneum forms the outermost layer of the
epidermis. It consists of dead cells converted to protein.
Chapter 6They are called keratinized cells because they have lost
most of their fluid. The organelles of the cell are now just
masses of the hard protein keratin that gives this layer its
structural strength. These cells are also covered and
surrounded with lipids to prevent any passage of -fluids
through this layer. These cells have only about 20% -water
as compared to cells in the lowermost layer that have about
70%. The cells resemble scales in shape and can consist of
up to 25 layers. By the time cells reach this layer, the
desmosomes have broken apart and, there-fore, these cells
are constantly being sloughed off. This -process is referred
to as excessive desquamation. The shedding of these cells
from the scalp produces what we call dandruff.This layer also functions as a physical barrier to light
and heat waves, microorganisms (e.g., bacteria, fungi,
protozoa, and viruses), and most chemicals. The thickness
of this layer is determined by the amount of stimulation on
the surface by abrasion or weight bear-ing, hence the
thickened palms of the hands and soles of the feet. When
skin is subjected to an excessive amount of abrasion or
friction, a thickened area called a callus (KAL-us) will
develop. Learners who do a lot of writing will -develop
small calluses on their fingers that hold their pens.
Abrasion on the bony prominences on the foot can produce
structures we call corns.The Stratum Lucidum
The stratum lucidum lies directly beneath the stratum
corneum but is difficult to see in thinner skin. It is only one
or two cell layers thick. Its cells are transparent and flat.The Stratum Granulosum
The stratum granulosum consists of two or three layers
of flattened cells. Because granules tend to accumulate in
these cells, it was named the granular layer. These gran-
ules have nothing to do with skin color. This layer is very
active in keratinization. In this layer, the cells lose their
nuclei and become compact and brittle.The Stratum Spinosum
The stratum spinosum consists of several layers of
prickly or spiny-shaped cells that are polyhedral in
structure. In this layer, desmosomes are still quite
prevalent. The out-line caused by the polyhedral shapes
causes the cell’s outlines to look spiny, hence the name. In
some classi-fication schemes, this layer is included with the
stratum germinativum.