Page 192 - Fundamentals of anatomy physiology

5-1 The epidermis is composed
of layers with various functions
Learning Outcome Describe the main structural features
of the epidermis, and explain the functional significance of each.
The epidermis is a stratified squamous epithelium. Recall
from Chapter 4 that such an epithelium provides physical
protection and helps keep microorganisms outside the body.
p. 146
Like all other epithelia, the epidermis is avascular. Because
there are no local blood vessels, epidermal cells rely on the
diffusion of nutrients and oxygen from capillaries within the
dermis. As a result, the epidermal cells with the highest meta-
bolic demands are found close to the basement membrane,
where the diffusion distance is short. The superficial cells,
far removed from the source of nutrients, are dead. Spotlight
Figure 5–3 describes the epidermal layers and functions.
Keratinocytes (ke-RAT-i-no. -sı.ts), the body’s most abun-
dant epithelial cells, dominate the epidermis. These cells form
several layers, or strata, and contain large amounts of the pro-
tein keratin (discussed shortly). Thin skin covers most of the
body surface. Thick skin is found on the palms of the hands
and the soles of the feet. Note that the terms thick and thin
refer to the relative thickness of the epidermis, not to the cuta-
neous membrane as a whole.
Spotlight Figure 5–3 shows the five layers of keratinocytes in
a section of the epidermis in an area of thick skin. The boundar-
ies between the layers are often difficult to see in a standard light ment by cells called melanocytes. p. 154 These pigment cells
micrograph. Notice that the various layers have Latin names.
The word stratum (plural, strata) means “layer,” and the rest of
the name refers to the function or appearance of the layer. The
strata, in order from the basement membrane toward the free
surface, are the stratum basale, the stratum spinosum, the stratum
granulosum, the stratum lucidum, and the stratum corneum.
Stratum Basale
The deepest layer of the epidermis is the stratum basale
(STRA-tum buh-SAHL-a. y) or stratum germinativum (STRA-tum
jer-mi-na-TE. -vum). Hemidesmosomes attach the cells of this
layer to the basement membrane that separates the epider-
mis from the areolar tissue of the dermis. p. 143 The stra-
tum basale and the underlying dermis interlock, increasing
the strength of the bond between the epidermis and dermis.
The stratum basale forms epidermal ridges, which extend
into the dermis and are adjacent to dermal projections called
dermal papillae (singular, papilla; a nipple-shaped mound)
that project into the epidermis (Spotlight Figure 5–3). These
ridges and papillae are important because the strength of the
attachment is proportional to the surface area of the basement
membrane. In other words, the more numerous and deeper
the folds, the larger the surface area becomes.
Chapter 5 The Integumentary System 179
The contours of the skin surface follow the ridge patterns,
which vary from small conical pegs (in thin skin) to the com-
plex whorls seen on the thick skin of the palms and soles.
Ridges on the palms and soles increase the surface area of the
skin and increase friction, ensuring a secure grip. The ridge
patterns on the tips of the fingers are the basis of fingerprints.
These ridge shapes are determined partially by the interaction
of genes and partially by the intrauterine environment. That
is, during fetal development our genes determine general char-
acteristics, but the contact with amniotic fluid, another fetus
(in the case of twins), or the uterine wall affect the ultimate
5pattern of a fetus’s fingerprints. So, identical twins do not have
identical fingerprints. However, the pattern of your epidermal
ridges is unique and does not change after birth and during
your lifetime. Basal cells, or germinative cells, dominate the
stratum basale. Basal cells are stem cells that divide to replace
the more superficial keratinocytes that are shed at the epithe-
lial surface.
Skin surfaces that lack hair also contain specialized epi-
thelial tactile cells known as Merkel cells scattered among the
cells of the stratum basale. Each Merkel cell together with a
sensory nerve terminal is called a tactile disc. The tactile cells
are sensitive to touch and when compressed release chemicals
that stimulate their associated sensory nerve endings. (The
skin contains many other kinds of sensory receptors, as we will
see in later sections.)
The brown tones of skin result from the synthesis of pig-
are distributed throughout the stratum basale, with cell pro-
cesses extending into more superficial layers.
Stratum Spinosum
Each time a stem cell divides, one of the daughter cells is
pushed superficial to the stratum basale into the stratum
spinosum. This stratum consists of 8 to 10 layers of keratino-
cytes bound together by desmosomes. p. 142 The name stra-
tum spinosum, which means “spiny layer,” refers to the fact that
the cells look like miniature pincushions in standard histo-
logical sections. They look that way because the keratinocytes
were processed with chemicals that shrank the cytoplasm but
left the cytoskeletal elements and desmosomes intact. Some of
the cells entering this layer from the stratum basale continue
to divide, further increasing the thickness of the epithelium.
The stratum spinosum also contains cells that participate
in the immune response. Called dendritic cells (dendron, tree)
because of their branching projections, they are also known
as Langerhans cells. They stimulate defense against (1) micro-
organisms that manage to penetrate the superficial layers of
the epidermis and (2) superficial skin cancers. We will con-
sider dendritic cells and other cells of the immune response in
Chapter 22.