Page 154 - Fundamentals of anatomy physiology

Epithelia have several important characteristics:
Cellularity. Epithelia are made almost entirely of cells
bound closely together by interconnections known as cell
junctions. In other tissue types, the cells are often widely
separated by extracellular materials.
Polarity. An epithelium has an exposed surface, which faces
the exterior of the body or an internal space, and a base,
which is attached to underlying tissues. The term polarity
refers to the presence of structural and functional differ-
ences between the exposed and attached surfaces. An epi-
thelium consisting of a single layer of cells has an exposed
apical surface and an attached basal surface. The two
surfaces differ in plasma membrane structure and function.
Attachment. The base of an epithelium is bound to a thin,
noncellular basement membrane, which is also called the
basal lamina. The basement membrane is a complex struc-
ture produced by the basal surface of the epithelium and
the underlying connective tissue.
Avascularity. Epithelia are avascular (a. -VAS-ku. -lar; a-, with- a whole to perform the functions just listed, individual epi-
out + vas, vessel), which means that they lack blood ves-
sels. Epithelial cells get nutrients by diffusion or absorption
across either the exposed or the attached epithelial surface.
Regeneration. Epithelial cells that are damaged or lost at the
exposed surface are continuously replaced through stem
cell divisions in the epithelium. Regeneration is a charac-
teristic of other tissues as well, but the rates of cell division
and replacement are typically much higher in epithelia
than in other tissues.
Functions of Epithelial Tissue
Epithelia perform four essential functions:
1. Provide Physical Protection. Epithelia protect exposed and
internal surfaces from abrasion, dehydration, and destruc-
tion by chemical or biological agents.
2. Control Permeability. Any substance that enters or leaves
your body must cross an epithelium. Some epithelia are
relatively impermeable. Others are easily crossed by com-
pounds as large as proteins. Many epithelia contain the
molecular “machinery” needed for selective absorption or
secretion. The epithelial barrier can also be regulated and
modified in response to stimuli. For example, hormones
can affect the transport of ions and nutrients through epi-
thelial cells. Even physical stress can alter the structure and
properties of epithelia. For example, calluses form on your
hands when you do manual labor for some time.
3. Provide Sensation. Most epithelia are extremely sensitive
to stimulation, because they have a large sensory nerve
supply. These sensory nerves continually provide infor-
mation about the external and internal environments. For
Chapter 4 The Tissue Level of Organization 141
example, the lightest touch of a mosquito will stimulate
sensory neurons that tell you where to swat. A neuroepi-
thelium is an epithelium that is specialized to perform a
4particular sensory function. Our neuroepithelia contain
sensory cells that provide the sensations of smell, taste,
sight, equilibrium, and hearing.
4. Produce Specialized Secretions. Epithelial cells that produce
secretions are called gland cells. Individual gland cells are
usually scattered among other cell types in an epithelium.
In a glandular epithelium, most or all of the epithelial
cells produce secretions. These cells either discharge their
secretions onto the surface of the epithelium (to provide
physical protection or temperature regulation) or release
them into the surrounding interstitial fluid and blood (to
act as chemical messengers).
Specializations of Epithelial Cells
How are epithelial cells different from other body cells? They
have several structural specializations. For the epithelium as
thelial cells may be specialized for (1) the movement of fluids
over the epithelial surface, providing protection and lubrica-
tion; (2) the movement of fluids through the epithelium, to
control permeability; or (3) the production of secretions that
provide physical protection or act as chemical messengers.
Specialized epithelial cells generally possess a strong polarity.
Figure 4–1 shows one common type of epithelial polarity.
The epithelial cell is often divided into two functional re-
gions. One is the apical surface, where the cell is exposed to
an internal or external environment. The other consists of the
basolateral surfaces, which include both the base, where the cell
attaches to underlying epithelial cells or deeper tissues, and
the sides, where the cell contacts its neighbors.
Many epithelial cells that line internal passageways have
microvilli on their exposed surfaces. p. 99 Just a few may be
present, or microvilli may carpet the entire surface. Microvilli are
especially abundant on epithelial surfaces where absorption and
secretion take place, such as along portions of the digestive sys-
tem and kidneys. The epithelial cells in these locations are trans-
port specialists. Each cell has at least 20 times more surface area
to transport substances than it would have without microvilli.
Cilia are characteristic of surfaces covered by a ciliated
epithelium. A typical ciliated cell contains about 250 cilia that
beat in a coordinated manner. The synchronized beating of the
cilia moves substances over the epithelial surface. For example,
the ciliated epithelium that lines the respiratory tract moves
mucus up from the lungs and toward the throat, as though
on an escalator. The sticky mucus traps inhaled particles, in-
cluding dust, pollen, and pathogens. The ciliated epithelium
then carries the mucus and the trapped debris to the throat.
There they are swallowed or expelled by coughing. If abrasion