Epithelial tissue Flashcards
epithelial tissue,
Epithelial tissue includes epithelia and glands. Epithelia
(ep-i-THE
.
-le
.
-a; singular, epithelium) are layers of cells that
cover external or line internal surfaces. Glands are structures
that produce fluid secretions. They are either attached to or
derived from epithelia.
Epithelia cover every exposed surface of your body. Epi-
thelia form the surface of the skin and line the digestive, respi-
ratory, reproductive, and urinary tracts. In fact, they line all
passageways that communicate with the outside world. The
more delicate epithelia line internal cavities and passageways,
such as the chest cavity, fluid-filled spaces in the brain, the inner
surfaces of blood vessels, and the chambers of the heart.
Histologists recognize the follow-
ing four basic types of tissue:
Epithelial tissue covers exposed surfaces, lines internal
passageways and chambers, and forms glands.
■ Connective tissue fills internal spaces, provides structural
support for other tissues, transports materials within the
body, and stores energy.
■ Muscle tissue is specialized for contraction and includes
the skeletal muscles of the body, the muscle of the heart,
and the muscular walls of hollow organs.
Nervous tissue carries information from one part of the
body to another in the form of electrical impulses.
In this chapter we introduce the basic characteristics of these
tissues.
Epithelia perform the following four essential functions:
Provide Physical Protection. Epithelia protect exposed and
internal surfaces from abrasion, dehydration, and destruc-
tion by chemical or biological agents.
■ 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 absorbing or secreting
specific substances. The epithelial barrier can also be regu-
lated and modified in response to stimuli. For example,
hormones can affect the transport of ions and nutrients
through epithelial cells. Even physical stress can alterthe structure and properties of epithelia. For example, cal-
luses form on your hands when you do manual labor for
some time.
■ Provide Sensation. Most epithelia are extremely sensitive to
stimulation, because they have a large sensory nerve supply.
These sensory nerves continually provide information about
the external and internal environments. For example, the
lightest touch of a mosquito will stimulate sensory neurons
that tell you where to swat. A neuroepithelium is an epithelium
that is specialized to perform a particular sensory function.
Our neuroepithelia contain sensory cells that provide the sen-
sations of smell, taste, sight, equilibrium, and hearing.
■ 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.
Epithelia have several important features:
Polarity. An epithelium has an exposed surface, either facing
the external environment 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. Often,
the apical surface has microvilli; sometimes it has cilia. The
functional polarity is also evident in the uneven distribu-
tion of organelles between the exposed surface and the
basement membrane. In some epithelia, such as the lining
of the kidney tubules, mitochondria are concentrated near
the base of the cell, where energy is in high demand for the
cell’s transport activities (Figure 4–2).
■ 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.Attachment. The base of an epithelium is bound to a thin,
noncellular basement membrane. This basement mem-
brane is formed from the fusion of several successive lay-
ers (the basal lamina and reticular lamina), a collagen
matrix, and proteoglycans (intercellular cement). The base-
ment membrane adheres to the basal surface and to the
underlying tissues to establish the cell’s border and resist
stretching.
■ Avascularity. Epithelia are avascular (a
.
-VAS-ku
.
-lar; a-, without
+ vas, vessel), which means that they lack blood vessels.
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.
How are epithelial cells different from other body cells?
They
have several structural specializations. For the epithelium as a
whole to perform the functions just listed, individual epithelial
cells may be specialized for (1) the movement of fluids over the
epithelial surface, providing protection and lubrication; (2) the
movement of fluids through the epithelium, to control perme-
ability; or (3) the production of secretions that provide physical
protection or act as chemical messengers.
The specialized epithelial cell is often divided into two
functional regions, which means the cell has a strong polarity.
One is the apical surface, where the cell is exposed to an inter-
nal or external environment. The other consists of the baso-
lateral surfaces, which include both the base (basal surface),
where the cell attaches to underlying epithelial cells or deeper
tissues, and the sides (lateral surfaces), where the cell contacts
its neighbors
microvilli
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 diges-
tive system and kidneys. The epithelial cells in these locations
are transport specialists. Each cell has at least 20 times more
surface area to transport substances than it would have without
microvilli.
Three factors help maintain the physi-
cal integrity of an epithelium:
(1) intercellular connections,
(2) attachment to the basement membrane, and (3) epithelial
maintenance and repair.
cell adhesion molecules (CAMs)
Large areas of opposing
plasma membranes are interconnected by transmembrane pro-
teins called cell adhesion molecules (CAMs), which bind to
each other and to extracellular materials. For example, CAMs
on the basolateral surface of an epithelium help bind the cell to
the underlying basement membrane. The membranes of adja-
cent cells may also be bonded by a thin layer of proteoglycans
that contain polysaccharide derivatives known as glycosamino-
glycans (GAGs), most notably, hyaluronan (hyaluronic acid).
Cell junctions
Cell junctions are specialized areas of the plasma mem-
brane that attach a cell to another cell or to extracellular
materials. The three most common types of cell junctions are
(1) gap junctions, (2) tight junctions, and (3) desmosomes
gap junction
At a gap junction (Figure 4–3b), two
cells are held together by two embedded interlocking trans-
membrane proteins called connexons. Each connexon is com-
posed of six connexin proteins that form a cylinder with a central
pore. Two aligned connexons form a narrow passageway that
lets small molecules and ions pass from cell to cell. Gap junc-
tions are common among epithelial cells, where the movement
of ions helps coordinate functions such as the beating of cilia.
Gap junctions are also common in other tissues. For example,
gap junctions in cardiac muscle tissue and smooth muscle tissue
are essential in coordinating muscle cell contractions.
Tight Junctions.
Tight junctions encircle the apical regions of
epithelial cells. At a tight junction (also known as an occlud-
ing junction), the lipid portions of the two plasma membranes
are tightly bound together by interlocking membrane proteins. Inferior to the tight junctions, a continuous
adhesion belt forms a band that encircles cells and binds them to
their neighbors. The bands are attached to the microfilaments
of the terminal web. p. 118
lumen
A lumen refers to the central, hollow space within a tubular structure, such as blood vessels or organs. It is lined by specialized cells, such as endothelial cells in blood vessels, and plays a crucial role in the transport of fluids, gases, and nutrients.
Desmosomes
Most epithelial cells are subject to mechani-
cal stresses—stretching, bending, twisting, or compression—so
they must have durable interconnections. At a desmosome
(DEZ-mo
.
-so
.
m; desmos, ligament + soma, body), CAMs and
proteoglycans link the opposing plasma membranes. Des-
mosomes are very strong and can resist stretching and twisting.
A typical desmosome is formed by components from two
cells. Within each cell, a complex known as a dense area is con-
nected to the cytoskeleton. This connection to the cytoskeleton
gives the desmosome—and the epithelium—its strength. For
example, desmosomes are abundant between cells in the super-
ficial layers of the skin. As a result, damaged skin cells are usually
lost in sheets rather than as individual cells. (That is why your
skin peels rather than sheds off as a powder after a sunburn.)
There are two types of desmosomes:
Spot desmosomes are small discs connected to bands of
intermediate filaments (Figure 4–3d). The intermediate
filaments stabilize the shape of the cell.
■ Hemidesmosomes resemble half of a spot desmosome. Rather
than attaching one cell to another, a hemidesmosome
attaches a cell to extracellular filaments in the basement mem-
brane (Figure 4–3e). This attachment helps stabilize the posi-
tion of the epithelial cell and anchors it to underlying tissues.
