Page 155 - Fundamentals of anatomy physiology
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142 Unit 1 Levels of Organization
4 Figure 4–1 The Polarity of Epithelial Cells. Many epithelial cells Intercellular connections involve either extensive areas of
opposing plasma membranes or specialized attachment sites,
have functional specializations associated with an apical surface that discussed shortly. Large areas of opposing plasma membranes
is exposed to either the external environment or internal spaces, and are interconnected by transmembrane proteins called cell ad-
basolateral surfaces (base and sides) that interact with deeper tissues hesion molecules (CAMs), which bind to each other and to
and adjacent cells. Often, the apical surface has microvilli; sometimes extracellular materials. For example, CAMs on the basolateral
it has cilia. This functional polarity is also evident in the uneven distri- surface of an epithelium help bind the cell to the underlying
bution of organelles between the free surface (here, the top) and the basement membrane. The membranes of adjacent cells may
basement membrane. In some epithelia, such as the lining of the kid- also be bonded by a thin layer of proteoglycans that contain
ney tubules, mitochondria are concentrated near the base of the cell, polysaccharide derivatives known as glycosaminoglycans (GAGs),
where energy is in high demand for the cell’s transport activities. most notably hyaluronan (hyaluronic acid).
Cilia Cell junctions are specialized areas of the plasma mem-
brane that attach a cell to another cell or to extracellular materi-
Microvilli als. The three most common types of cell junctions are (1) tight
Apical junctions, (2) gap junctions, and (3) desmosomes (Figure 4–2a).
surface
Some epithelial functions require rapid intercellular com-
Golgi munication. At a gap junction (Figure 4–2b), two cells are held
apparatus together by two interlocking transmembrane proteins called
connexons. Each connexon is composed of six connexin proteins
Nucleus that form a cylinder with a central pore. Two aligned connexons
form a narrow passageway that lets small molecules and ions pass
Mitochondria from cell to cell. Gap junctions are common among epithelial
Basement membrane cells, where the movement of ions helps coordinate functions
Basolateral such as the beating of cilia. Gap junctions are also common in
surfaces other tissues. For example, gap junctions in cardiac muscle tissue
and smooth muscle tissue are essential in coordinating muscle
or toxic compounds such as nicotine and carbon monoxide cell contractions. A number of human diseases are caused by mu-
in cigarette smoke injure the cilia or the epithelial cells, cili- tations in the genes that code for different connexin proteins.
ary movement can stop. Without the protective upward flow of
mucus, infection or disease become more likely. At a tight junction (also known as an occluding junc-
tion), the lipid portions of the two plasma membranes are
Maintaining the Integrity of Epithelia tightly bound together by interlocking membrane proteins
(Figure 4–2c). Inferior to the tight junctions, a continuous
To be effective as a barrier, an epithelium must form a com- adhesion belt forms a band that encircles cells and binds them
plete cover or lining. Three factors help maintain the physi- to their neighbors. The bands are attached to the microfila-
cal integrity of an epithelium: (1) intercellular connections, ments of the terminal web. p. 97 This kind of attachment
(2) attachment to the basement membrane, and (3) epithelial is so tight that tight junctions largely prevent water and solutes
maintenance and repair. from passing between the cells.
Intercellular Connections When the epithelium lines a tube, such as the intestinal
tract, the apical surfaces of the epithelial cells are exposed
Cells in an epithelium are firmly attached to one another, and to the space inside the tube, a passageway called the lumen
the epithelium as a unit is attached to extracellular fibers of (LOO-men). Tight junctions effectively isolate the contents of
the superficial clear layer of the basement membrane. Many the lumen from the basolateral surfaces of the cell. For exam-
cells in your body form permanent or temporary bonds with ple, tight junctions near the apical surfaces of cells that line
other cells or extracellular material. Epithelial cells, however, the digestive tract help keep enzymes, acids, and wastes in the
are specialists in intercellular connection. lumen from reaching the basolateral surfaces and digesting or
otherwise damaging the underlying tissues and organs.
Most epithelial cells are subject to mechanical 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. Desmosomes are very strong
and can resist stretching and twisting.
