118
UNIT 1
Organization of the Body
4
Polarity
All epithelia have an
apical surface
, an upper free surface ex-
posed to the body exterior or the cavity of an internal organ, and
a lower attached
basal surface
. Te two surfaces differ in both
structure and function. For this reason, we say that epithelia
exhibit
apical-basal polarity
.
Although some apical surfaces are smooth and slick, most
have
microvilli
, fingerlike extensions of the plasma mem-
brane. Microvilli tremendously increase the exposed surface
area. In epithelia that absorb or secrete (export) substances
(those lining the intestine or kidney tubules, for instance),
the microvilli are o±en so dense that the cell apices have a
fuzzy appearance called a
brush border
. Some epithelia, such
as that lining the trachea (windpipe), have motile
cilia
(tiny
hairlike projections) that propel substances along their free
surface.
Adjacent to the basal surface of an epithelium is a thin sup-
porting sheet called the
basal lamina
(lam
9
ĭ-nah; “sheet”).
Tis noncellular, adhesive sheet consists largely of glycopro-
teins secreted by the epithelial cells plus some fine collagen
fibers. Te basal lamina acts as a selective filter that determines
which molecules diffusing from the underlying connective tis-
sue are allowed to enter the epithelium. Te basal lamina also
acts as scaffolding along which epithelial cells can migrate to
repair a wound.
Specialized Contacts
Except for glandular epithelia (discussed on pp. 124–125),
epithelial cells fit closely together to form continuous sheets.
Lateral contacts, including
tight junctions
and
desmosomes
,
bind adjacent cells together at many points (these junctions
are described in Chapter 3). Te tight junctions help keep
proteins in the apical region of the plasma membrane from
diffusing into the basal region, and thus help to maintain
epithelial polarity.
Supported by Connective Tissue
All epithelial sheets rest upon and are supported by connective
tissue. Just deep to the basal lamina is the
reticular lamina
, a
layer of extracellular material containing a fine network of col-
lagen protein fibers that “belongs to” the underlying connective
tissue. Te two laminae form the
basement membrane
, which
reinforces the epithelial sheet, helps it resist stretching and tear-
ing, and defines the epithelial boundary.
Homeostatic Imbalance
4.1
An important characteristic of cancerous epithelial cells is their
failure to respect the basement membrane boundary, which
they penetrate to invade the tissues beneath.
Avascular but Innervated
Although epithelium is
avascular
(contains no blood vessels),
it is
innervated
(supplied by nerve fibers). Epithelial cells are
nourished by substances diffusing from blood vessels in the un-
derlying connective tissue.
manufacturers in the mid-1800s. Many dyes consist of negatively
charged molecules (acidic stains) or positively charged molecules
(basic stains) that bind within the tissue to macromolecules of the
opposite charge. Different parts of cells and tissues take up differ-
ent dyes, distinguishing different anatomical structures.
For transmission electron microscopy (²EM), tissue sec-
tions are “stained” with heavy metal salts. Tese metals deflect
electrons in the beam to different extents, providing contrast.
Electron-microscope images are in shades of gray because color
is a property of light, not of electron waves, but the image may
be artificially colored to enhance contrast. Another kind of elec-
tron microscopy, scanning electron microscopy (SEM), provides
three-dimensional pictures of an unsectioned tissue surface.
Preserved tissue we see under the microscope has been exposed
to many procedures that alter its original condition and introduce
minor distortions called
artifacts
. For this reason, most micro-
scopic structures we view are not exactly like those in living tissue.
Check Your Understanding
1.
What is the purpose of fixing tissue for microscopic viewing?
2.
What types of stains are used to stain tissues to be viewed
with an electron microscope?
For answers, see Appendix H.
Epithelial Tissue
List several structural and functional characteristics of
epithelial tissue.
Name, classify, and describe the various types of epithelia,
and indicate their chief function(s) and location(s).
Epithelial tissue
(ep
0
ĭ-the
9
le-ul), or an
epithelium
(plural: epi-
thelia), is a sheet of cells that covers a body surface or lines a body
cavity (
epithe
5
laid on, covering). ²wo forms occur in the body:
Covering and lining epithelium
, which forms the outer layer
of the skin; dips into and lines the open cavities of the uro-
genital, digestive, and respiratory systems; and covers the
walls and organs of the closed ventral body cavity
Glandular epithelium
, which fashions the glands of the body
Epithelia form boundaries between different environments,
and nearly all substances received or given off by the body must
pass through an epithelium. For example, the epidermis of the
skin lies between the inside and the outside of the body. Epithe-
lium lining the urinary bladder separates underlying cells of the
bladder wall from urine.
In its role as an interface tissue, epithelium accomplishes
many functions, including (1) protection, (2) absorption, (3)
filtration, (4) excretion, (5) secretion, and (6) sensory reception,
all of which will be touched upon later in this chapter.
Special Characteristics of Epithelium
Epithelial tissues have five distinguishing characteristics: polar-
ity, specialized contacts, supported by connective tissues, being
avascular but innervated, and having the ability to regenerate.
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