Organization of the Body
All epithelia have an
, an upper free surface ex-
posed to the body exterior or the cavity of an internal organ, and
a lower attached
. Te two surfaces diﬀer in both
structure and function. For this reason, we say that epithelia
Although some apical surfaces are smooth and slick, most
, 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
. Some epithelia, such
as that lining the trachea (windpipe), have motile
hairlike projections) that propel substances along their free
Adjacent to the basal surface of an epithelium is a thin sup-
porting sheet called the
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.
Except for glandular epithelia (discussed on pp. 124–125),
epithelial cells ﬁt closely together to form continuous sheets.
Lateral contacts, including
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
Supported by Connective Tissue
All epithelial sheets rest upon and are supported by connective
tissue. Just deep to the basal lamina is the
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
reinforces the epithelial sheet, helps it resist stretching and tear-
ing, and deﬁnes the epithelial boundary.
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
(contains no blood vessels),
(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
. For this reason, most micro-
scopic structures we view are not exactly like those in living tissue.
Check Your Understanding
What is the purpose of ﬁxing tissue for microscopic viewing?
What types of stains are used to stain tissues to be viewed
with an electron microscope?
For answers, see Appendix H.
List several structural and functional characteristics of
Name, classify, and describe the various types of epithelia,
and indicate their chief function(s) and location(s).
le-ul), or an
thelia), is a sheet of cells that covers a body surface or lines a body
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
, 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.