28
UNIT 2
Basic Tissues
a very limited distribution. Occasionally, it can be found in the
conjunctiva of the eye and in some large ducts of the exocrine
glands.
TRANSITIONAL EPITHELIUM
is stratiF ed epithelium, often
referred to as
urothelium
, which lines the excretory channels
leading from the kidney (renal calyces, ureters, bladder, and
proximal segment of the urethra). It may contain four to six cell
layers in the relaxed state. However, the histological appearance
of the epithelium can change when stretched (±ig. 3-17A–C).
In the empty bladder, the basal cells are mostly cuboidal and
the middle layer is polygonal, although surface cells bulge into
the lumen. Surface cells are often described as “dome shaped”
and are called
dome
cells
or
umbrella
cells
; they contain extra
cell membrane material near the superF cial (apical) surface. The
dome cells may contain two nuclei. In the stretched bladder, the
thickness of the epithelium is much reduced, and surface cells, as
well as the intermediate cells, are extremely fl attened.
Specializations of the Apical Surface
(Apical Domain)
Apical surfaces
of the epithelium may reveal
cilia
,
microvilli
, and
stereocilia
, depending on their function and location. (1)
Cilia
are elongated, motile structures that have a greater diameter
and length than microvilli. The core of a cilium is composed
of
microtubules
arranged in a consistent array of two central
microtubules surrounded by a circle of nine peripheral pairs of
microtubules (±igs. 3-9A to 3-11A). Cilia arise from electron-
dense, cylindrical structures, called
basal
bodies
, in the apical
cytoplasm just below the cell membrane. There are many mito-
chondria at the apical surface of cells with motile cilia. The
function of cilia is to aid in the transport of material along the
surface of epithelial cells. Cilia are present in the pseudostrati-
F
ed ciliated columnar epithelium in the respiratory tract and
ciliated simple columnar epithelium in the oviduct (fallopian
tube). (2)
Microvilli
are smaller than cilia; each has a core that
is composed of
actin
microF
laments
. Microvilli are anchored
to a network structure called the
terminal web
, which contains
actin F
laments to stabilize the microvillus (±igs. 3-7A,B and
3-11B). These specialized structures increase apical surface
area to aid in absorption. Microvilli are commonly seen in sim-
ple columnar epithelium lining the small intestine and simple
cuboidal epithelium
lining the proximal tubules in the kidney.
(3)
Stereocilia
are long microvilli, which consist of
actin micro-
F
laments
and help with absorption (±ig. 3-11C). They can
be found in the pseudostratiF
ed columnar epithelium in the
epididymis and vas deferens of the male reproductive system.
Specializations of the Lateral Surface
(Lateral Domain)
The
lateral surface
of epithelial cells contains cell junctions and
cell adhesion molecules that are responsible for the cohesive
nature of epithelial tissue. Intercellular connections of the epi-
thelial cells include (1)
tight junctions (zonula occludens)
, that
completely surround the apical cell borders to seal the underlying
intercellular clefts from the outside environment (±ig. 3-8A,B);
(2)
adhering junctions (zonula adherens)
, found just beneath the
tight junction, also forming a bandlike junction surrounding
the entire cell and serving to attach adjacent cells (±ig. 3-8A,B);
(3)
desmosomes (macula adherens)
, located beneath the adher-
ing junctions, also assist in cell to cell attachment (±ig. 3-8A,B)
(the
junctional complex
is composed of tight junction, adher-
ing junction, and desmosome); and (4)
gap junctions
, which are
communicating junctions
, provide a low-resistance channel to
permit passage of ions and small molecules between adjacent
cells (±ig. 3-8B). Gap junctions are present not only in epithelial
tissues, but they can also be found in many other tissues (smooth
muscle, cardiac muscle, and nerve tissues) in the body. However,
gap junctions are not present in skeletal muscle, blood cells, and
spermatozoa.
Specializations of the Basal Surface
(Basal Domain)
Epithelial cells rest on a
basement membrane
, consisting of a
basal lamina
and a
reticular lamina
, which provide an under-
lying foundation for the cells (±igs. 3-2B and 3-8B). The term
“basement membrane” is used in light microscopy observation,
although the basement membrane is often difF
cult to visualize
with the light microscope. The terms “basal lamina” and “retic-
ular lamina” are ultrastructural terms and refer to features that
require electron microscopy to be seen (see ±ig. 3-4). Epithelial
cells produce their own basement membrane. Cells are anchored
to the basement membrane by
hemidesmosomes
, junctions that
connect the cells to the underlying basement membrane (±ig.
3-8B).
Basal plasma membrane enfolding
may also be present
in some epithelial cells (e.g., salivary gland excretory duct epi-
thelium). This is a corrugation of the cell membrane in the basal
(and sometimes lateral) regions of the cell, which increases cell
surface area and is involved in ion and fl uid transport. There
are many
mitochondria
in the vicinity of the plasma membrane
enfolding. These produce adenosine triphosphate (ATP) for
active transport. The combination of the plasma membrane
enfolding and the concentration of mitochondria result in a
striated appearance in some of the epithelial cells.
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