40
UNIT 2
Basic Tissues
D. Cui
Basal
body
Plasma
membrane
Microtubules
Central
microtubule
A
D. Cui
Plasma
membrane
Actin
filaments
Terminal web
(actin filaments)
B
Figure 3-12A.
Cross section of cilia.
EM,
3
74,000
The structure of the
axoneme
(core) of a
cilium
can be readily appreciated from a
high-magniF
cation cross- sectional view.
The
microtubules
have an extremely
orderly, consistent arrangement into
two separate, central microtubules sur-
rounded by nine sets of doublet micro-
tubules. This is often termed the
9+2
arrangement
.
Figure 3-12B.
Cross
section
of
microvilli.
EM,
3
74,000
Many examples of columnar and
cuboidal epithelia comprise cells that
bear
microvilli
, but absorptive cells
lining the small intestine provide the
premier example of tightly packed
microvilli that provide a pronounced
increase in surface area of the
plasma
membrane
. Microvilli are less than half
the diameter of cilia, and their cores
have a much simpler structure. In cross
section, the
actin ±
laments
that extend
from the
terminal web
into the microvilli
appear as a cluster of small dots. The
fuzzy coating on the membranes of the
microvilli is the
glycocalyx
.
SYNOPSIS 3-1
Specialized Structures of the Epithelial Cell
Apical surface (domain)
: Exposed to a luminal or external environment; site of primary function (absorption,
protection, etc.).
Microvilli
, composed of actin microF
laments anchored to
terminal web
.
Microvilli increase apical surface area to aid
in absorption.
Cilia
, composed of microtubules, arise from basal bodies
.
Cilia aid in the transport of material across the surface of the
epithelium.
Stereocilia
, unusually long microvilli that aid in absorption.
Lateral surface (domain)
: Contains junctional complexes that connect cells to neighboring cells.
Tight junctions (zonula occludens)
, specialized membrane proteins between the apical and the lateral surfaces of the
cell. Surround the apical borders and serve as impermeable barriers.
Adhering junctions (zonula adherens)
, beneath the tight junctions, form bandlike junctions, and link the cytoskeleton
of one cell to neighboring cells. They provide mechanical stability of the cells.
Desmosomes (macula adherens)
, spotlike junctions, which assist in cell-to-cell attachment.
Gap junctions
, communicating junctions that permit passage of ions and small molecules between neighboring cells.
Basal surface (domain)
: Contains junctional complexes and basement membrane. Basolateral folds may also be present.
Hemidesmosome
, a junction (one half of a desmosome) connecting cells to the underlying basement membrane.
Basement membrane
, consists of basal lamina and reticular lamina, which provide an underlying sheet for epithelial
cells.
Basolateral folds
, corrugations of the cell membrane in the lateral and basal regions of the cell, which increase cell
surface area and are involved with ion and fl uid transport.
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