CHAPTER 2
Cell Structure and Function
19
Figure 2-6A.
Cell surface and cyto-
skeleton, intestinal absorptive cells.
EM,
3
73,000
This image is restricted to a very small part of
the surfaces of two
absorptive
cells
(
entero-
cytes
) in the wall of the small intestine. The
plane of section is tangential to the plane of
the surface, so that the
right side
of the image
shows numerous microvilli that project into
the lumen and function to greatly increase the
surface area exposed to the contents of the
intestine. The electron-dense dots in the
cores
of the
microvilli
are
actin ± laments
, which,
in this case, provide for stiffness rather than
motility. These actin F
laments extend from
the microvilli into the cytoplasm as part of
the
terminal web
of actin F laments near the
cell surface. Components of a
junctional
complex
provide a seal (
zonula occludens
)
and adhesion (
zonula adherens
and
macula
adherens
) between neighboring enterocytes.
Beneath the terminal web of actin F
laments
are thicker F laments,
intermediate ± laments
,
which provide mechanical strength to the
cell. Some of the intermediate F
laments are
anchored in the
macula adherens
.
Zonula
occludens
Terminal
web
(actin
filaments)
Intermediate
filaments
Macula
adherens
Zonula
adherens
Microvilli
with core
of actin
filaments
A
Microtubules
Microtubules
Microtubules
Intermediate
Intermediate
filaments
filaments
/neuro-
/neuro-
filaments)
filaments)
Intermediate
filaments
/neuro-
filaments)
Mitochondria
Mitochondria
Mitochondria
Synaptic
Synaptic
vesicles
vesicles
Synaptic
vesicles
B
Figure 2-6B.
Cytoskeleton, the dendrite
of a neuron.
EM,
3
20,000
The central structure in this view is a
dendrite of a neuron
. It is a process extend-
ing from the cell body of the cell. Dendrites
and
axons
, the other type of neural pro-
cess, require
cytoskeletal elements
both for
mechanical support and for conveying essen-
tial molecules, particles, and organelles over
distances that can be quite extensive. The
structural support is provided mainly by the
intermediate ± laments
, which are termed
neuro±
laments
in neurons because of their
specialized molecular structure. Intermediate
F laments appear in electron micrographs as
electron-dense single lines when they course
within the plane of the section or as electron-
dense dots when their orientation is vertical
to the plane of the section. The movement of
molecules and particles along the lengths of
dendrites and axons requires
microtubules
as tracks and
motor molecules
(dyneins
and kinesins) to transport the cargo struc-
tures and molecules along the microtubules.
Because of their tubular structure, microtu-
bules appear as a closely paired set of paral-
lel lines when they course within the plane of
the section or as circles when their orienta-
tion is vertical to the plane of the section.
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