CHAPTER 3
Epithelium and Glands
31
Figure 3-3A.
Simple squamous epithelium, blood
vessels. H&E,
3
219; inset
3
310
Simple squamous epithelium
lining the lumen surface of
all types of blood vessels and lymphatic vessels is called
an
endothelium
(sometimes a
vascular endothelium
). In
this example, a single layer of squamous cells lines the
inner layer, the
tunica intima
, of a medium artery. The
endothelial cells are fl attened and elongated, and they
always rest on a thin
basement membrane
.
Electron
microscopy is required to see the ultrastructural features
of the basement membrane. Endothelial cells of vessels
sense changes in blood pressure, oxygen tension, and
blood fl ow and respond to these changes by secreting
substances, which have effects on the tone of vascular
smooth muscle. Endothelial cells are also important in
the control of blood coagulation; the endothelium pro-
duces von Willebrand factor that mediates platelet adhe-
sion to collagen in subendothelial connective tissues at
an injury site to stop bleeding. They also produce antico-
agulant substances that prevent blood clotting and allow
unobstructed fl ow of blood in normal conditions.
Tunica
Tunica
intima
intima
Subendothelial
Subendothelial
connective
connective
tissue
tissue
Internal
Internal
elastic
elastic
lamina
lamina
Lumen of the artery
Endothelium
Tunica media
of the artery wall
Tunica
intima
Endothelial cells
Internal
elastic
lamina
Subendothelial
connective
tissue
A
D. Cui
Subendothelial connective tissue
Internal elastic lamina
Endothelial cell
Basement membrane
B
Figure 3-3B.
A representation of simple squamous
epithelium, artery endothelium.
These
endothelial cells
are fl
attened and elongated,
oriented parallel to the direction of blood fl ow, and
rest on a
basement membrane
. The cells and basement
membrane are linked by junctions called
hemidesmo-
somes
. Beneath the basement membrane is a
suben-
dothelial layer of connective tissue
. The wavy structure
is called the
internal elastic lamina
.
The endothelium,
subendothelial connective tissue, and the internal elas-
tic lamina comprise the
tunica intima
.
CLINICAL CORRELATION
Figure 3-3C.
Atherosclerosis.
Atherosclerosis
is the formation of deposits of yellowish
plaques that contain cholesterol, lipoid material, and
lipo-
phages
(
macrophages
). These deposits form the innermost
layers of large- and medium-sized arteries. Of particular
clinical signiF
cance are those plaques that form at the
bifurcation of the common carotid artery into the internal
and external carotid arteries and in the cerebral vessels.
Although there are many possible causes of plaques, the
more common are
endothelial dysfunction
,
dyslipidemia
,
infl
ammatory and immunologic factors, and
hypertension
.
As shown in this image, deposits of
cholesterol
and
fatty
material
accumulate in the inner layers of the vessel result-
ing in damage to the vessel wall, including disruption of the
endothelium
. These deposits, when hardened, may occlude
blood fl
ow to distant tissues, and blood clots may form
on exposed collagen in subendothelial connective tissue.
Clot formation or dislodged pieces of plaque may result in
vascular
occlusion and
stroke
.
D. Cui
Endothelial cell
Basement membrane
Internal elastic lamina
Cholesterol deposits
Foam macrophage (foam cells)
Fragmented internal elastic lamina
Fat
deposits
C
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