168
UNIT 3
Organ Systems
Figure 9-11.
Arteriole.
TEM,
3
5, 800; inset
3
3,800
The
arteriole
in the larger image was f xed in a dilated state, whereas the
inset
shows a slightly smaller arteriole that was f xed in a
contracted state. In both vessels, a thin elastic lamina separates the endothelium From the smooth muscle cells oF the tunica media. The
state oF contraction oF the smooth muscle cells is regulated largely by signals From the endothelial cells. In addition to chemical messen-
gers, such as nitric oxide and endothelin, signaling may also involve direct contacts between endothelial and smooth muscle cells. The
cell in the upper leFt corner oF the larger micrograph is a
Schwann cell
with several unmyelinated axons embedded in its cytoplasm.
Nucleus of smooth
Nucleus of smooth
muscle cell
muscle cell
Nucleus of
Nucleus of
Schwann cells
Schwann cells
Endothelium-smooth muscle
Endothelium-smooth muscle
cell junctions
cell junctions
Internal
Internal
elastic
elastic
lamina
lamina
Internal elastic
Internal elastic
lamina
lamina
Nucleus of smooth
Nucleus of smooth
muscle cell
muscle cell
Nuclei of
Nuclei of
endothelial
endothelial
cells
cells
Nucleus of
Nucleus of
endothelial
endothelial
cell
cell
Endothelium–smooth muscle
cell junctions
Internal
elastic
lamina
Internal elastic
lamina
Nucleus of smooth
muscle cell
Nucleus of
Schwann cell
Nuclei of
endothelial
cells
Nucleus of
endothelial
cell
Nucleus of smooth
muscle cell
SYNOPSIS 9-2
Functions of Endothelium in Blood Vessels
Endothelium provides a permeability barrier and controls the composition oF interstitial tissue fl
uid.
Endothelium allows the movement oF leukocytes, fl uid, and immunoglobulins into tissues.
Endothelium provides For angiogenesis in the Formation and diFFerentiation oF new blood vessels.
Endothelium provides signals that regulate the tone oF adjacent vascular smooth muscle cells that control blood pressure
by vasoconstriction (endothelin) and vasodilation (nitric oxide, prostacyclin).
Endothelium provides anticoagulant signals (thrombomodulin, nitrous oxide, and prostacyclin) that inhibit platelet attach-
ment and aggregation to prevent blood clotting and allow unobstructed fl
ow oF blood in normal conditions.
When endothelial cells are injured, they reduce the production oF platelet inhibitors and increase the release oF stimulators
oF platelet activation (e.g., von Willebrand Factor, tissue thromboplastin). Exposure oF the underlying basal lamina and con-
nective tissue Further stimulates platelets to adhere and aggregate. Damage to the endothelium will slow down oxidation and
clearance oF low-density lipoprotein, leading to many cardiovascular diseases such as
atherosclerosis
.
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