Biomedical Engineering Reference
In-Depth Information
Differences in pericyte morphology and distribution among vascular bed types
can be related to tissue-specific functions (heterogeneous functionality). The peri-
cyte coating (wrapping degree and pericyte density) depend on the perfused organ.
The greater the pericyte density, the stronger the microvascular barrier.
Among the types of pericytes, precapillary arteriolar, capillary, and postcapillary
venular have different features. Pre- and postcapillary pericytes have
α
-actinin
(SMC-pericyte transitional cells), whereas capillary pericytes do not.
Pericytes contain smooth muscle (like SMCs) and non-smooth muscle (like
endothelial cells) isoforms of actin and myosin. The former has higher levels in
pre- and postcapillary pericytes, whereas the concentration of the latter is greater in
capillary pericytes.
A basement membrane separates the endothelial cells and pericytes. However,
tight and gap junctions can develop between endothelial cells and pericytes. A
basement membrane can also be found along the outer surface of the pericytes.
Pericytes, by their contractile function, can regulate capillary bore, and then
the tissue perfusion, as well as transport from the blood via pericytic processes at
interendothelial clefts. Furthermore, pericytes mainly secrete vasoactive autoregu-
lating substances and release structural constituents of the basement membrane and
interstitial matrix.
Pericytes produce: (1) matrix structural proteins (collagen, fibronectin, laminin);
(2) agents involved in clotting and vasculogenesis, such as tenascin, throm-
bospondin,
41
and plasminogen activator inhibitor; (3) cell adhesion molecules;
(4) vasomotor factors such as endothelin-1; and (5) prostaglandins (TXa2, PGi2,
PGe2, PGf2
).
Pericytes have plasmalemmal receptors for growth factors (e.g., epidermal
growth
α
factor
receptor
[EGFR]
and
platelet-derived
growth
factor
receptor
PDGFR
) and various other ligands (e.g., endothelin-1 and histamine recep-
tors) [
682
].
Adenosine receptor A2R stimulates adenylate cyclase activity.
β
-Adrenergic and
muscarinic receptors have also been observed. Signaling leads to the activation of
cAMP, transient increase in IP
3
,andCa
2
+
influx. In opposition to smooth myocytes,
the membrane potential of the pericyte does not significantly vary after stimulation
by ET1, vasopressin, and acetylcholine.
Pericyte coverage is required during angiogenesis. Contacts between endothelial
cells and pericytes may control vascular growth, via platelet-derived growth factor
and transforming growth factor-
β
, and endothelium functioning. In cocultures,
pericytes modulate endothelial cell proliferation and the converse is also true.
Pericytes are recruited by endothelial cells that express PDGFb to stabilize the
vessel. Pericytes protect the endothelial cell, as they reduce their apoptosis rate.
β
41
Thrombospondins interact with blood coagulation and anticoagulant factors. They are involved
in cell adhesion, platelet aggregation, cell proliferation, angiogenesis, tissue repair, and tumor
metastasis.
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