Biomedical Engineering Reference
In-Depth Information
9.2.2
Lymphatic Endothelium
The lymphatic system has many functions. It conveys immunocytes. It drains fluids
from the interstitial spaces. It transports absorbed dietary lipids to metabolism
sites. Lymphatic vessels develop from specialized venous endothelial cells. During
embryogenesis, subpopulations of venous endothelial cells form lymphatic sacs in
the region of the primitive subclavian, inferior vena cava, and iliac veins. These
sacs then divide to create lymphatic networks. Mediators of embryonic lymphatic
development include the transcription factor Prospero-related protein-1 (Prox1)
and VEGFc. Separation of lymphatic and blood microvasculature in the intestinal
mucosa continues beyond fetal life.
Fasting-induced adipose factor (FIAF),
17
produced by enterocytes of the small
intestine, is required for separation between postnatal intestinal lymphatic and blood
vessels [
866
]. Signaling by FIAF implicates Prox1 effector in the postnatal intestinal
lymphatic endothelium. However, lymphaticovenous partitioning also uses Prox1-
independent pathways.
9.2.3
Endothelial Fenestrae
Fenestrae (caliber 60-70 nm) exist in the capillary endothelium, where large
molecule exchanges occur between flowing blood and perfused tissues. Fenestrae
hence increase the endothelium permeability for water, electrolytes, and small
macromolecules, especially in the nephron glomerulus, gastrointestinal tract, liver
sinusoids, ocular choriocapillaris, and endocrine glands.
Fenestrae form an array characterized by regular spacing, the so-called sieve
plate. The fenestra density in sieve plates can reach about 30 fenestrae per
m
2
.
The fenestra pore is made of 5- to 6-nm openings delineated by a diaphragm
with radial fibrils from a central node. Fenestrae are composed of the diaphragm
protein PV1, which is required for fenestra formation, as well as actin-filament
remodeling [
867
].
9.3
Endothelial Progenitor Cells
The uninterrupted endothelial lining is maintained and regenerated by both prolif-
eration of endothelial cells and migration of (blood) circulating cells and undiffer-
entiated cells from the subendothelial space. In particular, some blood mononuclear
CD34
cells can acquire endothelial-like characteristics and can home to angiogen-
esis sites [
868
].
+
17
Fasting-induced adipose factor is also called angiopoietin-like protein 4. It inhibits lipoprotein
lipase involved in the storage of triglycerides in adipocytes. It promotes endothelial cell survival.
It reduces VEGF-induced microvascular permeability.
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