Biomedical Engineering Reference
In-Depth Information
1) form vesicles and 2) transport vesicles across the cell. However, larger molecules that
cannot fit through the intercellular cleft are transported with this mechanism.
6.2 ENDOTHELIAL CELL AND SMOOTH MUSCLE
CELL PHYSIOLOGY
As already discussed, endothelial cells compose the interior lining of all blood vessels
within the body. These cells can partially regulate the transport of nutrients across the cell
and play a very critical role in coagulation. Endothelial cells join to form a sheet with one
of three different structures. Continuous endothelial cells are found in arteries, veins, and
capillaries throughout the body. The transport of nutrients across these endothelial cell
sheets is regulated by the type and quantity of junctions between neighboring cells.
Primarily, cadherin junctions mediate cell-cell adhesion and gap junctions regulate the
communication between cells. Endothelial cells are typically 10
m in diameter and
depending on their location within the circulatory system, their length can vary from 50 to
200
μ
m.
The second type of endothelial cell sheets is the discontinuous endothelial sheet. These
are endothelial cells that are not tightly connected to each other. This allows for a free dif-
fusion around the endothelial cells and is typically present in the liver and spleen. Most
large molecules including proteins can pass through this type of endothelial cell sheet. The
third type of endothelial cell sheet is the fenestrated endothelial sheet. These types of
endothelial cell sheets are located within the kidney's glomerulus and have similar inter-
cellular connections as continuous endothelial cells. However, there are channels that span
throughout the entire cells, which effectively expose the extracellular space to the blood
vessel lumen. These channels facilitate the movement of large molecules across the endo-
thelial cell wall.
VE-cadherin is the primary cadherin located on the endothelial cell membrane. This
protein is expressed as a dimer of two cadherin proteins. Each protein passes through the
cell membrane and has an intracellular portion that is connected to the cytoskeleton. This
strong cytoskeletal connection allows for force transmittance through the endothelial cell.
The association of these proteins is dependent on the local calcium concentration. With cal-
cium concentrations lower than approximately 0.5 mM, each cadherin dimer dissociates
and then the cadherins cannot make connections to other cells or the cytoskeleton. When
the calcium concentration increases, cadherin dimers associate and can then adhere to a
cadherin dimer from a neighboring cell. The role of cadherin is to connect cells together
and to mechanically couple the endothelial cell sheet with the intracellular load-bearing
components.
Vascular smooth muscle cells perform work in mostly the same way that skeletal mus-
cles perform work, except that the loads they transmit are typically orders of magnitude
smaller than the skeletal muscle loads. Smooth muscle cells contract when actin and myo-
sin form a cross-bridge. Similar to skeletal muscle, calcium regulates cross-bridge forma-
tion in vascular smooth muscle cells. However, the internal arrangement of proteins
within the smooth muscle cells is vastly different from the ubiquitous sarcomere structural
μ
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