Mechanosensory Pathways in Angiocrine Mediated Tissue Regeneration - Mechanical and Chemical Signaling in Angiogenesis - page 25

Table 1 Vascular endothelial growth factor function

Receptor Ligand Function

VEGFR1 (Flt-1) VEGF-A, VEGF-B Decoy receptor

of VEGF-R2, unknown

VEGFR2 (KDR/Flk-1) VEGF-A,VEGF-C, VEGF-E,

VEGF-D

Varied

VEGFR3 VEGF-C, VEGF-D Lymphangiogenesis

overexpression. An outline of VEGF isomers and receptors they interact with is
outlined in Table 1 .
VEGF-A binds to VEGFR1 (Flt-1) and VEGFR2 (KDR/Flk-1). VEGFR2 seems
to mediate most of the known cellular responses to VEGF [ 41 ]. The function of
VEGFR1 is less well-defined; it is thought to modulate VEGFR2 signaling.
VEGFR1 may also act as a decoy receptor, sequestering VEGF from VEGFR2
binding (particularly during embryonic vasculogenesis). VEGF-C and VEGF-D
are ligands for VEGFR3, which plays a role in lymphangiogenesis.

3.3 Parallel-Plate Flow System

To understand how shear stress is measured in vivo in small vessels, it is first
necessary to have an understanding of how shear stress can be measured in vitro.
The parallel-plate flow chamber is a commonly used platform for subjecting
monolayers of anchorage-dependent cells, such as endothelial cells, to relatively
uniform laminar shear stress. This approach has allowed scientists to recreate the
in vivo environment in which endothelial cells are exposed to shear stresses due to
blood flow.
A cell monolayer attached to one internal plate surface is subjected to fluid flow
by creating a pressure gradient across the chamber. In order to calculate the shear
stress on the cells, a Newtonian fluid is assumed. For steady flow between infi-
nitely wide parallel plates, shear stress is determined according to fluid flow,
viscosity of the media, and the depth of the chamber.
There are many advantages to using parallel-plate flow chambers. They pro-
mote uniform shear stress in a controlled environment. They can also simulate a
wide variety of flow rates, which associatively varies shear stress levels. This
enables simulation of blood vessels of many different sizes.
A parallel-plate flow chamber with a channel height of 1 mm was used to
subject Human Umbilical Vein Endothelial Cells (HUVECs) to shear stress
(Stovall Life Sciences, Inc). This chamber is optically transparent, allowing for
direct visualization of the cell behavior under flow on a microscope stage and real-
time monitoring. A high-precision peristaltic pump (Ismatec IPC, Model
#ISM931C) is connected with tygon tubing (1/16 in ID) to the flow cell. The pump
can impart a range of fluid shear stress by varying the flow rate. Cells are plated