Microfluidic Devices for Angiogenesis - Mechanical and Chemical Signaling in Angiogenesis - page 112

6 Quantification Methods and the Role of Modeling
An issue that often arises in studies of angiogenesis in microfluidic systems is how
best to quantify the nature of the networks. Various metrics have been developed
such as the projected area of the sprouts [ 89 , 90 ], the total length of sprouts or the
average length [ 90 ], the average number of cells contained in a sprout, and the
number of branches. Recently, a Matlab-based tool, Rapid Analysis of Vessel
Elements (RAVE) has been made available, that should prove useful in quanti-
tative analysis [ 109 ].
A new approach that has just recently been developed is to use a computational
model as a means of controlling in real time an in vitro experiment in angio-
genesis, with the objective of regulating the growing microvascular bed [ 110 ].
This approach would have obvious advantages in the creation of a vascular system
to meet the needs of a specific engineered tissue. In this approach, network growth
is observed over time, in this instance, simply in terms of network morphology,
and a simulation is used to predict future behaviors under a variety of stimulatory
patterns, such as the spatiotemporal distributions of angiogenic factors. Micro-
fluidic systems are particularly well suited to this method in that they allow both
for real-time imaging of vascular growth, and the ability to control the delivery of
growth factors over time and space in a precise manner. First attempts at this
employ a simple model that relates vessel diameter to the speed of tip cell
migration; by controlling the latter via changes in the VEGF gradient, the diameter
of the vessels as a function of position can, in theory, be controlled. Many hurdles
yet need to be overcome, however, if this method is to succeed, not least of which
is the fact that vessel grow at a particular instant in time is a consequence of
numerous signaling events that have occurred in the past. In order to effectively
control the process, it will be necessary to identify read-outs of earlier signaling
such as through the use of fluorescent reporters.
7 Future Directions
New microfluidic methods are being developed at a rapid pace, and it is clear that
future methods will enable further improvements in realism, control of the critical
biochemical and biomechanical factors, and imaging capabilities. From the current
studies of angiogenesis, assays are likely to emerge that facilitate the growth of
entire microvascular networks in co-culture with perivascular cells that are capable
of being perfused and stabilized for extended periods of time. These ''engineered''
networks will contribute an essential element in the quest for vascularized organs
for implantation and enable new, more detailed studies of angiogenesis. At the
same time, as our understanding of the underlying biological processes evolves,
new approaches to modeling can be developed that will eventually facilitate the
active regulation of in vitro angiogenesis.