Microfluidic Devices for Angiogenesis - Mechanical and Chemical Signaling in Angiogenesis - page 107

poly D lysine (PDL), both of which led to more sprouts with a more natural
appearance [ 90 ] (Fig. 5 C, D). Gel stiffness could be increased in type I collagen
either by using a higher collagen concentration or by gelation at low pH [ 92 ].
4 Factors that Influence the Properties of Vascular Sprouts
4.1 Biochemical Factors
In recent experiments, the combined effects of VEGF and ANG-1 were studied
[ 93 ]. Using a microfluidic system, an endothelial monolayer was induced to grow
into a type I collagen gel. When under the influence of a VEGF gradient alone, the
sprouts were observed to often become unstable, and the tip cell would frequently
break loose from the stalk and migrate separately. This tendency could be greatly
reduced by the additional of ANG-1, a known vascular stabilizing factor. While
this represents but one of many possible combinations, it demonstrates the enor-
mous potential of microfluidics to study the combined effects of multiple factors
that can either accelerate or slow and stabilize the growth of microvascular
sprouts.
While it is well established that biochemical factors influence angiogenesis, the
effects of physical factors on the sprouting of endothelial monolayers into gel have
been much less studied. Of studies conducted in microfluidic systems, two primary
matrix-related factors have been studied. One is the stiffness of the gel [ 92 ], which
can be varied either by the use of different gels (e.g., type I collagen, fibrin,
matrigel), by varying the concentration of gel, or by regulating the extent of cross-
links [ 92 ]. While the gel type has been extensively studied in macro-culture assays,
this has been relatively less studied in microfluidic systems.
4.2 Effects of Hemodynamic Shear
In one recently published set of experiments, Kang and co-workers [ 94 ], examined
the effect of shear stress on the apical (luminal) side of the monolayer on endo-
thelial sprouting. They found that, in the presence of sphingozine-1-phosphate
(S1P), one of the factors found to promote angiogenesis although it also has
stabilizing effects, shear stress on the apical surface of cells adherent to a gel
surface, more aggressively invaded into the gel, forming sprouts at a higher fre-
quency than monolayers lacking shear stress. In their experiments, the system
could be visualized only after fixation, so their data all examine a single time point
(24 h after the initiation of shear). Interestingly, in a different study, using VEGF
rather than S1P to induce sprouting, shear stress was found to exert a stabilizing
effect; that is, the monolayer was less likely to extend sprouts into the gel when