Microfluidic Devices for Angiogenesis - Mechanical and Chemical Signaling in Angiogenesis - page 111

somewhat disappointing, however, in that there was little evidence to support the
hypothesis.
It is well known that the hypoxic environment found in and around tumors is a
major factor influencing the transition of a quiescent endothelium to an activated,
angiogenic phenotype. These results from prolonged lifetime of HIF-1a caused by
intermittent hypoxia [ 104 ]. This, in turn, increases the synthesis and secretion of
angiogenic factors such as VEGF, bFGF, and the initiation of sprouting. This has
spawned an interest in developing microfluidic systems that can produce a
controlled hypoxic environment. In one of these [ 105 ], small alginate gel disks
seeded with U87 tumor cells were formed in PDMS microwells, then a layer of
endothelial cells were plated onto the surface. Since the rate of oxygen consumption
was known from separate experiments, computational methods could be used to
estimate the degree of hypoxia and the spatial distribution of oxygen tension.
When vascular networks grow in vivo, they are acted upon by numerous fac-
tors, both pro- and anti-angiogenic, as described above. Most of both types of
factors are synthesized and secreted by resident cells, such as in response to
hypoxic conditions. In addition, as the network matures, it undergoes a process of
pruning and stabilization, mediated largely by pericytes and smooth muscle cells
[ 106 ]. For this reason, some have begun to introduce other, accessory cells into
their microfluidic cultures in order to promote maturation and stabilization. In one
study [ 89 ], fibroblasts or mesenchymal stem cells were added to endothelial cells
to produce structures having many of the characteristics of a primitive vascular
plexus. In another study, a co-culture of endothelial cells and hepatocytes gave rise
to vascular sprouts with well-defined lumens and good long-term stability [ 102 ]
(Fig. 6 b). More studies of this type are certain to appear since the interactions
between multiple cell types have clear advantages in the formation and stabil-
ization of vascular structures.
5.3 Angiogenesis in Tissue Engineering
Angiogenesis, as mentioned earlier, is highly desired in engineered organs,
although the structure of the vascular network can be drastically different between
organ systems. This is nowhere more evident than in tissues such as the liver that
are both highly vascularized and possess a unique microvascular structure that is
central to organ function. For this reason, much work has been directed toward
inducing microvessles to grow and develop the desired structure in in vitro sys-
tems, either to function ex vivo or ultimately to be implanted. Several microfluidic
systems have been developed for this purpose, but only a few contain endothelial
cells and offer the hope of natural vascularization [ 102 , 107 , 108 ].