90
embryo and muscle versus kidney), which have different distributions of ECM components (as discussed in the previous section), it is evident that both β 1 and α v integrins can support vascular morphogenesis. For example, α v β 3 and α 2 β 1 integrins
associate with vascular morphogenesis in collagen-rich ECM, like adult tissue,
while α 5 β 1 and α 6 β 1 integrins involve fibronectin- and fibrin-rich ECM, like in embryonic tissue and healing wounds [50]. The binding of integrins onto RGD triggers
several downstream signaling events mediated by Rho GTPase, particularly Rac1
and Cdc42 [49]. Extensive work by Davis and his colleagues revealed the molecular
mechanism that regulates this EC morphogenesis in fibrin and collagen gels (an
excellent review of their work can be found in Chap. 20 of this book). This mechanism has also been observed and controlled in synthetic (HA-based) hydrogels [89].
To further substantiate the role of cell-ECM interactions, particularly those
mediated by integrin engagement, several groups have identified the importance of
integrin specificity in vascular regeneration. In tumor vessels, α v β 3 is preferentially
expressed, leading to formation of new, albeit disorganized, leaky vasculature [53].
In order to establish organized, mature neovessels, engagement of α 3 /α5β1, rather
than α v β 3 , was necessary [147]. While RGD peptides facilitate cell adhesion in synthetic matrices, it is important to consider the non-specific integrin engagement
potential of these peptides, which may influence vascular regeneration.
The number of RGD adhesion sites and the method of their presentation to the
vascular cells are also crucial in affecting cell migration [82] and vascular morphogenesis [110]. Using an in vitro angiogenesis model, Folkman and Ingber were able
to show that, when cultured on a moderate coating density that only partially resisted
cell traction forces, ECs could retract and differentiate into branching capillary networks [67, 110]. High ECM density was saturated with RGD adhesion peptide,
which allowed the ECs to spread and proliferate, while low ECM density resulted in
rounded and apoptotic cells. Interestingly, in medium ECM density, with the appropriate RGD adhesion peptide, ECs collectively retracted and differentiated into
branching capillary networks with hollow tubular structures. It is evident that the
ECs exerted mechanical forces on the surrounding ECM to create a pathway for
migration and branching in forming vascular structures [48]. Hence, both the quantity of RGD peptide and the method of presentation within the engineered synthetic
biomaterials determine the initial morphogenetic events in angiogenesis.
Scaffold Degradation Regulates Vascular Morphogenesis
Scaffolds made from ECM components, like collagen and fibrin gels, contain proteolytic degradable sequences which can be degraded by the MMPs and other proteases (e.g., cathepsins) secreted by vascular cells. This cell-mediated degradation
controls both structural integrity and temporal mechanical properties, which dictate
the presentation of chemical and mechanical cues at various stages of angiogenesis.
However, the degradation kinetics of these ECM-based scaffolds is determined by
their inherent cross-linking density which, in turn, limits their manipulation for vascular tissue engineering. In contrast, synthetic biomaterials can be engineered to
M. R. Blatchley et al.
embryo and muscle versus kidney), which have different distributions of ECM components (as discussed in the previous section), it is evident that both β 1 and α v integrins can support vascular morphogenesis. For example, α v β 3 and α 2 β 1 integrins
associate with vascular morphogenesis in collagen-rich ECM, like adult tissue,
while α 5 β 1 and α 6 β 1 integrins involve fibronectin- and fibrin-rich ECM, like in embryonic tissue and healing wounds [50]. The binding of integrins onto RGD triggers
several downstream signaling events mediated by Rho GTPase, particularly Rac1
and Cdc42 [49]. Extensive work by Davis and his colleagues revealed the molecular
mechanism that regulates this EC morphogenesis in fibrin and collagen gels (an
excellent review of their work can be found in Chap. 20 of this book). This mechanism has also been observed and controlled in synthetic (HA-based) hydrogels [89].
To further substantiate the role of cell-ECM interactions, particularly those
mediated by integrin engagement, several groups have identified the importance of
integrin specificity in vascular regeneration. In tumor vessels, α v β 3 is preferentially
expressed, leading to formation of new, albeit disorganized, leaky vasculature [53].
In order to establish organized, mature neovessels, engagement of α 3 /α5β1, rather
than α v β 3 , was necessary [147]. While RGD peptides facilitate cell adhesion in synthetic matrices, it is important to consider the non-specific integrin engagement
potential of these peptides, which may influence vascular regeneration.
The number of RGD adhesion sites and the method of their presentation to the
vascular cells are also crucial in affecting cell migration [82] and vascular morphogenesis [110]. Using an in vitro angiogenesis model, Folkman and Ingber were able
to show that, when cultured on a moderate coating density that only partially resisted
cell traction forces, ECs could retract and differentiate into branching capillary networks [67, 110]. High ECM density was saturated with RGD adhesion peptide,
which allowed the ECs to spread and proliferate, while low ECM density resulted in
rounded and apoptotic cells. Interestingly, in medium ECM density, with the appropriate RGD adhesion peptide, ECs collectively retracted and differentiated into
branching capillary networks with hollow tubular structures. It is evident that the
ECs exerted mechanical forces on the surrounding ECM to create a pathway for
migration and branching in forming vascular structures [48]. Hence, both the quantity of RGD peptide and the method of presentation within the engineered synthetic
biomaterials determine the initial morphogenetic events in angiogenesis.
Scaffold Degradation Regulates Vascular Morphogenesis
Scaffolds made from ECM components, like collagen and fibrin gels, contain proteolytic degradable sequences which can be degraded by the MMPs and other proteases (e.g., cathepsins) secreted by vascular cells. This cell-mediated degradation
controls both structural integrity and temporal mechanical properties, which dictate
the presentation of chemical and mechanical cues at various stages of angiogenesis.
However, the degradation kinetics of these ECM-based scaffolds is determined by
their inherent cross-linking density which, in turn, limits their manipulation for vascular tissue engineering. In contrast, synthetic biomaterials can be engineered to
M. R. Blatchley et al.