38
stimuli, in the form of fluid shear stress generated by flowing blood, and tension
generated by external contractile forces. Various in vitro models, consisting of ECs
cultured in 3D scaffolds composed of different biomaterials, have been established
to study the effect of mechanical stimuli on vascularization processes. Some of
these models are designed to manipulate the mechanical properties of the 3D matrix
such as matrix stiffness and boundary conditions, while others are designed to
actively apply fluid shear stress and external tensile forces using designated bioreactors. This chapter will summarize the main insights gained regarding the impact of
internal and external mechanical cues on the formation of vascular networks in 3D
culture systems (Fig. 2.1). Internal mechanical forces refer to cell-induced forces
regulated by the cellular environment, while external forces are those that are
actively applied on the engineered tissue by external sources.
2.2 Internal Forces
2.2.1 Cellular Forces
Mechanical interactions between cells and the extracellular matrix (ECM) play a
central role in regulation of cell division, motility, and differentiation [10, 32, 38].
Cells, including ECs, modify the mechanical and structural properties of their surrounding ECM by exerting contractile forces. Endothelial invasion and sprouting
involve three-dimensional (3D) matrix deformation, as demonstrated by fluorescent
particle displacement in gel [28, 49], anisotropic fibrillar structure of the ECM [30],
and local ECM stiffness [25]. These matrix alterations subsequently trigger feedback responses which dictate vascular network morphogenesis. For example, endothelial sprouts exert mechanical forces that reorganize the matrix to support tubelike
endothelial structures and branching point formation. It has also been shown that
cell contractile forces regulate sprouting directionality. Korff et al. demonstrated
that forces induced by sprouting vessels led to long-range deformation of the underlying collagen gel. Interestingly, sprouts of nearby EC spheroid followed the direction of tension-aligned fibers generated by the ECs [26]. Cell-cell mechanical
communication has also been demonstrated in an experimental model in which EC
sprouting correlated with substrate deformations generated by neighboring cells in
a compliant polyacrylamide gel (Fig. 2.2a) [43].
2.2.2 Matrix Stiffness
The role of ECM stiffness in regulating cellular morphology, differentiation, traction force generation, focal adhesion, and cell migration dynamics is well studied
[4, 10, 41, 42, 61]. Modification of the fibrin or collagen gel density has been a
common approach to manipulate stiffness of 3D matrices in vitro. Studies
B. Zohar et al.
stimuli, in the form of fluid shear stress generated by flowing blood, and tension
generated by external contractile forces. Various in vitro models, consisting of ECs
cultured in 3D scaffolds composed of different biomaterials, have been established
to study the effect of mechanical stimuli on vascularization processes. Some of
these models are designed to manipulate the mechanical properties of the 3D matrix
such as matrix stiffness and boundary conditions, while others are designed to
actively apply fluid shear stress and external tensile forces using designated bioreactors. This chapter will summarize the main insights gained regarding the impact of
internal and external mechanical cues on the formation of vascular networks in 3D
culture systems (Fig. 2.1). Internal mechanical forces refer to cell-induced forces
regulated by the cellular environment, while external forces are those that are
actively applied on the engineered tissue by external sources.
2.2 Internal Forces
2.2.1 Cellular Forces
Mechanical interactions between cells and the extracellular matrix (ECM) play a
central role in regulation of cell division, motility, and differentiation [10, 32, 38].
Cells, including ECs, modify the mechanical and structural properties of their surrounding ECM by exerting contractile forces. Endothelial invasion and sprouting
involve three-dimensional (3D) matrix deformation, as demonstrated by fluorescent
particle displacement in gel [28, 49], anisotropic fibrillar structure of the ECM [30],
and local ECM stiffness [25]. These matrix alterations subsequently trigger feedback responses which dictate vascular network morphogenesis. For example, endothelial sprouts exert mechanical forces that reorganize the matrix to support tubelike
endothelial structures and branching point formation. It has also been shown that
cell contractile forces regulate sprouting directionality. Korff et al. demonstrated
that forces induced by sprouting vessels led to long-range deformation of the underlying collagen gel. Interestingly, sprouts of nearby EC spheroid followed the direction of tension-aligned fibers generated by the ECs [26]. Cell-cell mechanical
communication has also been demonstrated in an experimental model in which EC
sprouting correlated with substrate deformations generated by neighboring cells in
a compliant polyacrylamide gel (Fig. 2.2a) [43].
2.2.2 Matrix Stiffness
The role of ECM stiffness in regulating cellular morphology, differentiation, traction force generation, focal adhesion, and cell migration dynamics is well studied
[4, 10, 41, 42, 61]. Modification of the fibrin or collagen gel density has been a
common approach to manipulate stiffness of 3D matrices in vitro. Studies
B. Zohar et al.