97
These studies underline the importance of engineering a tissue construct with a
matrix stiffness amenable to promote in vivo vascularization. However, investigating how matrix stiffness may affect in vivo vascularization remains challenging due
to the complexity of the system, which involves matrix remodeling, host capillary
ingrowth, as well as anastomosis of the vascular construct and contributions from
other cell types. For example, in vivo vascular ingrowth into Matrigel scaffolds was
found to be optimal at intermediate matrix stiffness, in sharp contrast to the observed
in vitro ingrowth [158]. Elegant work by Yoder’s research group also found that
increasing the collagen concentration yielded stiffer scaffolds, which in turn promoted host capillary ingrowth in vivo. Compared to stiffer scaffolds, softer scaffolds might have experienced excessive in vivo remodeling and failed to retain the
vascular constructs. Moreover, in vitro angiogenesis studies have found that ECMbased gels produce a much narrower range of stiffness [46, 158] than synthetic
hydrogels [88, 158]. Future investigations are needed to evaluate vascularization by
both the host capillary and the engineered vascular construct over a wider range of
physiologically relevant matrix elasticities. Despite the differences in scaffold composition (ECM-based gels versus synthetic hydrogels), culture conditions (in vitro
versus in vivo), assay type (2D versus 3D), and ranges of matrix stiffness, all of
these studies highlight the relevance of engineering scaffolds with mechanical elasticity suited to the specific needs of tissue vascularization.
4.2.3 The Effects of Oxygen Availability and the ECM
In this section, we will consider O 2 tension and the ECM as two interdependent factors determining the efficiency of vasculature formation. We will review currently
available O 2 measurement techniques and challenges, along with the mathematical
modeling approaches used to overcome some of these challenges in describing O 2
gradients in 3D environments. Then, we will discuss cellular adaptations and
responses to O 2 availability in 3D ECM constructs and the possible outcomes of
variations in O 2 distribution in 3D cultures of vascular cells.
4.2.3.1 Varying Oxygen Tensions in the ECM of Tissue and Matrix
Scaffolds: Measuring and Modeling
Oxygen Measurement Techniques and Challenges
Manipulation of oxygen, in order to direct pluripotent or vascular cells to form
blood vessels, requires knowing the precise O 2 tension that the cells are exposed to
under varying conditions. Many different O 2 measurement techniques have been
used in vitro and in vivo. The accuracy of these measurements is fundamental to
confidently describe the cellular responses under various O 2 availabilities, as well as
to controlling the O 2 tension in order to direct angiogenesis and vasculogenesis. An
4 Hypoxia and Matrix Manipulation for Vascular Engineering
These studies underline the importance of engineering a tissue construct with a
matrix stiffness amenable to promote in vivo vascularization. However, investigating how matrix stiffness may affect in vivo vascularization remains challenging due
to the complexity of the system, which involves matrix remodeling, host capillary
ingrowth, as well as anastomosis of the vascular construct and contributions from
other cell types. For example, in vivo vascular ingrowth into Matrigel scaffolds was
found to be optimal at intermediate matrix stiffness, in sharp contrast to the observed
in vitro ingrowth [158]. Elegant work by Yoder’s research group also found that
increasing the collagen concentration yielded stiffer scaffolds, which in turn promoted host capillary ingrowth in vivo. Compared to stiffer scaffolds, softer scaffolds might have experienced excessive in vivo remodeling and failed to retain the
vascular constructs. Moreover, in vitro angiogenesis studies have found that ECMbased gels produce a much narrower range of stiffness [46, 158] than synthetic
hydrogels [88, 158]. Future investigations are needed to evaluate vascularization by
both the host capillary and the engineered vascular construct over a wider range of
physiologically relevant matrix elasticities. Despite the differences in scaffold composition (ECM-based gels versus synthetic hydrogels), culture conditions (in vitro
versus in vivo), assay type (2D versus 3D), and ranges of matrix stiffness, all of
these studies highlight the relevance of engineering scaffolds with mechanical elasticity suited to the specific needs of tissue vascularization.
4.2.3 The Effects of Oxygen Availability and the ECM
In this section, we will consider O 2 tension and the ECM as two interdependent factors determining the efficiency of vasculature formation. We will review currently
available O 2 measurement techniques and challenges, along with the mathematical
modeling approaches used to overcome some of these challenges in describing O 2
gradients in 3D environments. Then, we will discuss cellular adaptations and
responses to O 2 availability in 3D ECM constructs and the possible outcomes of
variations in O 2 distribution in 3D cultures of vascular cells.
4.2.3.1 Varying Oxygen Tensions in the ECM of Tissue and Matrix
Scaffolds: Measuring and Modeling
Oxygen Measurement Techniques and Challenges
Manipulation of oxygen, in order to direct pluripotent or vascular cells to form
blood vessels, requires knowing the precise O 2 tension that the cells are exposed to
under varying conditions. Many different O 2 measurement techniques have been
used in vitro and in vivo. The accuracy of these measurements is fundamental to
confidently describe the cellular responses under various O 2 availabilities, as well as
to controlling the O 2 tension in order to direct angiogenesis and vasculogenesis. An
4 Hypoxia and Matrix Manipulation for Vascular Engineering