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brane contains network-organizing proteins, such as collagen IV, collagen XVIII,
laminin, nidogen, entactin, and the proteoglycan perlecan. The tunica media contains vascular SMCs (v-SMCs) and elastic tissue composed of elastin, fibrillins,
fibulins, emilins, and microfibril-associated proteins. The tunica adventitia contains
fibroblasts and elastic laminae and has its own blood supply, known as the vasa
vasorum [57]. SMCs and elastic laminae contribute to the vessel tone and regulate
vessel diameter and blood flow. This generic blood vessel architecture is modified
with various ECM components to fulfill their individual tasks. Arteries, which function to deliver oxygenated blood, usually have a thick tunica media with numerous
concentric layers of v-SMCs, whereas veins have a thick tunica adventitia layer
enriched in ECM components with elastic properties, such as elastin and fibrillin.
As described, the composition of the ECM is inherently dynamic throughout
development as well as vascular regeneration, positing the importance of remodeling and deposition of new ECM as these processes progress. Additionally, stability
of mature vessels requires a different ECM composition than developing or regenerating vasculature. Several studies have highlighted these changes in ECM deposition and have identified regulators of these important mechanisms. Much of the
work to date has established the role of perivascular cells, including pericytes and
smooth muscle cells, in ECM production [232]. Crucially, ECs also produce ECM
as blood vessels form. Of particular interest, endothelial progenitor populations and
mature ECs produce ECM differently; EPCs produce collagen IV, fibronectin, and
laminin, while mature ECs have limited ECM production in standard cell culture
conditions. However, when subjected to hypoxic conditions, mature ECs adopt an
ECM secretome similar to the pro-regenerative EPCs, wherein they secrete collagen
IV, fibronectin, and laminin at low O 2 (1%). At moderate hypoxia (5% O 2 ), both cell
types produce collagen I [134]. When developing engineered vasculature, these factors are critical to consider to obtain mature, long-lasting blood vessels, as ECM
composition is an important parameter governing vascular stability.
4.2.2.2 Properties of the ECM that Affect Vascular Morphogenesis
Recent decades have vastly expanded our understanding of how ECM properties
affect vascular assembly, primarily due to newly available, well-defined in vitro
models. The most common models are cultures of ECs in gels made of different
ECM components, such as collagen, fibrin, fibronectin, and Matrigel. These ECM
components contain instructive physical and chemical cues that direct vascular morphogenesis, which involves several steps: (1) proteolytic degradation of basement
membrane proteins by both soluble and membrane-bound matrix metalloproteinases (MMPs); (2) cell activation, proliferation, and migration; (3) vacuole and
lumen assembly into a tube with tight junctions at cell-cell contacts; (4) branching
and sprouting; (5) synthesis of basement membrane proteins to support the formation of capillary tube networks; and (6) tube maturation and stabilization by pericytes. These complex processes require a delicate balance between various
immobilized and soluble GFs, as well as endothelial and perivascular cell
M. R. Blatchley et al.
brane contains network-organizing proteins, such as collagen IV, collagen XVIII,
laminin, nidogen, entactin, and the proteoglycan perlecan. The tunica media contains vascular SMCs (v-SMCs) and elastic tissue composed of elastin, fibrillins,
fibulins, emilins, and microfibril-associated proteins. The tunica adventitia contains
fibroblasts and elastic laminae and has its own blood supply, known as the vasa
vasorum [57]. SMCs and elastic laminae contribute to the vessel tone and regulate
vessel diameter and blood flow. This generic blood vessel architecture is modified
with various ECM components to fulfill their individual tasks. Arteries, which function to deliver oxygenated blood, usually have a thick tunica media with numerous
concentric layers of v-SMCs, whereas veins have a thick tunica adventitia layer
enriched in ECM components with elastic properties, such as elastin and fibrillin.
As described, the composition of the ECM is inherently dynamic throughout
development as well as vascular regeneration, positing the importance of remodeling and deposition of new ECM as these processes progress. Additionally, stability
of mature vessels requires a different ECM composition than developing or regenerating vasculature. Several studies have highlighted these changes in ECM deposition and have identified regulators of these important mechanisms. Much of the
work to date has established the role of perivascular cells, including pericytes and
smooth muscle cells, in ECM production [232]. Crucially, ECs also produce ECM
as blood vessels form. Of particular interest, endothelial progenitor populations and
mature ECs produce ECM differently; EPCs produce collagen IV, fibronectin, and
laminin, while mature ECs have limited ECM production in standard cell culture
conditions. However, when subjected to hypoxic conditions, mature ECs adopt an
ECM secretome similar to the pro-regenerative EPCs, wherein they secrete collagen
IV, fibronectin, and laminin at low O 2 (1%). At moderate hypoxia (5% O 2 ), both cell
types produce collagen I [134]. When developing engineered vasculature, these factors are critical to consider to obtain mature, long-lasting blood vessels, as ECM
composition is an important parameter governing vascular stability.
4.2.2.2 Properties of the ECM that Affect Vascular Morphogenesis
Recent decades have vastly expanded our understanding of how ECM properties
affect vascular assembly, primarily due to newly available, well-defined in vitro
models. The most common models are cultures of ECs in gels made of different
ECM components, such as collagen, fibrin, fibronectin, and Matrigel. These ECM
components contain instructive physical and chemical cues that direct vascular morphogenesis, which involves several steps: (1) proteolytic degradation of basement
membrane proteins by both soluble and membrane-bound matrix metalloproteinases (MMPs); (2) cell activation, proliferation, and migration; (3) vacuole and
lumen assembly into a tube with tight junctions at cell-cell contacts; (4) branching
and sprouting; (5) synthesis of basement membrane proteins to support the formation of capillary tube networks; and (6) tube maturation and stabilization by pericytes. These complex processes require a delicate balance between various
immobilized and soluble GFs, as well as endothelial and perivascular cell
M. R. Blatchley et al.