5
information suggests that some laminin isoforms have inhibitory activity toward
ECs during morphogenic events [71]. The vasculature appears to predominantly
express laminin-8 (α4,β1,γ1- 411), laminin-9 (α4,β2,γ1-421), laminin-10 (α5,
β1,γ1- 511), and laminin-11 (α5,β2,γ1- 521) isoforms [34, 50, 76]. In the past and
more recently, we have reported that these subunits are differentially expressed by
both ECs and pericytes during vascular tube morphogenesis and maturation events
[12, 94]. The biological role of each isoform has not yet been elucidated in sufficient detail during these processes.
One critical question that similarly has not been investigated in sufficient detail
is the nature of the embryonic ECM environment where vascular development takes
place [35, 57]. It is clear that there is much less fibrillar collagen during development, while the matrices are known to be rich in glycosaminoglycans, such as hyaluronic acid, proteoglycans, and fibronectin. It appears that developing embryos
strongly depend on the presence of fibronectin (perhaps its importance relates to its
mechanosensitive ability to self-assemble) [57]. Fibronectin knockout mice show
severe defects in vascular development along with other abnormalities [6, 42].
Fibronectin is also alternatively spliced, and several splice isoforms (IIIA and IIIB)
appear to play a critical functional role to promote vascular tube assembly and maturation during development [6]. One of the problems with investigating such issues
in a more molecular detail is that there currently are no 3D systems available that
mimic an embryonic ECM environment, an important future direction for in vitro
model development using vascular or other cell types.
1.3 Review of Work
1.3.1 Molecular Events Regulating Vascular Tube
Morphogenesis and EC Sprouting in 3D Matrices
A major effort of our laboratory has been to elucidate the molecule and signaling
requirements for ECs to form tube networks when suspended within 3D matrices
and to sprout and form tubes from a monolayer surface into 3D matrices [34, 36, 60,
84] (Fig. 1.2). To this end, we have developed 3D matrix microassay systems to
assess both of these phenomena in assays that mimic vasculogenesis and angiogenic
sprouting events, using either collagen or fibrin matrices [29, 68]. Other laboratories
have developed related systems to investigate these events [3, 78]. The majority of
our work has focused on models that mimic embryonic vasculogenesis, whereby
human ECs are seeded as single cells within a 3D matrix [28, 29, 36, 68]. Using
appropriate media conditions, ECs undergo dramatic morphologic changes that lead
to the development of interconnecting networks of EC-lined tubes (Fig. 1.1). For
example, two ECs are observed in a time-lapse series to form intracellular vacuoles,
which fuse within each cell and then through exocytic events; the two cells then
interconnect to form a multicellular lumen structure (Fig. 1.1a). There is no
1 Molecular Control of Capillary Tube Morphogenesis and Maturation Through…
information suggests that some laminin isoforms have inhibitory activity toward
ECs during morphogenic events [71]. The vasculature appears to predominantly
express laminin-8 (α4,β1,γ1- 411), laminin-9 (α4,β2,γ1-421), laminin-10 (α5,
β1,γ1- 511), and laminin-11 (α5,β2,γ1- 521) isoforms [34, 50, 76]. In the past and
more recently, we have reported that these subunits are differentially expressed by
both ECs and pericytes during vascular tube morphogenesis and maturation events
[12, 94]. The biological role of each isoform has not yet been elucidated in sufficient detail during these processes.
One critical question that similarly has not been investigated in sufficient detail
is the nature of the embryonic ECM environment where vascular development takes
place [35, 57]. It is clear that there is much less fibrillar collagen during development, while the matrices are known to be rich in glycosaminoglycans, such as hyaluronic acid, proteoglycans, and fibronectin. It appears that developing embryos
strongly depend on the presence of fibronectin (perhaps its importance relates to its
mechanosensitive ability to self-assemble) [57]. Fibronectin knockout mice show
severe defects in vascular development along with other abnormalities [6, 42].
Fibronectin is also alternatively spliced, and several splice isoforms (IIIA and IIIB)
appear to play a critical functional role to promote vascular tube assembly and maturation during development [6]. One of the problems with investigating such issues
in a more molecular detail is that there currently are no 3D systems available that
mimic an embryonic ECM environment, an important future direction for in vitro
model development using vascular or other cell types.
1.3 Review of Work
1.3.1 Molecular Events Regulating Vascular Tube
Morphogenesis and EC Sprouting in 3D Matrices
A major effort of our laboratory has been to elucidate the molecule and signaling
requirements for ECs to form tube networks when suspended within 3D matrices
and to sprout and form tubes from a monolayer surface into 3D matrices [34, 36, 60,
84] (Fig. 1.2). To this end, we have developed 3D matrix microassay systems to
assess both of these phenomena in assays that mimic vasculogenesis and angiogenic
sprouting events, using either collagen or fibrin matrices [29, 68]. Other laboratories
have developed related systems to investigate these events [3, 78]. The majority of
our work has focused on models that mimic embryonic vasculogenesis, whereby
human ECs are seeded as single cells within a 3D matrix [28, 29, 36, 68]. Using
appropriate media conditions, ECs undergo dramatic morphologic changes that lead
to the development of interconnecting networks of EC-lined tubes (Fig. 1.1). For
example, two ECs are observed in a time-lapse series to form intracellular vacuoles,
which fuse within each cell and then through exocytic events; the two cells then
interconnect to form a multicellular lumen structure (Fig. 1.1a). There is no
1 Molecular Control of Capillary Tube Morphogenesis and Maturation Through…