Microfluidic Devices for Angiogenesis - Mechanical and Chemical Signaling in Angiogenesis - page 92

the 1970s has spawned an entirely new field of both basic and translational
research. Moreover, there is considerable interest in understanding how to mod-
ulate vascularization and insights from angiogenesis are used to design new
therapies for cardiovascular diseases [ 3 ] or more boldly, the in vitro creation of
vascularized tissues or organs by implementing tissue engineering approaches for
replacement therapies.
1.1 In Vitro Versus In Vivo Models of Angiogenesis
Over time, investigators have implemented different types of in vivo and in vitro
models in an effort to recapitulate natural angiogenesis. The requirement for more
affordable, reliable, reproducible and well-characterized model systems has con-
tributed to vast progress over the years. Despite these advances, there still remain
challenges with either approach, evident by the limited success in the translation of
basic research to the clinic or bedside. Nevertheless, the research community is
aware of these shortcomings, and has identified the extensive criteria that must be
met [ 4 ] for successfully recreating angiogenesis in the lab. Thus, the field con-
tinues to evolve, as newer models are developed and older ones are refined.
In vivo systems: Three main types of in vivo angiogenesis assay have been
described; (1) microcirculatory preparations in chick embryo and rodents; (2)
recruitment of vessels by biocompatible polymer matrix implants; and (3) excision
of vascularized tissues (see [ 4 ] for an extensive review). One of the earliest in vivo
angiogenesis models dates back to the late 1930s, where Ide et al. [ 5 ] demonstrated
the vascularization of an implant of Brown-Pearce epitheliaoma using the trans-
parent rabbit ear window developed by Sandison [ 6 ]. Since then, several other in
vivo assays have been developed including cranial windows, chick chorioallantoic
membrane (CAM), corneal micropocket assays among others [ 4 ].
In vitro systems: In vitro assays traditionally take what could be viewed as a
minimalistic approach; the angiogenesis cascade is decomposed and investigated
as the sum of its individual steps, namely migration, proliferation and tube for-
mation. Two research groups, Jaffe et al. and Gimbrone et al. were among the first
to report of the successful long-term culture of endothelial cells in vitro [ 7 , 8 ].
However, it was not until methods for the successful culture and clonal expansion
of endothelial cells (ECs) in vitro [ 9 ] were developed that the first in vitro angi-
ogenesis assay was established. In 1980, Folkman and Haudenschild demonstrated
that cloned capillary ECs cultured on a gelatinized Cuprak dish in tumour-
conditioned medium could initiate angiogenesis [ 10 ]. Since then a variety of in
vitro models have been developed in an attempt to simulate and analyze the
process of neovascularization. These in vitro assays can be broadly classified as
two-dimensional (2D) or three-dimensional (3D). In 2D models, cells are plated on
culture surfaces that are coated with thin layers of adhesion proteins whereas in 3D
models cells are cultured on or in 3D matrices. In 3D cultures, cells are able to
invade or migrate within the matrix, which better recapitulates the 3D nature of the