Biomedical Engineering Reference
In-Depth Information
Three-dimensional matrices have also provided new insight into tissue organization
and more accurate models for pathogenesis than traditional 2D culture techniques,
such as Petri dishes. Work done studying human breast cancer has shown the impor-
tance of moving from traditional, nonphysiological 2D systems to 3D ones. Culture of
human breast epithelial cells in collagen gels has demonstrated the ability of the cells
to re-express their in vivo organization and differentiation, and recapitulate histology.
These studies also allowed for the identifi cation of previously unknown phenomena at
many different levels; cell-cell interactions, gene expression and ECM affects on cel-
lular organization and polarity which were not present in traditional 2D cultures
(Bissell et al.
2002,
2003
; Gudjonsson et al.
2003
). Three-dimensional gel culture sys-
tems also facilitate natural, complex 3D tissue development in vivo. Co-cultures of
human umbilical-vein epithelial cells and 10T1/2 mesenchymal cells in collagen-
fi bronectin gels when implanted into mice developed a 3D branching network of tubes
that connected with the mouse's own vasculature and became perfused (Koike et al.
2004
). Arteriolar and venular patterns of blood fl ow were observed in the construct
and the new vessels responded naturally to local administration of a vasoconstrictor.
The use of 3D gels has also been shown to provide an improved environment for dif-
ferentiation of embryonic stem cells. Chondrogenic differentiation of embryoid bodies
in polyethylene glycol (PEG) based gels in comparison to monolayer cultures showed
induction of chondrocytic phenotype and upregulation of cartilage relevant markers
(Alhadlaq and Mao
2004
). Histological analysis demonstrated basophilic ECM depo-
sition characteristic of neocartilage (Hwang et al.
2005
). It is clear that providing cells
with the cues present in a 3D environment is essential to creating complex engineered
tissue systems.
The most common way of creating a 3D cell culture is to suspend cells in Type I
collagen. This provides not only physical support but also a natural biochemical and
physical environment for the cells to interact with. Other natural gels such as gelatin
and Matrigel (a commercially available basement membrane) or synthetic gels such
as PEG are also commonly used. These gels can provide a nanotopgraphy to cells
that is similar to what they would naturally fi nd in vivo (Abrams et al.
2000a
) . This has
the advantage of infl uencing individual cells to grow in a more natural manner.
Unfortunately, cellular organization in three dimensions of cell populations is not
possible with these traditional 3D scaffolds because many of the factors infl uencing
tissue morphogenesis are still unknown, and in an isolated system, it is not possible to
recreate the entire dynamic physical and biochemical milieu with which a developing
or healing organ would be supplied. However, if an engineered ECM can be provided
with natural details at the scale with which individual cells can interact, with the ECM
still properly ordered at a larger scale, it may be possible to build a tissue or organ in
the proper manner without going through the entire developmental or healing process.
The traditional manner of seeding cells into a 3D gel is to mix the cells into a pre-
polymer solution in order to disperse them through the matrix. Then the polymer is
cured to entrap the cells and provide form to the construct. Alternatively, the cells
may be placed on the surface of a cured polymer matrix into which they can then
migrate. These methods of seeding, however, provide little to no control over
the resultant cellular organization. Therefore, a technique is needed to move from
