Harnessing Cell–Biomaterial Interactions for Osteochondral Tissue Regeneration - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 76

prehypertrophic to a hypertrophic state during condensation [ 32 , 51 ]. Overall,
these findings suggest that chondrogenic differentiation of MSCs promoted by
cell-cell contact during in vitro culture can be enhanced by exposure to chon-
drogenic or cartilage-like factors.
The microenvironment has also been shown to greatly influence chondrogenic
differentiation of MSCs. For example, when MSCs are entrapped in alginate gels,
they are able to maintain a rounded morphology and express type II collagen in the
presence of TGF-b 1 , and type X collagen expression was found to be lower that for
MSCs grown in pellet culture [ 48 , 54 ]. A lower level of type X collagen has been
shown to be expressed by MSCs when ECM components are present, such as
native cartilage-derived matrix with TGF-b 3 and/or bone morphogenetic protein
(BMP)-6, versus culture in alginate alone [ 55 ], indicating the importance of
cell-matrix interactions in MSC differentiation. Exposure of MSCs to methacry-
lated HA has been found to upregulate type II collagen expression and, by also
incorporating TGF-b 3 , higher levels of type II collagen, SOX9, and aggrecan are
observed by 14 days of culture [ 56 ]. When the methacrylated HA constructs were
implanted subcutaneously in mice, the presence of TGF-b 3 with MSCs in the
constructs resulted in better neocartilage expression than observed with MSCs in
poly(ethylene glycol) (PEG) hydrogels. Other work has shown that HA in
poly(ethylene oxide) diacrylate hydrogels allows entrapped MSCs to express type
II collagen and cartilage proteoglycans, but incorporation of TGF-b 3 was able to
lower the expression of type I collagen [ 57 ]. Fibrin hydrogels with heparinized
nanoparticles releasing TGF-b 3 were able to induce higher levels of type II
collagen, aggrecan, and SOX9 expression compared with fibrin hydrogels, fibrin
hydrogels with TGF-b 3 , and fibrin hydrogels with nanoparticles alone. In a rabbit
full-thickness articular cartilage defect, the MSCs in the fibrin construct containing
the nanoparticles and TGF-b 3 showed the best neocartilage formation [ 58 ].
Overall, interactions of cells with growth factors and ECM molecules are essential
for chondrogenic differentiation of MSCs.
5.1.2 Adipose-Derived Stem Cells
Adipose-derived stem cells (ADSCs) are part of the embryonic mesoderm and can
be isolated from fat tissue, which is commonly removed from the body by lipo-
suction. ADSCs have a fibroblastic morphology and are positive for cell markers
CD90 and CD105 and negative for CD14, CD34, and CD45, which is similar to
the criteria suggested for selection of human MSCs [ 59 ]. Additionally, ADSCs can
be expanded for long periods of time with low levels of senescence. Cartilage
nodules containing sulfated proteoglycan-rich matrix and type II collagen
have been observed when ADSCs are cultured in a micromass culture [ 60 ].
The chondrogenic culture medium with dexamethasone containing TGF-b 1 , BMP-6,
and/or TGF-b 3 has been found to be important to facilitate the differentiation of
ADSCs into chondrocytes [ 55 , 61 ].