Harnessing Cell–Biomaterial Interactions for Osteochondral Tissue Regeneration - Tissue Engineering: Cell-Surface Interactions for Tissue Culture - page 78

Directed differentiation of ESCs to chondrocytes has also been accomplished by
culturing the cells with exogenous growth factors that may allow them to first
differentiate into primitive streak mesendoderm [fibronectin matrix, WNT3A,
activin A, fibroblast growth factor (FGF)-2, BMP-4], then to differentiate into a
mesoderm population (fibronectin matrix, FGF-2, BMP-4, neurotrophin-4,
follistatin) [ 75 ]. Culturing the mesoderm cells with fibronectin and gelatin, and
weaning them off BMP-4 while supplying FGF-2, neurotrophin-4, and growth
differentiation factor 5, led the cells to express the chondrogenic marker SOX9 as
well as to produce type II collagen and sulfated GAGs [ 75 ]. Additionally, new
approaches have begun to investigate how to bypass the formation of an EB,
because of the lack of control of EB size and the associated cell number. Cell-cell
interactions promoted by pellet or micromass culture of ESCs in combination with
growth factors can further enhance the formation of type II collagen [ 73 , 74 , 76 ].
Co-culture of ESCs with chondrocytes can also aid in chondrogenic differentiation
in vitro and in vivo [ 77 , 78 ]. ESCs were initially co-cultured with irradiated
chondrocytes and TGF-b 3 . Co-culturing these cells with fresh ESCs in Hyaff-11,
a hyaluronan gel, and TGF-b 1 showed positive alician blue-van Gieson staining
for collagen and GAGs [ 78 ]. In addition, human ESCs cultured in RGD-modified
PEG hydrogels showed an increase in synthesis of GAG and collagen as well as a
stimulated gene expression level of link protein and type II collagen versus cells
cultured in unmodified PEG hydrogels [ 79 ].
5.1.4 Other Stem Cells
Stem cells can be isolated from other tissues as well, such as the muscle and
periosteum. These stem cells have shown the potential to become chondropro-
genitor cells. Muscle-derived stem cells (MDSCs) are located in the muscle tissue,
have been shown to be multipotent, and can proliferate quickly with limited
senescence. New cartilage was able to form in full-thickness osteochondral defects
in rats after they had been treated for 5 weeks with a type I collagen gel scaffold
containing MDSCs [ 80 ]. The presence of 10 lg BMP-2 in a MDSC pellet culture
in a diffusion chamber resulted in expression of type II collagen and aggrecan [ 81 ].
Additionally, when this pellet was placed in an in vivo rat patellar groove defect,
the newly formed tissue covered the defect area with GAGs and collagen, as seen
by histological sections stained with hematoxylin and eosin as well as with tolu-
idine blue. MDSCs retrovirally transduced with a BMP-4 gene have been found to
express type II collagen in vitro when cultured in chondrogenic medium supple-
mented with TGF-b 1 [ 82 ]. Injection of the MDSCs with acellular fibrin glue into a
rat osteochondral defect resulted in glossy cartilage being formed after 24 weeks.
The periosteum is located on the surface of the bone cortex and contains two
distinct layers: a fibrous outer layer and the cambium. Chondroprogenitor cells
have been isolated in the thin, inner, cambium layer, which is located adjacent to
the bone surface [ 83 ]. Surface markers that are present for MSCs have also been