Biomedical Engineering Reference
In-Depth Information
allograft bone alone had failed to heal these
defects (Fig.
Stem-cell differentiation toward a chondro-
genic phenotype also depends on activation of
the TGF-
C and D). As more clinical
trials are conducted, bone derived from stem-
cell grafts may make the challenges of auto-
graft and allograft transplants a thing of the
past.
1
.
2
β
/BMP cell-signaling pathways [
11
,
71
,
103
]. Thus, human ADSCs that had been
predifferentiated in the presence of TGF-
,
126
in
an alginate construct, when implanted sub-
cutaneously, produced signifi cantly more car-
tilaginous matrix than cells not so treated [
β
].
Other signaling mechanisms involving the
parathyroid hormone-related peptide (PTHrP)
receptor, glucocorticoid receptor, hyaluronic
acid, and sonic hedgehog pathways have also
been found to stimulate stem-cell chondrogen-
esis [
27
1.6 Stem-Cell-Engineered
Cartilage:
Microenvironmental Factors
Influence Stem-Cell
Chondrogenesis
]. The relative importance of
growth and differentiation factors and of the
resulting signaling pathways is, however,
model-dependent. Therefore, the same devel-
opmental challenges that must be overcome to
generate bone also apply to stem-cell chondro-
genesis [
25
,
27
,
32
,
103
As with bone, embryonic and adult-derived
stem cells can give rise to cartilage in vitro [
27
,
].
Cartilage generation by stem cells also
depends on the type of substrate or scaffold
used. Conventional scaffolds are composed of
collagen and proteoglycans or other hydrated
organic molecules such as agarose, alginate, or
hydrogels. Some articular cartilage defects,
however, are repaired with the aid of constructs
that contain calcium/phosphate salts [
48
43
]. And similarly
to bone, the ability of stem cells to form carti-
lage in vitro depends on both physical and
chemical stimuli. These include growth and
differentiation factors, cell-cell interactions,
cell-matrix interactions, and inorganic chemi-
cal and physical factors such as oxygen tension
and the three-dimensional organization of
the cells. Unlike bone, however, the physical
elements of the cartilage microenvironment
appear to be more critical for stem cells to dif-
ferentiate into chondrocytes than for stem cells
to become osteoblasts.
Three-dimensional interactions between
cells are required for chondrocyte differentia-
tion and subsequent cartilage tissue formation.
When stem cells are plated as a monolayer, vir-
tually no chondrogenesis results, even with
added growth factors such as TGF-
,
56
,
71
,
93
,
107
,
116
,
125
,
126
11
,
32
,
36
, a large articular
cartilage defect in sheep was almost completely
repaired when autologous BM-MSCs were
loaded onto a
,
80
]. As shown in Fig.
1
.
3
β
-tricalcium phosphate scaffold
[
].
Understanding the properties of the scaf-
folds or matrices in which the cells are deliv-
ered is especially critical for cartilage formation
in vitro and its implantation. The microenvi-
ronment within the scaffold must be such as to
support stem-cell growth and differentiation.
As discussed in other chapters of this volume,
the scaffold material itself must have physical
properties comparable to those of the host car-
tilage and last until enough new cartilage has
been produced to replace or supplement the
implanted scaffold. For the scaffold to be
replaced, it must be biodegradable.
Gelatin and agarose-based scaffolds have
been used most commonly. When stem cells
are seeded into scaffolds with nonosteogenic
matrices such as gelatin and agarose, the matri-
ces undergo changes in stress and compression
that correlate with increased cartilage matrix
accumulation. [
36
β
and BMP
[
]. However, if the cells can estab-
lish three-dimensional polarity when cultured
as condensed cell pellets or seeded into semi-
solid matrices such as alginate or hydrogel,
they express proteoglycans and collagen iso-
forms, and a cartilage matrix is formed [
10
,
27
,
42
,
45
11
,
27
,
71
]. The oxygen level in the culture is a
second, important physicochemical parameter
that affects chondrogenesis. Reducing the
oxygen level in the culture to that which char-
acterizes the cartilage environment in vivo
enhances cartilage formation by ADSCs [
,
125
14
,
120
], decreases cell proliferation, and increases
the secretion of the essential protein, type II
collagen, and of chondroitin-
] When ADSCs are loaded
onto gelatin scaffolds, their equilibrium
11
4
-sulfate [
120
].
