Biomedical Engineering Reference
In-Depth Information
treated by these autograft methods, however,
are still unacceptably high. As is the case for
chondrocyte-loaded cartilage autografts, the
supply of autologous tenocytes and ligament
fi broblasts is limited, and their harvest often
leads to donor-site morbidity.
The fi eld of stem-cell-engineered tendons
and ligaments is still in its infancy, even though
the observation that BM-MSCs can differenti-
ate into tendons and ligaments was made over
a decade ago [
[
]. The average repair stiffness and modulus
values were
53
%, respectively, of the
values in normal patellar tendon [
30
% and
20
]. Simi-
larly, rabbit BM-MSCs loaded onto collagen
gels and contracted onto sutures possessed
only
53
% of the maximum stress capacity of the
normal tendon when implanted in a patellar
tendon defect model. More disconcerting was
the observation that bone formed in
25
28
% of the
patellar implant sites [
]. The results point to
the obvious conclusion that the mesenchymal
tissue engineer must continue efforts to iden-
tify the relevant mechanisms involved in
tendon and ligament differentiation.
The recent utilization of silk-fi ber-based
delivery scaffolds with BM-MSCs has improved
stem-cell-engineered ligaments [
7
]. As is the case for cartilage
and bone, the abundance of stem cells makes
up for the limited availability of donor tissue
and the high donor-site morbidity. Stem-cell-
generated grafts, however, like ligament fi bro-
blast- and tenocyte-seeded grafts, must be able
to synthesize and remodel collagen, elastin,
and other ECM proteins so that physiologically
relevant levels of mechanical resistance and
organization can be attained. Secondly, they
must be delivered on a scaffold that is initially
strong enough to endure cyclic stresses yet can
undergo gradual degradation, thereby allowing
the stem cells to differentiate and to secrete
matrix proteins that can replace the scaffold.
Finally, the new tissue must integrate with the
host tissue so as to avoid recurrence of the
injury.
Identifying optimal in vitro conditions that
permit implantation has been challenging.
Three factors seem essential: the absolute
number of stem cells, the ratio of cells to colla-
gen, and the ability of the cultured cells to
synthesize the collagen in vitro prior to implan-
tation [
20
]. The
silk fi bers have superior mechanical properties
and biodegrade within a more compatible
timeframe. When woven into a six-cord rope
confi guration, the constructs display mechani-
cal properties similar to the anterior cruciate
ligament, and the constructs possess a greater
surface area for cell attachment and ECM depo-
sition [
4
,
115
].
Integration of tendons and ligaments into
the bone is critical for the long-term success
of any engineered graft. Autografts and
allografts used for ligament and tendon recon-
struction have a poor record in this regard.
Because of their ability to differentiate into
multiple tissues, stem cells have the potential
to generate the different tissues required for
appropriate integration into the host tissue.
When tendon autografts coated with fi brin
glue were loaded with MSCs, cartilage cells
covered a large area at the tendon-bone junc-
tion within
4
,
115
]. Furthermore, as with ligament
fi broblast-loaded constructs, exposure to
appropriate cyclic strain is important to estab-
lish appropriate orientation and cross linking
of matrix fi bers within the new tissue [
8
,
9
,
53
weeks, a mature
zone of cartilage blended from bone into the
tendon grafts. At
2
weeks [
70
]. By
8
].
To date, the characteristics that have been
attained by stem-cell-engineered tendons and
ligaments have fallen short of the desired
outcome. In early studies, BM-MSCs initially
seeded onto collagen scaffolds did not induce
ligament regeneration, because the collagen
scaffold did not stimulate the stem cells to
produce adequate amounts of ligament matrix.
In addition, the collagen fi ber scaffolds did not
support long-term anchoring of the grafts in
vivo [
34
weeks, the MSC-enhanced
grafts had a signifi cantly higher failure load
and greater stiffness than the grafts loaded
with fi brin glue.
For stem-cell-based therapeutics to be a
success in clinical trials, research must be done
to address key factors known to infl uence stem-
cell effi cacy. For example, autologous stem cells
may be infl uenced by both the health status and
the age of the patient. Bruder and colleagues
have observed that the number of stem cells in
bone marrow appears to decline with age [
8
]. In a rabbit full-length, full-thickness
tendon-defect model, the average maximum
force and stress values of the BM-MSC-
engineered collagen implants were approxi-
mately
115
];
however, whether this is true of adult stem cells
from other body sites remains to be deter-
mined. Fewer stem cells are available as an
17
30
% that of normal patellar tendons
