Biomedical Engineering Reference
In-Depth Information
Given that adipose-derived mesenchymal precursors can
be harvested in abundant quantities under local anes-
thesia with little patient discomfort, they may emerge as
an important source for cell-based therapy. One can en-
vision a scenario in which a mesenchymal cell fraction is
purified from a patient's bone marrow or liposuction
aspirate, is exposed to oxygen and other environmental
conditions optimized for differentiation along a certain
lineage, and is ultimately returned to the same patient to
fill a tissue defect.
Some other tissues have been shown to be sources of
somatic stem cells. Periosteum, as well as adipose tissue
and peripheral blood, is a good source that removes the
confounding effect of hematopoietic stem cells. Perios-
teum has been shown to be an effective source of cells in
the repair of osteochondral defects in an animal model
but again suffers from painful procurement procedures
and low cell yield. On the basis of the currently available
therapeutic options, the ideal reconstitutive measure
would cause insignificant donor morbidity, regenerate
quickly the harvested tissue, have no size limitations, be
readily available, have no issues of immunogenicity, and
be of low cost. Good cell volume would obviate the need
for
ex vivo
expansion and easy availability would allow for
expensive preliminary
in vitro
human cell testing before
clinical trials.
Friedenstein
et al.
[29]
. They placed samples of whole
bone marrow in plastic culture dishes, and, after 4 h,
poured off the cells that were nonadherent. In effect,
they discarded most of the hematopoietic stem cells.
The small number of adherent cells were heterogenous in
appearance, but the most tightly adherent cells were
spindle shaped and formed foci of two to four cells. The
cells in the foci remained dormant for 2-4 days and then
began to multiply rapidly. After passage several times in
culture, the adherent cells became more uniformly
spindle shaped in appearance. The cells had the ability to
differentiate into colonies that resembled small deposits
of bone or cartilage.
Adult or somatic MSCs, commonly referred to as
BMSCs, are stem cells originated from embryonic me-
soderm and are a unique class of multipotent cells that
are noncommitted and remain in an undifferentiated
state. When induced by the appropriate biological cues
including right chemicals, hormones, and growth factors,
BMSCs are capable of extensive proliferation and dif-
ferentiation into several phenotypes including fibrous
tissues (e.g., tendon), and hematopoiesis-supporting
reticular stroma, and represent a heterogeneous cell
population likely containing a range of progenitor cells.
Of most importance from a tissue engineering standpoint
is the fact that stem cells that have been transformed to
osteoblasts are much like bone cells of a developing or-
ganism, and are capable of secreting large amounts of
ECM. Moreover, the ease of isolation of MSCs makes
them very attractive for tissue engineering applications;
aspiration of bone marrow is only slightly more compli-
cated than a blood donation, and MSCs can be enriched
to obtain a relatively pure population of cells. The tech-
nique using bone marrow cells (BMCs) does not require
cell culture with serum from other species, which might
be associated with a risk of infection. In addition, as large
numbers of cells can be obtained from BMCs without
culture, fewer steps in the preparation of tissues would
mean a lower risk of contamination and less work and
time. Fewer materials for culture would mean cost ben-
efits, and the technique can be applied even in emergency
cases in a clinical setting, as cell seeding requires only
a few hours. In culture of cells generated from suspen-
sions of marrow, colonies form from a single precursor
cell termed the ''colony-forming-unit fibroblast'' (CFU-F).
The progeny of these CFU-Fs is what have been defined
as BMSCs. During prolonged cultivation
ex vivo,
adult
BMSCs undergo two possible interdependent pro-
cedures: replicative aging and a decline in differentiation
potential.
One strategy for tissue engineering is to use these
MSCs or BMSCs harvested from marrow stroma. The
MSCs have attracted extensive interest due to the sur-
prising finding that they can also commit to neural cell
lineages in both animal models and cell cultures. Under
7.2.8.2.1 MSCs
Circulating blood cells survive for only a few days or
months. This means that hematopoietic stem cells in
bone marrow must provide a continuous source of pro-
genitors for blood cells. Bone marrow also contains cells
that meet the criteria for stem cells of non-hematopoietic
tissues. The stem-like cells for non-hematopoietic tissues
are referred to as MSCs, because they can be differen-
tiated in culture into osteoblasts, chondrocytes, adipo-
cytes, and myoblasts. The marrow stromal cells can be
isolated from other cells in marrow by their tendency to
adhere to tissue culture plastic.
The presence of stem cells for non-hematopoietic
cells in bone marrow was first suggested by a German
pathologist Cohnheim 140 years ago
[28]
. He studied
wound repair by injecting an insoluble dye into the veins
of animals and then looking for the appearance of dye-
containing cells in wounds he created at a distant site. He
concluded that most of the cells appearing in the wounds
came from the bloodstream and, by implication, from
bone marrow. This work raised the possibility that bone
marrow might be the source of fibroblasts that deposit
collagen fibers as part of the normal process of wound
repair.
Evidence that bone marrow contains cells that can
differentiate into fibroblasts as well as other mesenchy-
mal cells has been available since the pioneering work of
