J Musculoskel Neuron Interact 2002; 2(4):309-320
Adult mesenchymal stem cells: Potential for muscle and
tendon regeneration and use in gene therapy
M. Pittenger, P. Vanguri, D. Simonetti, R. Young
Osiris Therapeutics, Inc., Baltimore, Maryland, USA
The expansion potential and plasticity of stem cells, adult or embryonic, offer great promise for their use in medical
therapies. Recent provocative data suggest that the differentiation potential of adult stem cells may extend to lineages beyond
those usually associated with the germ layer of origin. In this review, we describe recent developments related to adult stem
cell research and in particular, in the arena of mesenchymal stem cell (MSC) research. Research demonstrates that transduced
MSCs injected into skeletal muscle can persist and express secreted gene products. The ability of the MSC to differentiate into
cardiomyocytes has been reported and their ability to engraft and modify the pathology in infarcted animal models is of great
interest. Research using MSCs in tendon repair provides information on the effects of physical forces on phenotype and gene
expression. In turn, MSCs produce changes in their matrix environment in response to those biomechanical forces. Recent
data support the potential of MSCs to repair tendon, ligament, meniscus and other connective tissues. Therapeutic
applications of adult stem cells are approaching clinical use in several fields, furthering the possibility to regenerate damaged
and diseased tissue.
Keywords: Stem Cells, Mesenchymal Stem Cells, Adult Stem Cells, Differentiation, Tissue Regeneration, Myogenic, Cardiac,
Gene Therapy, Tendon
Clearly, one of the most exciting recent developments in
biology and medicine is the many reports that stem cells exist
in the adult that can regenerate damaged and diseased tissue.
Many adult tissues are known to contain stem cells that
provide for the new cells for the normal tissue turnover that is
known to occur. As examples, this includes liver parenchymal
cells that can regenerate large segments of that organ, the
regenerative satellite cells resident in skeletal muscle,
several epithelial cell types such as the intestinal crypt cells
that provide for the continued turnover of the intestinal
epithelia. Stem cells also have been isolated from the embryo
that can likely generate many tissues1,2and the isolation of
the human embryonic stem (ES)3and embryonic germ
(EG)4cells brings this prospect closer to reality. While there
has been some effort to direct ES cells to different lineages5,
the indication that differentiation of ES cells can be
controlled in a predictable manner is not yet evident and
various cell types are seen in the same culture dish. Almost
concurrent with the human ES and EG cell reports were
several publications describing stem cells from adult
tissues6-8. Our own work characterized human bone marrow-
derived cells that could be greatly expanded to a
homogeneous population (or grown clonally), would
differentiate to several discrete lineages and fulfilled the
criteria to be called mesenchymal stem cells6(Figure 1).
Another report offered evidence that while hepatocytes have
great ability to regenerate functioning liver tissue, there are
also stem cells in blood with this capability7. A provocative
study utilized neural stem cells to repopulate the
hematopoietic lineages and rescue a lethally irradiated mouse,
perhaps one of the first reports that stem cells may have
great plasticity beyond the lineages for which they were first
characterized8. Particularly important is the predictability
and uniformity of the isolation process from one preparation
of stem cells to the next. In this regard, the adult stem cells
isolated from bone marrow continue to offer promising
Corresponding author: Mark Pittenger, Osiris Therapeutics, Inc., 2001
Aliceanna Street, Baltimore, MD 21231, USA
Accepted 5 March 2002
results for tissue regeneration. The challenge is to characterize
these cells, understand their regenerative potential, and to
develop useful therapies. Whether the adult bone marrow-
derived MSCs can be used to regenerate tissues outside the
mesodermal lineage is also under investigation in several
laboratories. Below, we briefly review the developments in
this field that support the prospects for therapeutic use of
MSCs and highlight work on muscle and tendon regeneration.
Adult bone marrow has proven to be a reliable source of
multipotential cells, first the hematopoietic stem cell (HSC)
and now the mesenchymal stem cell (MSC).
Bone marrow stroma is a tissue composed of many cell types
that fills the intramedullary space of bone. It contains
osteoblasts, adipocytes, reticular cells, endothelial cells,
macrophages, monocytes etc., as well as progenitors for these
cell types. Bone marrow contains niches that provide support
for the maintenance of hematopoietic stem cells, and the
mesenchymal stem cells. Therefore, an important function of
marrow stroma is to provide an environment that prevents the
differentiation of stem cells. It is important to keep in mind
that while bone marrow contains mesenchymal stem cells,
clearly not all bone marrow stromal cells are stem cells.
Bone marrow can be drawn easily under local anesthetic
from accessible sites such as the posterior iliac crest. While
other tissues may contain MSCs, their collection is more
difficult and/or traumatic for the donor. It is likely that such
stem cells can be isolated from many tissues, but which tissues
provide the most accessible cells that retain the greatest
potential with the least trauma to the donor? Keeping in mind
that we are only beginning to understand the potential of stem
cells that persist in the adult, much evidence to date would
recommend bone marrow MSCs. We routinely utilize a modified
version of the isolation procedure first developed in the Caplan
laboratory and this has been presented in detail elsewhere6,9,10.
The process of tissue healing has always fascinated the
observer. Literature on new tissue formation before 1900
comes largely from embryologists and from the accounts of
surgeons, often in military service. For the embryologist,
there were few reagents available such as antibodies,
purified proteins, or gene probes that we rely on heavily
today for following cellular and molecular events during
tissue remodeling. Moreover the first antibiotics,
sulfonamides, were not available until the 1930s, so many
in vitro experiments were hampered by the necessity to
maintain absolute sterility. Therefore, many of these early
investigations were performed in vivo where the immune
system provided bacterial surveillance. The surgeons’
descriptions of events in the ongoing healing process were
largely observational at limited time points, and the process
could not easily be followed longitudinally. Moreover,
publications, including textbooks, made limited use of
pictures and figures.
Studies on effects of exposure to radioactive substances in
the 1940s led to the identification of spleen and bone
marrow as the source of progenitor cells for the
hematopoietic system11-13. These early studies led to the
development of marrow ablation and therapeutic bone
marrow transplantation and the realization that there must
be an isolatable hematopoietic stem cell (HSC). The search
for greater understanding of the characteristics of this adult
stem cell is 30-40 years old and continues to be a very active
field at both the research and clinical levels14.
Early tissue transplantation studies revealed that cells
responded to being placed in a new environment by altering
their phenotype15. The transplantation of whole bone
marrow to an ectopic site demonstrated the formation of
bone and cartilage16,17. In the 1960s, Alexander Friedenstein
cultured cells from guinea pig bone marrow and studied the
cells that became attached to the culture dish18,19. These cells
were mitotically very active and formed bone when
implanted in syngeneic hosts. They were described as
fibroblastic colony forming cells (FCFC) or osteogenic
progenitor cells (OPCs), later more commonly referred to as
colony forming units-fibroblastic (CFU-F). Experiments
M. Pittenger et al.: Adult stem cells for connective tissues
Figure 1. Human mesenchymal stem cells isolated from bone marrow taken from the iliac crest can be induced to differentiate to several
lineages. A) phase micrograph of log phase hMSCs B) Confluent hMSCS can be readily differentiated into adipocytes as shown here with
the lipid vacuoles stained with oil red O C) A micromass of hMSCs can be induced to become chondrocytes as shown here, stained with anti-
type II collagen D) Monolayers of hMSCS differentiate to osteoblasts and elevate expression of alkaline phosphatase (red stain) and
accumulate calcium deposits (dark areas) E) hMSCs can serve a stromal function to support HSCs and their progeny which form
"cobblestones" on top of the hMSCS (hMSCS are underneath and not visible).
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