Mesenchymal stem cells, or
MSCs, are multipotent
stem cellStem cells are cells found in most, if not all, multi-cellular organisms. They are characterized by the ability to renew themselves through mitotic cell division and differentiating into a diverse range of specialized cell types. Research in the stem cell field grew out of findings by Canadian...
s that can
differentiateIn developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized cell type. Differentiation occurs numerous times during the development of a multicellular organism as the organism changes from a single zygote to a complex system of...
into a variety of cell types. Cell types that MSCs have been shown to differentiate into
in vitroA procedure performed in vitro is performed not in a living organism but in a controlled environment, such as in a test tube or Petri dish...
or
in vivoIn vivo refers to experimentation using a whole, living organism as opposed to a partial or dead organism, or an in vitro controlled environment. Animal testing and clinical trials are two forms of in vivo research...
include
osteoblastAn osteoblast is a mononucleate cell that is responsible for bone formation. Osteoblasts produce osteoid, which is composed mainly of Type I collagen. Osteoblasts are also responsible for mineralization of the osteoid matrix...
s,
chondrocyteChondrocytes are the only cells found in cartilage. They produce and maintain the cartilaginous matrix, which consists mainly of collagen and proteoglycans...
s,
myocyteA myocyte is the type of cell found in muscles. They arise from myoblasts.Each myocyte contains myofibrils, which are long chains of sarcomeres, the contractile units of the cell....
s,
adipocyteAdipocytes, also known as lipocytes and fat cells, are the cells that primarily compose adipose tissue, specialized in storing energy as fat.There are two types of adipose tissue, white adipose tissue and brown adipose tissue , which are also known as white fat and brown fat, respectively, and...
s,
endotheliumThe endothelium is the thin layer of cells that line the interior surface of blood vessels, forming an interface between circulating blood in the lumen and the rest of the vessel wall. Endothelial cells line the entire circulatory system, from the heart to the smallest capillary...
s, and, as described lately,
beta-pancreatic islets cellsright|thumbnail|300px|A porcine islet of Langerhans. The left image is a brightfield image created using [[hematoxylin]] stain; nuclei are dark circles and the [[acinar]] pancreatic tissue is darker than the islet tissue...
.
While the terms Mesenchymal Stem Cell and Marrow Stromal Cell have been used interchangeably, neither term is sufficiently descriptive as discussed below:
- Mesenchyme
Mesenchyme, or mesenchymal connective tissue, is an example of reticular connective tissue, a type of loose connective tissue, which is derived from all three germ layers and located within the embryo . Mesenchyme is characterized morphologically by a prominent ground substance matrix containing a...
is embryoAn embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
nic connective tissueConnective tissue is a form of fibrous tissue.. It is one of the four types of tissue in traditional classifications ....
that is derived from the mesodermIn humans, the mesoderm is one of the three primary germ cell layers - the other two are the ectoderm and endoderm - in the very early embryo. The mesoderm is the middle layer. It differentiates to gives rise to a number of tissues and structures including bone, muscle, connective tissue, and the...
and that differentiates into hematopoieticHaematopoiesis is the formation of blood cellular components. All cellular blood components are derived from haematopoietic stem cells...
and connective tissue, whereas MSCs do not differentiate into hematopoietic cells.
- Stromal cells are connective tissue cells that form the supportive structure in which the functional cells of the tissue reside.
Mesenchymal stem cells, or
MSCs, are multipotent
stem cellStem cells are cells found in most, if not all, multi-cellular organisms. They are characterized by the ability to renew themselves through mitotic cell division and differentiating into a diverse range of specialized cell types. Research in the stem cell field grew out of findings by Canadian...
s that can
differentiateIn developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized cell type. Differentiation occurs numerous times during the development of a multicellular organism as the organism changes from a single zygote to a complex system of...
into a variety of cell types. Cell types that MSCs have been shown to differentiate into
in vitroA procedure performed in vitro is performed not in a living organism but in a controlled environment, such as in a test tube or Petri dish...
or
in vivoIn vivo refers to experimentation using a whole, living organism as opposed to a partial or dead organism, or an in vitro controlled environment. Animal testing and clinical trials are two forms of in vivo research...
include
osteoblastAn osteoblast is a mononucleate cell that is responsible for bone formation. Osteoblasts produce osteoid, which is composed mainly of Type I collagen. Osteoblasts are also responsible for mineralization of the osteoid matrix...
s,
chondrocyteChondrocytes are the only cells found in cartilage. They produce and maintain the cartilaginous matrix, which consists mainly of collagen and proteoglycans...
s,
myocyteA myocyte is the type of cell found in muscles. They arise from myoblasts.Each myocyte contains myofibrils, which are long chains of sarcomeres, the contractile units of the cell....
s,
adipocyteAdipocytes, also known as lipocytes and fat cells, are the cells that primarily compose adipose tissue, specialized in storing energy as fat.There are two types of adipose tissue, white adipose tissue and brown adipose tissue , which are also known as white fat and brown fat, respectively, and...
s,
endotheliumThe endothelium is the thin layer of cells that line the interior surface of blood vessels, forming an interface between circulating blood in the lumen and the rest of the vessel wall. Endothelial cells line the entire circulatory system, from the heart to the smallest capillary...
s, and, as described lately,
beta-pancreatic islets cellsright|thumbnail|300px|A porcine islet of Langerhans. The left image is a brightfield image created using [[hematoxylin]] stain; nuclei are dark circles and the [[acinar]] pancreatic tissue is darker than the islet tissue...
.
Definition
While the terms Mesenchymal Stem Cell and Marrow Stromal Cell have been used interchangeably, neither term is sufficiently descriptive as discussed below:
- Mesenchyme
Mesenchyme, or mesenchymal connective tissue, is an example of reticular connective tissue, a type of loose connective tissue, which is derived from all three germ layers and located within the embryo . Mesenchyme is characterized morphologically by a prominent ground substance matrix containing a...
is embryoAn embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
nic connective tissueConnective tissue is a form of fibrous tissue.. It is one of the four types of tissue in traditional classifications ....
that is derived from the mesodermIn humans, the mesoderm is one of the three primary germ cell layers - the other two are the ectoderm and endoderm - in the very early embryo. The mesoderm is the middle layer. It differentiates to gives rise to a number of tissues and structures including bone, muscle, connective tissue, and the...
and that differentiates into hematopoieticHaematopoiesis is the formation of blood cellular components. All cellular blood components are derived from haematopoietic stem cells...
and connective tissue, whereas MSCs do not differentiate into hematopoietic cells.
- Stromal cells are connective tissue cells that form the supportive structure in which the functional cells of the tissue reside. While this is an accurate description for one function of MSCs, the term fails to convey the relatively recently-discovered roles of MSCs in repair of tissue.
- Because the cells, called MSCs by many labs today, can encompass multipotent cells derived from other non-marrow
Bone marrow is the flexible tissue found in the hollow interior of bones. In adults, marrow in large bones produces new blood cells. It constitutes 4% of total body weight, i.e...
tissues, such as adult muscleMuscle is the contractile tissue of the body and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
or the dental pulp of deciduous baby teeth, yet do not have the capacity to reconstitute an entire organ, the term Multipotent Stromal Cell has been proposed as a better replacement.
History
In 1924, Russian-born morphologist
Alexander A. MaximowAlexander A. Maximow , was a scientist renowned for his experimental work confirming the unitarian theory of hematopoiesis: all blood cells develop from a common precursor cell....
used extensive histological findings to identify a singular type of precursor cell within mesenchyme which develops into different types of blood cells.
Scientists
Ernest A. McCullochDr. Ernest Armstrong McCulloch, OC, O.Ont, FRSC is a University of Toronto cellular biologist, best known for demonstrating – with James Till – the existence of stem cells.-Biography:...
and
James E. TillJames Edgar Till, OC, O.Ont, FRSC is a University of Toronto biophysicist, best known for demonstrating – with Ernest McCulloch – the existence of stem cells.-Early work:...
first revealed the clonal nature of marrow cells in the 1960s. An
ex vivoEx vivo means that which takes place outside an organism. In science, ex vivo refers to experimentation or measurements done in or on tissue in an artificial environment outside the organism with the minimum alteration of natural conditions...
assay for examining the clonogenic potential of multipotent marrow cells was later reported in the 1970s by Friedenstein and colleagues. In this assay system, stromal cells were referred to as
colony-forming unit-fibroblasts (CFU-f).
Subsequent experimentation revealed the plasticity of marrow cells and how their fate could be determined by environmental cues. Culturing marrow stromal cells in the presence of osteogenic stimuli such as
ascorbic acidAscorbic acid is a sugar acid with antioxidant properties. Its appearance is white to light-yellow crystals or powder, and it is water-soluble. One form of ascorbic acid is commonly known as vitamin C. The name is derived from a- and scorbutus , the disease caused by a deficiency of vitamin C...
,
inorganic phosphateA phosphate, an inorganic chemical, is a salt of phosphoric acid. In organic chemistry, a phosphate, or organophosphate, is an ester of phosphoric acid. Organic phosphates are important in biochemistry and biogeochemistry or ecology. Inorganic phosphates are mined to obtain phosphorus for use in...
, and
dexamethasoneDexamethasone is a potent synthetic member of the glucocorticoid class of steroid hormones. It acts as an anti-inflammatory and immunosuppressant...
could promote their differentiation into
osteoblastAn osteoblast is a mononucleate cell that is responsible for bone formation. Osteoblasts produce osteoid, which is composed mainly of Type I collagen. Osteoblasts are also responsible for mineralization of the osteoid matrix...
s. In contrast, the addition of
transforming growth factor-betaTransforming growth factor is used to describe two classes of polypeptide growth factors, TGFα and TGFβ....
(TGF-b) could induce chondrogenic markers.
Morphology
Mesenchymal stem cells are characterized morphologically by a small cell body with a few cell processes that are long and thin. The cell body contains a large, round
nucleusIn cell biology, the nucleus , also sometimes referred to as the "control center", is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as...
with a prominent
nucleolusThe nucleolus is a non-membrane bound structure composed of proteins and nucleic acids found within the nucleus. Ribosomal RNA is transcribed and assembled within the nucleolus...
which is surrounded by finely dispersed
chromatinChromatin is the complex combination of DNA and protein that makes up chromosomes. It is found inside the nuclei of eukaryotic cells. It is divided between heterochromatin and euchromatin forms....
particles, giving the nucleus a clear appearance. The remainder of the cell body contains a small amount of
Golgi apparatusThe Golgi apparatus is an organelle found in most eukaryotic cells...
, rough endoplasmic reticulum, mitochondria, and polyribosomes. The cells, which are long and thin, are widely dispersed and the adjacent
extracellular matrixIn biology, the extracellular matrix is the extracellular part of animal tissue that usually provides structural support to the animal cells in addition to performing various other important functions. The extracellular matrix is the defining feature of connective tissue in animals.Extracellular...
is populated by a few
reticularReticular fibers or reticulin is a histological term used to describe a type of fiber in connective tissue composed of type III collagen. Reticular fibers crosslink to form a fine meshwork...
fibrils but is devoid of the other types of
collagenCollagen is the main protein of connective tissue in animals and the most abundant protein in mammals, making up about 25% to 35% of the whole-body protein content. It is naturally found exclusively in metazoa, including sponges. In muscle tissue it serves as a major component of endomysium...
fibrils.
Detection
There is no test that can be performed on a single cell to determine whether that cell is an MSC. There are surface antigens that can be used to isolate a population of cells that have similar self-renewal and differentiation capacities, yet MSCs, as a population, typically do not all express the proposed markers; and it is not certain which ones must be expressed in order for that cell to be classified as an MSC. It may be that the therapeutic properties attributed to MSCs result from the interaction between the different cells that make up an MSC culture, suggesting that there is no one cell that has all the properties.
Differentiation capacity
MSCs have a large capacity for self-renewal while maintaining their multipotency. Beyond that, there is little that can be definitively said. The standard test to confirm multipotency is differentiation of the cells into osteoblasts, adipocytes, and chondrocytes as well as myocytes and possibly neuron-like cells. However, the degree to which the culture will differentiate varies among individuals and how differentiation is induced, e.g. chemical vs. mechanical; and it is not clear whether this variation is due to a different amount of "true" progenitor cells in the culture or variable differentiation capacities of individuals' progenitors. The capacity of cells to
proliferateThe word proliferation literally means the process of "giving life to" something, either an abstract or a concrete. Depending on the context used, proliferation can refer to a number of different topics, such as:*Nuclear proliferation...
and
differentiateIn developmental biology, cellular differentiation is the process by which a less specialized cell becomes a more specialized cell type. Differentiation occurs numerous times during the development of a multicellular organism as the organism changes from a single zygote to a complex system of...
is known to decrease with the age of the donor, as well as the time in culture. Likewise, whether this is due to a decrease in the number of MSCs or a change to the existing MSCs is not known.
Immunomodulatory effects
Numerous studies have demonstrated that human MSC avoid
allorecognitionTransplant rejection occurs when a transplanted organ or tissue is not accepted by the body of the transplant recipient. This is explained by the concept that the immune system of the recipient attacks the transplanted organ or tissue...
, interfere with
dendritic cellDendritic cells are immune cells that form part of the mammalian immune system. Their main function is to process antigen material and present it on the surface to other cells of the immune system, thus functioning as antigen-presenting cells....
and T-cell function and generate a local immunosuppressive microenvironment by secreting
cytokineCytokines are any of a number of substances that are secreted by specific cells of the immune system which carry signals locally between cells, and thus have an effect on other cells. They are a category of signaling molecules that are used extensively in cellular communication. They are proteins,...
s. It has also been shown that the immunomodulatory function of human MSC is enhanced when the cells are exposed to an inflammatory environment characterised by the presence of elevated local
interferon-gammaInterferon-gamma is a dimerized soluble cytokine that is the only member of the type II class of interferons. This interferon was originally called macrophage-activating factor, a term now used to describe a larger family of proteins to which IFN-γ belongs...
levels. Other studies contradict some of these findings, reflecting both the highly heterogeneous nature of MSC isolates and the considerable differences between isolates generated by the many different methods under development.
Culturing
The majority of modern culture techniques still take a CFU-f approach, where raw unpurified bone marrow or ficoll-purified bone marrow monocytes are plated directly into
cell cultureCell culture is the process by which cells are grown under controlled conditions. In practice the term "cell culture" has come to refer to the culturing of cells derived from multicellular eukaryotes, especially animal cells...
plates or flasks. Mesenchymal stem cells, but not red blood cells or haematopoetic progenitors, are adherent to tissue culture plastic within 24 to 48 hours. However, at least one publication has identified a population of non-adherent MSCs that are not obtained by the direct-plating technique.
Other
flow cytometryFlow cytometry is a technique for counting and examining microscopic particles, such as cells and chromosomes, by suspending them in a stream of fluid and passing them by an electronic detection apparatus. It allows simultaneous multiparametric analysis of the physical and/or chemical...
-based methods allow the sorting of bone marrow cells for specific surface markers, such as STRO-1. STRO-1+ cells are generally more homogenous, and have higher rates of adherence and higher rates of proliferation, but the exact differences between STRO-1+ cells and MSCs are not clear.
Clinical use
The mesenchymal stem cells can be activated and
mobilizedCell mobility generally refers to motility, but may also refer to other ways of activation, such as cell differentiation and cell proliferation....
if needed. However, the efficiency is very low. For instance, damage to muscles heals very slowly. However, if there were a method of activating the mesenchymal stem cells then such wounds would heal much faster.
Direct injection or placement of cells into a site in need of repair may the preferred method of treatment, as vascular delivery suffers from a "pulmonary first pass effect" where intravenous injected cells are sequestered in the lungs. Clinical case reports in orthopedic applications have been published, though the number of patients treated is small and these methods still lack rigorous study demonstrating effectiveness. Wakitani has published a small case series of nine defects in five knees involving surgical transplantation of mesenchymal stem cells with coverage of the treated chondral defects.