Mesenchymal stem cells, or MSCs, are multipotent stromal cells that can differentiate into a variety of cell types,[1] including: osteoblasts (bone cells),[2]chondrocytes (cartilage cells),[3]myocytes (muscle cells)[4] and adipocytes (fat cells). This phenomenon has been documented in specific cells and tissues in living animals and their counterparts growing in tissue culture.
While the terms mesenchymal stem cell and marrow stromal cell have been used interchangeably, neither term is sufficiently descriptive:
In 1924, Russian-born morphologist Alexander A. Maximow used extensive histological findings to identify a singular type of precursor cell within mesenchyme that develops into different types of blood cells.[9]
Scientists Ernest A. McCulloch and James E. Till first revealed the clonal nature of marrow cells in the 1960s.[10][11] An ex vivo assay for examining the clonogenic potential of multipotent marrow cells was later reported in the 1970s by Friedenstein and colleagues.[12][13] In this assay system, stromal cells were referred to as colony-forming unit-fibroblasts (CFU-f).
The first clinical trials of MSCs were completed in 1995 when a group of 15 patients were injected with cultured MSCs to test the safety of the treatment. Since then, over 200 clinical trials have been started. However, most are still in the safety stage of testing.[7]
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 acid, inorganic phosphate and dexamethasone could promote their differentiation into osteoblasts. In contrast, the addition of transforming growth factor-beta (TGF-b) could induce chondrogenic markers.[citation needed]
The youngest, most primitive MSCs can be obtained from umbilical cord tissue, namely Wharton's jelly and the umbilical cord blood. However MSCs are found in much higher concentration in the Whartons jelly compared to cord blood, which is a rich source of hematopoietic stem cells. The umbilical cord is easily obtained after a birth. It is normally thrown away and poses no risk for collection. The cord MSCs have more primitive properties than other adult MSCs obtained later in life, which might make them a useful source of MSCs for clinical applications.
A rich source for mesenchymal stem cells is the developing tooth bud of the mandibular third molar. While considered multipotent, they may prove to be pluripotent. They eventually form enamel, dentin, blood vessels, dental pulp and nervous tissues, a minimum of 24 other different unique end organs. Because of ease in collection at 810 years of age before calcification and minimal-to-no-morbidity, they probably constitute a major source for research and multiple therapies. These stem cells have been shown capable of producing hepatocytes.
Additionally, amniotic fluid has been shown to be a rich source of stem cells. As many as 1 in 100 cells collected during amniocentesis has been shown to be a pluripotent mesenchymal stem cell.[14]
Adipose tissue is one of the richest sources of MSCs. There are more than 500 times more stem cells in 1 gram of fat than in 1 gram of aspirated bone marrow.[citation needed] Adipose stem cells are actively being researched in clinical trials for treatment of a variety of diseases.
The presence of MSCs in peripheral blood has been controversial. A few groups have successfully isolated MSCs from human peripheral blood and been able to expand them in culture.[15] Australian company Cynata claims the ability to mass-produce MSCs from induced pluripotent stem cells obtained from blood cells.[16][17]
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 nucleus with a prominent nucleolus, which is surrounded by finely dispersed chromatin particles, giving the nucleus a clear appearance. The remainder of the cell body contains a small amount of Golgi apparatus, rough endoplasmic reticulum, mitochondria and polyribosomes. The cells, which are long and thin, are widely dispersed and the adjacent extracellular matrix is populated by a few reticular fibrils but is devoid of the other types of collagen fibrils.[18][19]
The International Society for Cellular Therapy (ISCT) has proposed a set of standards to define MSCs. A cell can be classified as an MSC if it shows plastic adherent properties under normal culture conditions and has a fibroblast-like morphology. In fact, some argue that MSCs and fibroblasts are functionally identical.[20] Furthermore, MSCs can undergo osteogenic, adipogenic and chondrogenic differentiation ex-vivo. The cultured MSCs also express on their surface CD73, CD90 and CD105, while lacking the expression of CD11b, CD14, CD19, CD34, CD45, CD79a and HLA-DR surface markers.[21]
MSCs have a great 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 neurons. MSCs have been seen to even differentiate into neuron-like cells,[22] but there is lingering doubt whether the MSC-derived neurons are functional.[23] The degree to which the culture will differentiate varies among individuals and how differentiation is induced, e.g., chemical vs. mechanical;[24] 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 proliferate and differentiate 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.[citation needed]
Numerous studies have demonstrated that human MSCs avoid allorecognition, interfere with dendritic cell and T-cell function and generate a local immunosuppressive microenvironment by secreting cytokines.[25] 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-gamma levels.[26] 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.[27]
The majority of modern culture techniques still take a colony-forming unit-fibroblasts (CFU-F) approach, where raw unpurified bone marrow or ficoll-purified bone marrow Mononuclear cell are plated directly into cell culture 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.[28]
Other flow cytometry-based methods allow the sorting of bone marrow cells for specific surface markers, such as STRO-1.[29] 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.[30]
Methods of immunodepletion using such techniques as MACS have also been used in the negative selection of MSCs.[31]
The supplementation of basal media with fetal bovine serum or human platelet lysate is common in MSC culture. Prior to the use of platelet lysates for MSC culture, the pathogen inactivation process is recommended to prevent pathogen transmission.[32]
Mesenchymal stem cells in the body can be activated and mobilized if needed. However, the efficiency is low. For instance, damage to muscles heals very slowly but further study into mechanisms of MSC action may provide avenues for increasing their capacity for tissue repair.[33][34]
A statistical-based analysis of MSC therapy for osteo-diseases (e.g., osteoarthritis) noted that most studies are still underway.[35] Wakitani published a small case series of nine defects in five knees involving surgical transplantation of MSCs with coverage of the treated chondral defects.[36]
At least 218 clinical trials investigating the efficacy of mesenchymal stem cells in treating diseases have been initiated - many of which study autoimmune diseases.[37] Promising results have been shown in conditions such as graft versus host disease, Crohn's disease, multiple sclerosis, systemic lupus erythematosus and systemic sclerosis.[38] While their anti-inflammatory/immunomodulatory effects appear to greatly ameliorate autoimmune disease severity, the durability of these effects are unclear.
However, it is becoming more accepted that diseases involving peripheral tissues, such as inflammatory bowel disease, may be better treated with methods that increase the local concentration of cells.[39]
Many of the early clinical successes using intravenous transplantation came in systemic diseases such as graft versus host disease and sepsis. Direct injection or placement of cells into a site in need of repair may be the preferred method of treatment, as vascular delivery suffers from a "pulmonary first pass effect" where intravenous injected cells are sequestered in the lungs.[40] Clinical case reports in orthopedic applications have been published, though the number of patients treated is small and these methods still lack demonstrated effectiveness.
Scientists have reported that MSCs when transfused immediately a few hours post thawing may show reduced function or show decreased efficacy in treating diseases as compared to those MSCs which are in log phase of cell growth, so cryopreserved MSCs should be brought back into log phase of cell growth in in vitro culture before these are administered for clinical trials or experimental therapies, re-culturing of MSCs will help in recovering from the shock the cells get during freezing and thawing. Various clinical trials on MSCs have failed which used cryopreserved product immediately post thaw as compared to those clinical trials which used fresh MSCs.[41]
Mesenchymal stem cells have been shown to contribute to cancer progression in a number of different cancers, particularly the hematological malignancies because they contact the transformed blood cells in the bone marrow.[42]
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