Mesenchymal stem cells and it ' s Characterization

Although acceptable for cell based therapeutic applications, a rigorous understanding of the Mesenchymal Stem Cells (MSC) requires a better definition of what an MSC is. Inspite of the fact that no unique markers are known for MSCs, many attempts have been made to develop a cell-surface antigen profile for the better purification and identification of MSCs, particularly important is whether MSCs isolated from different tissues are identifiable by the same immunophenotype. While the precise identity of MSCs remains a challenge, further understanding of their biological properties will be greatly advanced by analyzing the mechanisms that govern their differentiation potential. The ability of MSCs to differentiate into a variety of connective tissue cell types has rendered them an ideal candidate cell source for clinical tissue regeneration strategies. This review focuses specifically in the context of these advances in characterization of adult stem cells via selection of unique cell surface markers and regulation of lineagespecific differentiation of mesenchymal stem cells.


Introduction
hematopoietic cells by their rapid adherence to tissue culture vessels and by the fibroblast-like appearance Mesenchymal stem cells or multipotent stromal of their progeny in culture, pointing to their origin cells (MSCs) are adult stem cells which have been from the stromal compartment of BM.In addition to isolated from almost every type of connective tissue establishing BM stroma as the haystack in which to (da Silva Meirelles et al., 2006).They can self-renew search for the proverbial needle, the work of and under appropriate in vitro conditions have the Friedenstein and coworkers provided a second major capacity to differentiate into mesodermal, endodermal, breakthrough by showing that seeding of BM cell and even ectodermal lineages (Lakshmipathy and suspensions at clonal density results in the Verfaillie, 2005).They evoke only minimal immunoestablishment of discrete colonies initiated by single reactivity (Tse et al., 2003) and secrete bioactive cells (the colony-forming unit fibroblastic, CFU-Fs factors with anti-inflammatory and immuno- (Friedenstein et al., 1966).Combined with the timing modulatory effects in vivo (Caplan and Dennis, 2006). of the isolation of human embryonic stem (ES) cells, Hence, MSCs appear to have a remarkable clinical the term mesenchymal stem cell (MSC), proposed potential in tissue regeneration and immunoregulatory previously as an alternative to ''stromal'' or therapeutic applications.
''osteogenic'' stem cell, gained wide popularity.To It was Friedenstein and coworkers, in a series of date, the best characterized source of MSCs is the seminal studies in the 1960s and 1970s, who bone marrow (BM) and most of the knowledge demonstrated that the osteogenic potential, as revealed regarding MSCs is based on BM studies.by heterotopic transplantation of BM cells, was MSC have a great appeal for cell therapy and associated with a minor subpopulation of BM cells.tissue engineering for numerous reasons: These cells were distinguishable from the majority of 1) They are relatively easy to procure.
2) They expand rapidly in culture.
therefore is that all steps of MSC manufacturing from 3) They show only minor spontaneous differenti-starting material up to potency testing in the intended ation during ex vivo expansion.indications have to be highly standardized to assure a 4) They are multi potential.
required and reproducible cellular quality and 5) They form supportive stroma for hematopoiesis.potency.6) They seem to be largely immunologically inert, Surface markers of MSC paving the way for allogenic and xenogenic Coating the surface of every cell in the body are transplantation.
specialized proteins, called receptors that have the 7) They are immunosuppressive.capability of selectively binding or adhering to other 8) They secrete numerous trophic factors which "signaling" molecules.There are many different types modulate inflammation, remodeling and of receptors that differ in their structure and affinity for apoptosis.
the signaling molecules.Normally, cells use these In contrast to other cell types that express receptors and the molecules that bind to them as a way specific cell surface markers such as hematopoietic of communicating with other cells and to carry out cells (CD14, CD34, CD45) (Huss, 2000) and endothelial their proper functions in the body.These same cell cells (CD31) (Hristov et al.,2003) ).Fluorescence-activated cell sorting fibroblasts.Furthermore MSC's are plastic adherent of human bone marrow mononuclear cells showed and should in vitro exert at least tri-lineage the expression of CD49b, CD105, CD90, CD73, differentiation potential, ie.towards the adipogenic, CD130, CD146, CD200, and V/5 integrin and the osteogenic and chondrogenic lineages.In addition expression of CD73, CD146.But CD200 was found to MSC may trans-differentiate in to other lineages as be down regulated during differentiation (Gang et al.,

well. Any variation in isolation protocols and 2007). characterization strategies may result in the isolation
Negative markers: There is a consensus that MSCs and expansion of different sub-population of cells, or do not express CD11b (an immune cell marker) may change the characteristics of the cells.Given that (Pittenger et al.1999), glycophorin-A (an erythroid MSC used in clinical trials are produced using a lineage marker) and CD45 (a marker of all variety of different protocols, the results may not be hematopoietic cells) (Colter et al., 2001).CD34, the interpretable or reproducible.An essential requirement primitive hematopoietic stem cell (HSC) marker, is potential role of MSCs in blood vessel formation rarely expressed in MSCs, although it is positive in has also been evaluated.Enhanced neovascularimice (Honczarenko et al., 2006) advantage of their immune privileges, MSCs promise Adipogenic differentiation of MSCs is induced by the nuclear receptor and transcription factor, peroxisome tremendous therapeutic potential.proliferator-activated receptor-gamma (PPAR-) as Choice of stem cells for therapy well as fatty acid synthetase.
5-Azacytidine induction of myogenesis was Both embryonic and adult stem cells have its reported by Taylor and Jones for embryonic and adult own pros and cons.Embryonic stem cells are cells (Taylor & Jones, 1982) and by Wakitani et al. for pluiripotent (blastocyst stage embryos) or totipotent rat stromal cells (Wakitani, et al., 1995).Phinney et (early stage embryos) in nature and contains the al.1999 found that exposure of mouse MSCs to genetic blue print of each and every type of body cells, amphotericin B, but not 5-azacytidine, resulted in so as margin of safety will be more while going for the formation of multinucleated fibers resembling therapeutics, but cannot say impeccable as it's proneness myotubes (Phinney and Prockop, 2007).Treatment of to immune rejection and teratoma formation (Verma O MSCs with isobutylmethylxanthine and dibutyryl P., 2010).Cuncurrenlty mesenchymal stem cells are cyclic AMP induced expression of early markers of easy to culture and large number of cells will be neuronal differentiation (Deng et al., 2001).The available for therapeutics purpose even though they . CD31 (expressed on genesis has been associated with regeneration of endothelial and hematopoietic cells) (Pittenger et infracted myocardium by bone marrow derived al.1999) and CD117 (a hematopoietic stem/progenitor stem cells (Orlic et al., 2001).cell marker) (da Silva Meirelles et al., 2006) are Immune privileged properties of Mesenalmost always absent from MSCs.Currently, the thorn chymal stem cells in the side of the MSC biologist is the lack of a Stem cells are envisioned to be a major source definitive positive marker for MSCs; there is a myriad of reported positive markers, with each research group for cell based therapies.Survival of transplanted cells using a different subset of markers.Without a correlates with the number of differences in major definitive marker, in vivo studies on cell lineage and histocompatibility (MHC) antigens between donor and recipient that trigger T-cell responses and rejection niche are difficult.of cells with disparate MHC profiles (Janeway et al., Multilineage Differentiation Potential 1999).The expression of MHC antigens on tissues The differentiation potential of MSCs into determines the outcome of alloantigen-specific T-cell bone, cartilage and fat has been described and responses in vitro and in vivo (Janeway et al., 1999).characterized by multiple laboratories (Barry et Although most mammalian cells express MHC class I al.,2001; Muraglia et al., 2000; Pittenger et al.,1999).antigens, expression of MHC class II molecules Osteogenic activation requires the presence of glycerol are more restricted (Viret and Janeway, 1999).phosphate, ascorbic acid-2-phosphate, dexamethasone Recently, unique immunologic properties of MSCs have been described, such as their poor and fetal bovine serum .When cultured in monolayer immunogenicity in vitro and in vivo (Ryan et al., in the presence of these supplements the cells acquire 2005), inhibition of the proliferation and cytotoxicity an osteoblastic morphology with upregulation of of natural killer (NK) cells (Spaggiari et al., 2008), alkaline phosphatase activity and deposition of a inhibit the function of mature, naive and memory T-calcium rich mineralized extracellular matrix.cells whereas increase the proportion of regulatory T-Chondrogenic differentiation occurs when cells (di Nicola et al., 2002), and suppression of B cell MSCs are grown under conditions that include (1) proliferation and differentiation (Corcione et al., a three-dimensional culture format, (2) a serum-2006).In addition MSC have been shown to be free nutrient medium and (3) the addition of a decreasing the tumor necrosis factor secretion and member of the TGF-super family .When these conditions increase IL-10 secretion from dendritic cells (DC) , are met the cells rapidly lose their fibroblastic block the antigen-presenting cells (APC) maturation morphology and begin to initiate expression of a and these immature APC, in turn, secrete immuno-number of cartilage specific extracellular matrix suppressive cytokine IL-10 ( Oh et al.,2008).To date, components.MSCs cultured in monolayer in the the mechanism of MSC-mediated immuno-presence of isobutylmethylxanthine become adipocytes suppression has not been fully explained.Taking with the production of large lipid-filled vacuoles .
(Huss , 2000)pic surface receptors are the stem cell markers.identify of MSCs is not unique, sharing features of Positive markers: As part of the minimal criteria multiple cell lineages, including mesenchymal, proposed by the Mesenchymal and Tissue Stem hematopoietic, endothelial, epithelial, and muscle Cell Committee of the International Society for cells (Majumdar et al.,2000).In addition, efforts to Cellular Therapy to define MSCs (Dominici et characterize phenotypic features of MSCs have been al.,2006), cells must be positive for CD105, CD73, confounded by the fact that MSCs display a variety of and CD90 and negative for CD45, CD34, CD14 morphological characteristics and express various cell or CD11b, CD79a or CD19, and HLA-DR.The lineage-specific antigens that can vary between identification of a definitive marker allowing the different preparations and as a function of time in prospective isolation of MSCs from fresh tissue would culture(Huss , 2000).
be of the utmost importance.Stro-1 is by far the best-International Society for Cellular Therapies known MSC marker is not exclusive to these cells and(ISCT)is lost during culture(Kolf et al., 2007), so that it is not a general MSC marker.The cell population negative