Mesenchymal Stem Cells
10.4 Potential Concerns
Despite the increasing knowledge and characterizations of MSCs, and the mounting enthusiasm for the use of MSCs in regenerative therapies in humans, the mechanisms of MSCs proliferation and dif-ferentiation are still not fully understood. The current lack of reliable methods to control MSC cell fate, especially in the in vivo condition, is one of the main problems that needs to be addressed.
MSCs selection is another issue, due to the lack of a single definitive stem cell marker. Current isola-tion methods can only achieve heterogeneous cell populaisola-tions, and the proporisola-tions of MSC progenitors in enriched MSC populations can vary even when the samples are obtained from the same donor at the same time (Digirolamo et al., 1999). Individual MSC clones derived from the same source showed completely differently cell characteristics, including size, morphology, proliferation, and differentia-tion potentials (Friedenstein et al., 1987, Owen and Friedenstein, 1988). Gronthos et al. (1994) revealed that only 48% of the CFU of fibroblasts clones derived from STRO-1+ hBMSCs showed the capacity to differentiate into adipocytes in vitro. Previous research also indicated that only one-third of the initial adherent bone marrow-derived MSCs clones showed tri-lineage (osteo/adipo/chondro) differentiation potential (Pittenger et al., 1999). A later report demonstrated that only 30% of the BMSC clones had tri-lineage differentiation potential, while the rest exhibited a bi-linage (osteo/chondro) or a uni-lineage (osteo) potential (Muraglia et al., 2000). Even highly purified BMSC populations contained clones with differential capacity to form bone in vivo (Gronthos et al., 2003). Further confounding this issue is the fact that the cell properties of clones generated from a single clone are also variable. Kuznetsove et al.
(1997) reported that bone formation was observed in only 58.8% of the single colony-derived clones transplanted with hydroxyapatite-tricalcium phosphate ceramic scaffolds in vivo. Even the results of current clinical trials proved that the purified cell population is unnecessary to get positive result (Prockop, 2007), obtain the more purified MSCs will help to achieve more reliable result.
Moreover, MSCs tend to spontaneously differentiate into osteoblastic cell, adipocytes, and stromal cells under the current culturing condition (Banfi et al., 2000, Izadpanah et al., 2008). Attempts to maintain the “stemness” of MSCs showed higher possible population doublings. One of the methods is to culture the cells in a condition similar to the in vivo circumstance of stem cell niche, such as culturing cells on fibronectin matrices under low oxygen tension (3%) (Chow et al., 2001a,b). Another method is to cultured the MSCs at low seeding density in low serum (Sekiya et al., 2002). However, no differentiation potential report was included in any of those publications.
Most of the published research of MSCs was performed on heterogeneous cell populations, making it difficult, if not impossible, to determine whether multilineage differentiation potential is a property of a single MSC, versus a variety of progenitor cells. To solve the inconsistencies of MSCs researches, it is necessary to develop standardized protocols for MSCs isolation and expansion. Identified the unique gene will benefit the purification and enrichment of homogenous MSC populations.
Although fetal bovine serum (FBS) is a common supplement for culture medium used to achieve high cell proliferation, the use of FBS and other animal products increases the potential risk of trans-mitting animal diseases. Currently, alternative media for human MSC culture has generated a great deal of enthusiasm, and a variety of serum-free media have been reported (Muller et al., 2006, Lindroos
et al., 2009, Agata et al., 2009). Various substitutes for FBS, such as autologous serum, fresh frozen plasma, and human platelet lysates, have also been tested (Lange et al., 2007, Le Blanc et al., 2007).
The low frequency of MSCs in harvested tissue is another problem. In vivo cell therapy requires large numbers of cells, and insufficient cell numbers will not provide positive outcome (Habisch et al., 2007).
For most of the cases, in vitro expansion is necessary to achieve adequate cells. Previous study proved that BMSCs express telomerase, which maintained telomere length many cell doublings but not thought to be immortal (Morrison et al., 1996). Moreover, some publications indicated that MSCs may lose their multipotentiality after six or seven passages in vitro culture (Colter et al., 2000, Sekiya et al., 2002).
Increase the lifespan of MSCs by viral transduction of human telomerase reverse transcriptase may help to solve the problem.
Tumorigenesis potential of MSCs is much less than ESCs. However, MSCs still show the possibility of cancer initiation. Because those self-renewable stem cells can spontaneously transform in vitro (Rubio et al., 2005), which may cause the formation of malignant cells under the in vivo environment.
10.5 Conclusion
The mechanism behind the special cell properties of these multifunctional cells remains unclear.
The therapeutic potential of MSCs is not fully predictable, especially under the in vivo environment.
However, the self-renewal capacity, plasticity, and tumor-homing ability still make MSCs one of the most promising cell sources for tissue engineering and cell-based therapies. Further studies are neces-sary for better understanding and control of MSCs.
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