Stromal cells in bone marrow were described by Friedenstein et al.in 1990, as cells that proliferate from colonies and have spindle shape. They candifferentiate into many cell types that could be present in many differenttissues such as: osteoblasts, adipocytes, chondroblasts, ets.

under specific invitro conditions attached to plastic flasks. These cells that are later obtainedfrom postnatal bone marrow are called stromal stem cells or mesenchymal stem cells(MSC). It has been demonstrated that the presence of similar cells in many othertissues as well, despite the extensive attempts to characterize these cells, thedefinitive in vivo markers of MSC are still unknown.

Although, the demonstrationof functional data that support that common adult stem cells exist and have theability to differentiate with several specific differentiation lineages.Because MSC could be easily isolatedfrom bone marrow and could be prolonged in vitro, they became a primetarget for tissue regeneration for researchers. They are now broadly used in manyexperiments and transplantation in animals and some human therapeutic trials. But,more new research is required to learn more about the molecular mechanisms ofthe differentiation of MSC to estimate the full capacity for tissue regeneration(Vaananen, H.

; 2005).  Based on their ability for immune response suppression, they areused to study regenerative medicine. They have paracrine effects which includesimmunomodulation occurring by the secretion of soluble mediators, that includeshuman leukocyte antigen G5, transforming growth factor-beta and nitric oxide. Inbone marrow, MSCs regulate immunological memory by the recognition of defined amountsof specialized niches for memory T cells and plasma cells in the bone marrowand are contacting with T and B cells to do so. All stromal cells probablyshare all of these biological effects, which includes stem cells that are isolatedfrom exfoliated deciduous teeth and fibroblasts. (Yamaza T.

et al.; 2010).

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