These small RNAs bind to mRNA and either inhibit translation or cause degradation of transcripts. A more efficient method was found to be the combination of lentiviruses and micro RNAs (miRNAs) to reprogram cells (Anokye-Danso et al. Viral-mediated introduction of transcription factors is very inefficient. 2012), have been used as alternative vectors to transduce transcription factors because they do not integrate into the genomic DNA of the cell. 2008) and Sendai virus, an RNA virus (Seki et al. Depending on the integration site, integration can have deleterious effects on the cells, altering gene expression and increasing the risk of tumor formation. A major consideration for using retroviruses to generate iPSCs is that the viruses integrate into the host DNA. However, it was demonstrated that silencing was often incomplete and viral genes could still be expressed. Silencing is an epigenetic process that suppresses transcription. Previous studies of retroviral infection of embryonic cells had suggested that retroviruses are silenced in these cells. However, there are concerns with using these viral vectors. Most iPSCs have been generated using retroviral and lentiviral vectors to introduce transcription factors into stem cells. Various combinations of these transcription factors have since been used by other investigators. IPSCs were first generated by the introduction of the transcription factors Oct3/4, Sox2, Klf4, and c-Myc in cells maintained in culture conditions used for ESC (Takahashi and Yamanaka 2006). Practically, it has been found that some are relatively straightforward and others are more technically challenging. Theoretically, any somatic cell could be reprogrammed. Induced pluripotent stem cells (iPSCs) are somatic cells that have been reprogrammed to become pluripotent. Sources of somatic stem cells in the human body. It has been estimated that 1 g of adipose tissue yields 5,000 MSCs, whereas bone marrow aspirate contains 100–1,000 MSCs per ml (Strem et al. Adipose tissue is becoming an important source of MSCs because adipocytes are easily accessible and present in relatively high numbers in the body. Mesenchymal stem cells (MSCs) were originally isolated from bone marrow stroma but were subsequently found to be present in most tissues of the body including cord blood, adipose tissue, skin, and periodontal ligaments, which attach teeth to the jaw. Prior to apheresis, the donor is injected with granulocyte-colony stimulating factor to mobilize stem cells from the bone marrow. HSCs for bone marrow transplantation are collected either directly from bone marrow or by apheresis, the removal of white blood cells from peripheral blood. For example, cardiac stem cells are quite rare, while hematopoietic stem cells (HSCs) occur in high enough numbers that they are routinely isolated and used in bone marrow transplantation. The number of cells that can be isolated varies greatly depending upon the cell and tissue type. Positive and negative sorting for cell surface markers can quickly generate enriched populations. Many isolation and purification protocols involve flow cytometry and cell sorting. The methods of isolation and culture are dependent on the source and lineage. Somatic (adult) stem cells are found in most major organs and tissues, and are currently being isolated from many tissues in the body.
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