Induced-Pluripotent Stem Cells

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The embryonic stem cells, which are pluripotent cells that are derived from the inner cell mass of the pre-implantation embryo at the blastocyst stage. They are considered as an important finding as they have the potential of self-renewal and differentiate indefinitely into the three germ layers, the endoderm, mesoderm and the ectoderm. The human embryonic stem cells were considered to be useful in clinical applications, as they have the potential to be used in cell therapies on diseases like Parkinson’s disease, diabetes and spinal cord injury. However, this research faced a huge ethical difficulty in the use of human embryos, especially in some conservative countries and the religious issue. Also there was the problem on tissue rejection from the transplantation to the patients. In 2006, Yamanaka and his colleagues had discovered that differentiated cells could be reprogrammed back to an embryonic-state like by combining several transcriptional factors. They were able to generate what is now called the induced pluripotent stem cells from either mouse embryonic or the adult fibroblast cultures, which demonstrated huge similarities to embryonic stem cells, such as the morphology and growth properties. The induced-pluripotent stem cells could have do the same job as embryonic stem cells, while avoiding the ethical issues and tissue rejection as they were made from the patient’s own cell. In this essay, we will discuss the production and the concept of induced-pluripotent stem cells, the difference and similarities between the induced-pluripotent stem cells, the possible usage of induced-pluripotent stem cells and the barrier they are facing. Yamanaka and his colleagues in 2006 found that 4 transcriptional factors, Oct3/4, Sox2, c-Myc and Klf4 plays an important role in making the induced-pluripotent stem cells. They made this observation by introducing 24 candidate genes into mouse embryonic fibroblasts from Fbx15ßgeo/ßgeo embryos by retroviral transduction separately. An assay system was set up using G418, whereas induced pluripotent state could lead to resistance to it. Out of the 24 candidate genes, the 4 factors mentioned above stood out and suggested induced-pluripotent stem cells can be induced with the introduction of them. The product produced was shown to be similar to embryonic stem cells in morphology, proliferation and teratoma formation. Yet there were difference between them in the gene expression and DNA methylation. Yamanaka and co conducted further experiments in 2007, replacing the Fbx15 with Nanog, as it is more tightly associated with pluripotency. This result in generating ’higher quality’ induced-pluripotent stem cells, where it is more similar to embryonic cells in morphology, proliferation, teratoma formation, gene expression and competency for adult chimaeras. A comparison between induced-pluripotent stem cells and embryonic stem cells will be discussed in the later part of this essay. In the same experiment, they also discovered that the reactivation of c-myc retrovirus might lead to tumour formation. Yamanaka suggested retrovirus-mediated system might be replaced by transient expression such as adenovirus-mediated system. He also suggested that high-throughput screening of chemical libraries might be used for identifying small molecules to replace the four factors. More recent studies have shown that different approaches to make induced-pluripotent stem cells. The old approach done by Yamanaka requires the four factors delivered by retroviruses, yet experiments have revealed that only Oct3/4 is absolutely required, which is used on the maintenance of embryonic stem cell pluripotency. The others three factors can use alternative factors to replace them. For example Sox1 can replace Sox2, Klf2 or Klf5 can replace Klf4 and c-Myc can be replaced by N-Myc or L-Myc. This gives the suggestion where Sox2, Klf4 and c-Myc are not necessary in induced-pluripotent stem cells generation. In 2009, Kim and...
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