![]() 7 Expression of high levels of telomerase explains their immortality in culture.ĮSC research focuses mainly on two issues, both of which have shown significant progress in the past few years. Molecular characterization of ESCs is well developed, and they are known to express surface markers such as CD9, CD24, and alkaline phosphatase, and several genes involved with pluripotency, including Oct-4, Rex-1, SOX-2, Nanog, LIN28, Thy-1, and SSEA-3 and -4. However, ethical considerations still remain as it has to be tested whether the remaining cells can develop into a normal human being.Ĭultured ESCs show defined characteristics: they are pluripotent, capable of differentiating into cells derived from all three germ layers they are immortal in culture and may be maintained for several hundred passages in the undifferentiated state and they maintain a normal chromosomal composition. An alternative method involves the production of ESCs by collection of only one cell from the inner cell mass, allowing implantation of the remaining cells in the womb. The process of establishing an ESC line requires, however, the destruction of the blastocyst, raising ethical issues as scientific investigation alone is not capable of determining whether blastocysts constitute human beings. It is estimated that there are currently around 250 human ESC lines in the world, widely shared among different groups. 6 Once established, ESC lines may be maintained in permanent culture, frozen and thawed, and transported between laboratories. This is particularly important for human ESCs. 5 Although seemingly simple, the procedure is technically demanding because of the need for strictly controlled conditions necessary for the maintenance of the cells in the undifferentiated state. In 1981, two groups established the first ESC lines from mouse blastocysts, and in 1998 the first human ESC line was generated. These are capable of differentiating into any of the mature cell types present in the adult organism. Embryonic stem cells are immortal in culture, having been established from one pluripotent cell collected from the inner cell mass. In the blastocyst, the trophoblast at its periphery generates the embryonic membranes and placenta, whereas the inner cell mass develops into the fetus. The zygote undergoes successive mitotic divisions until a sphere of cells-the blastocyst-is formed.
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