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uni'wissen 02-2013_ENG

Pluripotent stem cells are all-rounders. They have the potential to develop into any type of body cell. The hopes that have been pinned on the use of stem cells for medical purposes are gigantic. Among other things, doctors want to use them to improve their understanding of the development of cancer, to replace dead tissue after events such as a heart attack, or to repair nerve fibers after a spinal cord injury. Until 2006, however, scientists could only obtain stem cells by removing them from embryos, a process that entails ethical challenges, particularly for re- search on humans. In 2012, the Japanese professor of medicine Shinya Yamanaka received the Nobel Prize in Medicine for his discovery of the technique for converting specialized body cells back into pluripotent cells. The research on so-called in- duced – artificially reprogrammed – pluripotent stem cells is considered to be the fastest-grow- ing field of research in biology. In creating plu- ripotent stem cells in accordance with Yamanaka’s technique, scientists used viruses to introduce a cocktail of genes to the newly start- ed cell, including the transcription factor Oct-4. Preparing Cells for Graduation Scientists are not yet certain what causes the specialization of the cell to be erased, changing it back into a stem cell. Prof. Dr. Wolfgang Driev- er and Dr. Daria Onichtchouk from the Cluster of Excellence BIOSS Centre for Biological Signal- ling Studies and the Department of Developmen- tal Biology of the University of Freiburg and their team have uncovered a further piece of this com- plicated puzzle – with the help of the model or- “There is a very brief transitional phase in which the cells have the potential to become anything at all” Lined up like on a pearl necklace: Zebrafish eggs are over one half of a millimeter large and easy to see under the microscope. Photo: Peter Mesenholl ganism zebrafish. The researchers demonstrate in an article in the journal Science that Pou5f1, the counterpart of the gene Oct-4 in zebrafish, serves not only to reprogram a specialized cell and convert it back into a pluripotent stem cell; in addition, Pou5f1 also switches on the embryo’s own genes at a very early stage in development. This stage is called the “maternal to zygotic tran- sition.” The zygote is the first cell of a new organism, created through the fusion of an egg with a sperm cell. The development of the zygote is controlled initially by genes given to it by the egg cell. The maternal to zygotic transition is the pro- cess by which the maternal genes are separated from the embryo’s own genes. In humans this occurs after the first cell division in the two-cell stage, while in fish it does not happen until the embryo has reached a size of around 1000 cells. Afterwards, the cells in the embryo are pluripo- tent for a brief developmental stage. The way this mechanism functions in the ze- brafish is very similar to the way it functions in humans, which points to a common heritage in the distant evolutionary past. “The zebrafish is an ideal model organism,” says Onichtchouk. 13