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ePaper created 2014-02-05, 10:56:17 | version 1.26.00
The eggs are over one half of a millimeter large and easy to see under the microscope. Scien- tists can create up to 10,000 eggs in a compara- tively short period of time and collect a great variety of molecular data as they develop syn- chronously. Mammalian zygotes, on the other hand, only complete the transition to the embry- onic stage after they have lodged themselves in the uterus – where it is no longer possible to ob- serve them. “There is a very brief transitional phase in which the cells have the potential to become anything at all,” says Driever. “Extending this moment artificially is like trying to hold a seesaw in perfect balance.” At almost exactly the mo- ment when the pluripotent state is reached, the cells are sent gradually onto the path of special- ization. The seesaw starts swinging. The cells differentiate roughly into three layers: so-called ectodermal cells, which later become part of the skin or the nervous system; mesodermal cells, which form blood or muscles; and entodermal cells, which line the digestive tract. Making Artificial Stem Cells Stable The state of pluripotency is thus not a com- plete lack of purpose but rather a positive poten- tial for the cell: first the potential to become any kind of cell and a moment later the potential to belong to one of the three basic types of cells that make up an organism. “The image of the transcription factor Pou5f1 as an eraser is not quite correct,” says Driever. “What happens is not that everything is erased but that, on the contrary, everything is prepared for something new.” The maternal to zygotic transition is better described as the cell’s high school graduation. On the one hand, a high school diploma qualifies one to enroll in college and major in the field of one’s choice. On the other hand, the advanced placement courses and electives one took on the way to graduation already represent a rough specialization based on one’s talents and inter- ests. “Our experience has been that the regula- tion of pluripotency and the activation of the zygote’s own genes are inextricably entwined with each another,” says Driever. As an example of the way in which Pou5f1 participates in the network of rules governing the “What happens is not that everything is erased but that, on the contrary, everything is prepared for something new” Researchers can use zebrafish to collect a large variety of molecular data: They can create up to 10,000 eggs in a comparatively short period of time and observe them as they develop synchronously. Photos: Wolfgang Driever’s research group 14