Female embryonic stem cells differentiated for 2 days. Red: X-chromosomes, green: X-inactivation centers (Xics)

Forty-five years ago, British scientist Mary Lyon already described this chromosome inactivation typical of female cells. Lyon proposed a hypothesis: With two copies of the X chromosome, all X-linked genes are present in two copies. However, in a male organism, which is equipped with a set of one X and one Y chromosome, the X genes are present in only one copy in each cell. To restore genetic balance, a female cell inactivates one of its two X chromosomes.

During development of a female embryo, inactivation of either of the X chromosomes, the one inherited from the father or the one inherited from the mother, occurs at random. To coordinate inactivation, the cell first needs to determine whether it contains more than one X chromosome and then make a choice which of the two to switch off. Since the mid-1980s it has been known that a specific region of the X chromosome termed X inactivation center (Xic) is crucial for a correct inactivation process.

Professor Dr. Roland Eils, who leads the bioinformatics departments at the German Cancer Research Center and at the Institute of Pharmacy and Molecular Biotechnology of Heidelberg University, suspected that the spatial arrangement of the Xics within the nucleus is key to inactivation. Working together with colleagues of the Curie Institute, Paris, he searched different cells for distinctive features in the distribution of Xic regions. The scientists compared developing female embryonic stem cells of mice just before X inactivation, with mouse cells in which X inactivation had already taken place. Using a 3-dimensional visualization of fluorescent labels of the Xic regions, they observed that the Xics of both X chromosomes in the developing stem cells were located very close to each other in up to 15 percent of cells. In the comparative cell line, this was found in only about three percent of cells, which constitutes a random result. The formation of pairs (co-localization) was particularly noticeable in the stem cells after about one and a half days of development, i.e. shortly before X inactivation.

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Part of the plasma chamber of Wendelstein 7-X: Twenty of these elements are joined to form a ring shaped in keeping with the twisting contours of the plasma. The ports in the vessel grant access for heating and measuring facilities. (Photo: IPP, W. Filser)

The aim of fusion research is to derive energy from fusion of atomic nuclei like the sun. To ignite the fusion fire the fuel in a future power plant, a hydrogen plasma, has to be confined in magnetic fields and heated to temperatures of over 100 million degrees. Wendelstein 7-X, which on completion will be the biggest fusion device of the stellarator type in the world, is aimed at investigating the suitability of this concept for application in power plants.

The plasma vessel was produced in 20 sections by MAN DWE GmbH at Deggendorf, Germany, four of which are already in use at IPP for installing the experiment. The assembled ring-shaped plasma chamber with a diameter of about 12 metres will later confine the plasma heated to 100 million degrees. The shape of the vessel is matched to the twisting plasma ring. This peculiar shape in conjunction with the high precision required constitutes a major challenge for production: tolerances of less than 3 millimetres being called for in some cases.

In order to fashion the bizarre shape from steel, the 35-ton vessel was assembled from 200 individual rings. Each ring, in turn, is composed of several finger-thick steel strips 18 centimetres wide which are multiply bent in keeping with the twisting contours. The more than 800 individual components of the vessel are joined vacuum-proof by more than 1,600 metres of brazing. For this purpose the components to be connected were placed in exactly defined positions two millimetres apart, the gaps being closed by hand with several adjacent and superposed brazed layers of wire – a total of several kilometres of brazing wire being used in the process.

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