© 2006 Nature Publishing Group NATURE|Vol 439|12 January 2006 NEWS & VIEWS145spatial and temporal characteristics of mag-netic reconnection will fuel the sometimesheated debate over what the phenomenon is like and what it can do. With the launch of NASA’s STEREO mission, expected later this year, larger baselines will become available, and there is hope that the study ofsolar-wind reconnection can be extended tomuch larger scales. At the smallest scales,NASA’s Magnetospheric Multi-Scale (MMS)mission, to be launched in 2013, will investi-gate the kinetic plasma processes near the X-line that allow the frozen field condition to be broken and reconnection to occur. The prospects for finally understanding thenature of reconnection, its ability to couplesmall- to large-scale phenomena, and the crucial role it plays in various cosmic settings,are excellent. ■Götz Paschmann is at the Max-Planck-Institut für extraterrestrische Physik, 85748 Garching,Germany.e-mail: [email protected]. Phan, T. D. et al. Nature 439, 175–178 (2006).2. Dungey, J. W. Phys. Rev. Lett. 6, 47–48 (1961).Figure 1 | The ins and outs of magneticreconnection. a, Magnetic field lines aregenerally frozen into the plasma flow (blue arrows),so two charged particles, A and B, connected by afield line at time t1remain connected by the samefield line at all later times. b,Two oppositelydirected field lines, identified by particles A, Band C, D, respectively, are moving towards eachother at time t1. When they touch at time t2, theybreak and cross-link (reconnect) at the so-calledX-point, leaving A, C and B, D, connected at timet3. The highly bent field lines act like a slingshot,and plasma flows out from the region at highspeeds. c, A perspective view of magnetic fieldlines reconnecting along an X-line (plasma inflow,green arrows; high-speed plasma outflow, redarrows). The times t1to t3refer to the same phasesof the process as in b. Phan et al.1investigate thelength of the X-line in the solar wind. ABt1ABt2ABt3t1ACDBt3ACDBt2ACDBX-pointt1t1t2t2t3X-lineabcMEDICINE Politic stem cellsIrving L. WeissmanResearch on embryonic stem cells holds huge promise for understandingand treating disease. Many people oppose such research on religious andethical grounds, but two new methods may bypass some of these objections.In this issue are two new methods1,2for pro-ducing pluripotent stem-cell lines — the greatfuture hope of regenerative medicine*. Bothpapers report proof-of-principle tests in miceof techniques that might be used for makinghuman pluripotent stem-cell lines. The proto-cols each aim to satisfy the religious, ethicaland/or political objections of groups that areopposed to some of the methods used inembryonic stem-cell research. Pluripotent stem-cell lines come from themost primitive cells in vertebrate develop-ment. They are prized because they can bothrenew themselves continuously in culture and,once released from this self-renewal cycle, can go on to form most mature cell types in the body (hence ‘pluripotent’, meaning manypotentials). Their ability to make a range of functional cell types makes them crucial tothe study of tissue development and degener-ative diseases, and they are considered to bepromising as a possible treatment for such disorders.Pluripotent stem-cell lines can be derivedfrom early embryos before they implant in theuterus (Fig. 1a, overleaf). These cells are calledembryonic stem cells (or ES cells). The preim-plantation embryo (a blastocyst) has an outershell of cells used for uterus implantation (thetrophectoderm) and an inner cell mass ofpluripotent cells that will give rise to the devel-oping embryo once it has implanted. To createES cell lines, cells from the inner mass areremoved and cultured, but this process meansthat the embryo cannot implant in the uterus.To get around this, Lanza and colleagues1(page 216) have adapted a method commonlyused in assisted-reproduction clinics forpreimplantation genetic diagnosis. Thisinvolves removing a cell from the eight-cellstage of development (before the blastocysthas formed) (Fig. 1b); this ‘blastomere’ cell isthen analysed for genetic defects. Instead,Lanza and colleagues1use the blastomere cellto produce ES cell lines — without compro-mising the embryo from which the blastomerewas obtained. The single blastomeres are co-cultured with established ES cell lines, andthen separated from them to form fully com-petent ES cell lines. The ES cells produced using Lanza and colleagues’ technique would have the samegenes as the embryo, essentially a mix from thetwo parents undergoing in vitro fertiliza-tion treatment. However, the goal for manyresearchers is to be able to produce pluripotentcells that represent the full genetic diversity ofhumans, or that are genetically identical to aparticular donor (a patient with a genetic dis-order, for example). The production of suchstem-cell lines would enable the study of thecellular and genetic bases of disease develop-ment3. For example, stem-cell lines generatedfrom mice that are immunodeficient becauseof a defect in a single gene are themselvesimmunodeficient, and this might hold true forcomplex multigene disorders such as amyo-trophic lateral sclerosis. These lines might alsobe ‘fixed’ in culture by replacing the defectivegene with healthy copies, and thereby onecould validate the role of particular drug targets or the efficacy of certain therapies. Inthe long term, healthy cells derived fromrepaired stem cells might aid the regenerationof tissues from the donor patient. At present, creating pluripotent cells from aspecified donor can only be achieved by aprocess called nuclear transfer (NT) (Fig. 1c).This involves removing the nucleus from adonor body cell (say, a skin cell) and injectingit into an egg that has had its own chromosomesremoved. The egg is then encouraged to forman embryo-like, or embryoid, blastocyst, during which process the body-cell nucleusundergoes ‘reprogramming’, changing fromexpressing skin genes to expressing more*The two papers concerned1,2and this article were publishedonline on 16 October 2005. Since then, ref. 4 by W. S. Hwanget al. has been brought into question, and its authors haverequested retraction of the paper.3. Petschek, H. E. in AAS–NASA Symp. Physics of Solar Flares(ed. Hess, W. N.) 425–439 (NASA, Washington DC,1964).4. Sonnerup, B. U. Ö. J. Plasma Phys. 4, 161–174 (1970).5. Paschmann, G. et al. Nature 282, 243–246 (1979).6.