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Hold the champagne

发布时间:2019-03-08 10:01:04来源:未知点击:

By Phillip Cohen IT HAS been hailed as a medical revolution. Two teams have developed methods of culturing human embryonic stem cells, which they hope will lead to ways of growing tissues or even organs for transplants. But much needs to be done to prove that their cultures have any therapeutic value—and that tissue grown in this way would be safe. Embryonic stem cells can differentiate to form any of the body’s tissues, such as skin or muscle. But keeping human embryonic stem cells dividing in culture proved difficult. Now that obstacle seems to have been overcome. “It’s exciting work,” says Mario Capecchi of the University of Utah in Salt Lake City, a pioneer of research on mouse stem cells. “But it opens many questions that aren’t going to be easy to answer.” John Gearhart of Johns Hopkins University in Baltimore isolated his stem cells from aborted fetuses. James Thomson at the University of Wisconsin, Madison, obtained his from embryos created by IVF and grown for about five days until each developed into a hollow ball of cells called a blastocyst. Gearhart revealed early data on his cells last year (This Week, 19 July 1997, p 4). But his paper (Proceedings of the National Academy of Sciences, vol 95, p 13 726) was narrowly beaten into print by Thomson’s (Science, vol 282, p 1145). The two papers are the culmination of years of effort. Thomson and Gearhart eventually succeeded by growing the stem cells over a “feeder” layer of irradiated mouse cells. “Take the feeder cells away, and the stem cells start to differentiate,” says Gearhart. “But we don’t know why.” In mice, embryonic stem cells are used for genetic engineering. After their genes are altered, they are injected into an embryo which develops into a chimera that has both engineered and unaltered cells. Pure engineered mice can be bred from these chimeras. Thomson and Gearhart don’t want to create transgenic people. Their goal is to grow cells and tissues to reinvigorate diseased or ageing bodies, perhaps by combining their methods with cloning. For example, cells could be taken from a person, used to clone a blastocyst and thus obtain stem cells. Tissues grown from stem cells taken from the blastocyst would match that person’s own tissues—eliminating problems with rejection. At least one biotechnology company was already thinking along these lines before Thomson’s paper appeared (This Week, 11 July, p 4). Turning such dreams into reality won’t be easy, however. No one knows how to make a stem cell differentiate to form a specific tissue, says Roger Pedersen of the University of California, San Francisco. “The language is one of growth factors and molecular signals. What we don’t know are the magic words needed to create each tissue.” Gearhart and Thomson can’t even be sure that their cells really can form any tissue. Ethical concerns mean that they can’t perform the acid test: deliberately creating a human chimera. The cells do have many of the right properties, however. They possess high levels of telomerase, an enzyme needed for cells to divide indefinitely. And they can form cells representative of the three main types of tissue: endoderm, mesoderm and ectoderm. But some differentiated cells, such as those from mouse yolk sacs,