Editor's Note: This is the first of two articles on questions raised by recent stem cell discoveries. The second article, focusing on various organs and the nervous system, will appear in the Nov. 27 issue of The Scientist.

Researchers first isolated embryonic stem cells (ESCs) from mouse blastocysts almost 20 years ago, and a paper announcing the discovery of human ESCs emerged in 1998. Adult-derived stem cells (ASCs) have since become the rage in certain quarters of biology, with unexpected--and sometimes downright weird--findings surfacing regularly in the top journals. Last month, a typical paper reported that neural ASCs can be coaxed into differentiating into skeletal muscle cells.1

As long-held notions about biological development are challenged,2 the therapeutic and ethical implications of stem cell work are also generating controversy. Scientific and "cultural" debates promise to sharpen further once reviewers, under National Institutes of Health guidelines issued in September,3...

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