It's been six years since human embryonic stem cells appeared on the public radar screen, and we still don't know what to call them. Take the word "embryo," which means different things to different people. In the 1980s, an embryo wasn't an embryo until it 1) had a primitive streak, 2) sported three layers, and 3) could no longer split to yield twins. Then somehow an inner cell mass became a full-fledged embryo, although it is only a mere smear of undifferentiated cells hugging the interior of a blastocyst.

The problem is apples-and-oranges dichotomies and definitions. The "intent" dichotomy distinguishes reproductive from therapeutic efforts, and the "nuclear source" dichotomy introduces DNA from in vitro fertilization (IVF) leftovers or somatic cells, both routed through an inner cell mass. And the "developmental timeframe" dichotomy of embryonic vs. adult makes no biological sense. What do you call a fibroblast, busy pumping out its...

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