results can be confounded in many ways, sources of human stem cells that can be cultured in vitro are perhaps the most critical need of investigators. They will permit many more questions to be posed and answered. Many more experiments can be completed with cultured cells in the same amount of time and with the same degree of effort as in living organisms. Moreover, data from in vitro studies allow more insightful and better-defined experiments to be developed in living organisms. Access to ESCs is likely to ultimately determine the rate at which scientists make progress in this field. In fact, the successful cultivation of postnatal and adult sources of stem cells for regenerative medicine is likely to advance more rapidly if the study of ESCs proceeds and cells from different sources can be compared. ESCs exhibit many properties whose improved understanding could assist researchers in modifying adult stem cells to achieve better growth in culture and greater capacity for controlled differentiation.

USE OF EMBRYOS FOR STEM CELL RESEARCH IS CONTROVERSIAL

A second major obstacle to the development of new medical therapies based on stem cells is opposition to ESC research on ethical, moral, or religious grounds. No field of biological science has been more controversial than that involving human reproduction. Contraception, abortion, and in vitro fertilization have all provoked major debate and controversy in this country and abroad. Stem cell research also touches on some of the most fundamental issues with which society has grappled over the centuries, including the definition of human life and the moral and legal status of the human embryo.

The workshop provided an opportunity for the committee to hear both from those who support ESC research and from those who oppose it on ethical grounds. The various speakers on the workshop panel articulated the main ethical arguments, which are summarized below. The committee acknowledges the importance and value of a dialogue that respects the many differing perspectives. Although it is not within



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