from genetically non-identical people, and immunological rejection poses serious clinical risks that can be life-threatening. Overcoming the threat of immunological rejection is thus one of the major scientific challenges to stem cell transplantation and, indeed, for transplantations of any sort. The SCNT technique offers the possibility of deriving stem cells for transplantation from the recipient’s own cells. Such cells would produce only the patient’s own proteins and would not cause an immunological reaction when transplanted into that patient.

The committee is respectfully mindful of the wide array of social, political, legal, ethical, and economic issues that must be considered in policy-making in a democracy. And it is impressed by the commitment of all parties in this debate to life and health, regardless of the different conclusions they draw. The committee hopes that, by addressing questions about the scientific potential of stem cell and how that potential can be best realized, it can contribute usefully to the debate and to the enhancement of treatments for disabling human diseases and injuries.

WHAT ARE STEM CELLS? BASIC DEFINITIONS

Stem cells are unspecialized cells that can self-renew indefinitely and that can also differentiate into more mature cells with specialized functions. In humans, stem cells have been identified in the inner cell mass of the early embryo; in some tissues of the fetus, the umbilical cord and placenta; and in several adult organs. In some adult organs, stem cells can give rise to more than one specialized cell type within that organ (for example, neural stem cells give rise to three cell types found in the brainneurons, glial cells, and astrocytes). Stem cells that are able to differentiate into cell types beyond those of the tissues in which they normally reside are said to exhibit plasticity. When a stem cell is found to give rise to multiple tissue types associated with different organs, the stem cell is referred to as multipotent.1

1  

The word “pluripotent” is sometimes used to describe stem cells that can differentiate into a very wide range of tissue types. In this report the term multipotent encompasses this type of stem cell.



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