What is a stem cell?

The nature, origin, sources and possible modes of derivation of human embryonic stem cells are scrutinized. Can cells with a capacity for differentiation equal to that of embryonic stem cells be derived from adult organisms and what are the appropriate sources for such cells? Do stem cells found in adult organs and tissues possess developmental plasticity, i.e. are they able to transdifferentiate across germ layer boundaries? What are the anticipated short- and long-term uses of embryonic and adult stem cells? The current state of science and contentious issues as related to these questions are discussed