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Stem cells are defined by their self-renewal capacity and their ability to generate diverse kinds of specialized progeny. Stem cells can be classified by their differentiation potential as totipotent, pluripotent, or multipotent, and by their tissue of origin in embryonic or adult stem cells. A great deal of interest has risen regarding their distinct differentiation models, from the conventional straightforward mother stem cell-daughter cell, to the considerably more complex transdifferentiation and undifferentiation models. These models are currently being applied to understand the phenomenon of cell plasticity that characterizes these cells. Stem cell plasticity accounts for the capacity to generate cells types different from their original tissue, a good example being the hematopoietic stem cells, which can give origin to hepatocytes and myocytes under highly regulated conditions. There are challenges and controversies regarding diverse aspects of research in stem cell plasticity studies, as a great deal of them are performed on donated ova from human fertility centers, leading to heated ethic arguments that require being dealt with in order to further improve stem cell research. Currently, it is possible to obtain pluripotent stem cells from sources other than human embryos, for example the amniotic fluid. New and spectacular developments in stem cell therapy and regenerative medicine are continuously being investigated. International legislation regarding manipulation and research on stem cells is heterogeneous and frequently divergent, whereas national laws are limited in their capacity to deal with the evolving challenges in these research and therapeutic field. [ABSTRACT FROM AUTHOR]