ABSTRACT
INTRODUCTION Definition of Stem Cells All stem cells share two main properties, first, they can self-renew, that is, they can divide in a nearly unlimited way without loosing their phenotype and characteristics; second, they are capable of exiting their self-renewal and become specific mature cells with a different phenotype in a process called differentiation. This differentiation potential allows stem cells to generate cells from varied cell lineages. Stem cells isolated from different tissues and organs, and even from different developmental stages, show these properties, but differ in the way they express them. Thereby it is possible to isolate stem cells, which have a more limited differentiation potential, only to few cell types (multipotent) or even one or two (uni-and bipotent, respectively) or a broad one giving rise to all cell lineages found in an organism (pluripotent). Only the zygote can be considered a totipotent stem cell, as it is capable of generating all cell types found in the body, including those cells supporting the development of the embryo in the uterus (trophoblast, placenta, etc.) (1).
