Biological cells that can divide and differentiate into diverse specialized cell types, capable of self-renewal, differentiate into other cell types and to proliferate in culture. The stem cells are derived either from the embryo or fetus (embryonic stem cells), or adult tissue (adult stem cells). They can also be obtained by nuclear transfer.
Totipotent stem cells: undifferentiated cells that can theoretically give rise to a whole organism; cells from the first divisions of the fertilized egg (up to 4th day).