What is a stem cell?

A cell must meet two basic key criteria in order to be considered a stem cell:

1. Able to differentiate into multiple cell types upon induction.

2. Able to remain in an undifferentiated state over numerous cell divisions when not induced to differentiate.

There are numerous "stem cells", some with more potential than others. An embryonic stem cell is pluripotent by definition because it is able to become any cell in the body (though this is difficult in practice). Other stem cells are more restricted, such as the hematopoietic stem cell, which can only form the cell types found in blood and the immune system.

A new form of cell called an induced pluripotent stem cell (iPS), can be generated by turning on at least 2 specific genes in skin or other non-stem cells. This is commonly done through use of a viral vector such as a retrovirus although it is now possible to avoid the use of virus. Preliminary studies indicate that iPS cells have identical capabilities as embryonic stem cells which may negate the need to derive embryonic stem cells from embryos. Still, everything we know about pluripotent stem cells comes from studies of embryonic stem cells and thus they are an integral part of the quest for therapeutic uses for stem cells.