Stem cells are defined by their unlimited developmental potential. A?totipotent? cell is able to generate all the cells required in life thereafter. Since the reigning dogma is that all cells come from other cells, there must be an undifferentiated, unspecialized cell that renews itself and turns into highly differentiated cell types on demand. For a very long time, we have considered development as an inexorable process, with irreversible restrictions and finally, a highly specialized community of cells with few if any options. Now, we have come to accept that there is an implicit potential in most mature tissues for developmental plasticity. This holds true for malignant cells too; eventually, single tumor cells can generate progeny with entirely different characteristics and often catastrophic consequences.
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