The classical definition of a stem cell requires that it possess two properties
Self-renewal
The ability to go through numerous cycles of cell division while maintaining the undifferentiated state.
Potency
The capacity to differentiate into specialized cell types. This requires stem cells to be either totipotent or pluripotent - to be able to give rise to any mature cell type.
This is the strictest definition*.
*Multipotent or Unipotent progenitor cells are sometimes referred to as stem cells.
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