The classical definition of a stem cell requires that it possesses two properties:
- Self-renewal: the ability to go through numerous cycles of cell growth and cell division, known as cell proliferation, while maintaining the undifferentiated state.
- Potency: the capacity to differentiate into specialized cell types. In the strictest sense, this requires stem cells to be either totipotent or pluripotent—to be able to give rise to any mature cell type, although multipotent or unipotent progenitor cells are sometimes referred to as stem cells. Apart from this, it is said that stem cell function is regulated in a feedback mechanism.
Self-renewal[edit]
Two mechanisms ensure that a stem cell population is maintained (doesn’t shrink in size):
1. Asymmetric cell division: a stem cell divides into one mother cell, which is identical to the original stem cell, and another daughter cell, which is differentiated.
When a stem cell self-renews, it divides and does not disrupt the undifferentiated state. This self-renewal demands control of cell cycle as well as upkeep of multipotency or pluripotency, which all depends on the stem cell.[13]
2. Stochastic differentiation: when one stem cell grows and divides into two differentiated daughter cells, another stem cell undergoes mitosis and produces two stem cells identical to the original.
Stem cells use telomerase, a protein that restores telomeres, to protect their DNA and extend their cell division limit (the Hayflick limit).[14]
