The classical definition of a stem cell requires that it possesses 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. 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 feed back mechanism.
Two mechanisms exist to ensure that a stem cell population is maintained:
1. Obligatory asymmetric replication: a stem cell divides into one mother cell that is identical to the original stem cell, and another daughter cell that 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.
2. Stochastic differentiation: when one stem cell develops into two differentiated daughter cells, another stem cell undergoes mitosis and produces two stem cells identical to the original.
Pluripotent, embryonic stem cells originate as inner cell mass (ICM) cells within a blastocyst. These stem cells can become any tissue in the body, excluding a placenta. Only cells from an earlier stage of the embryo, known as the morula
, are totipotent, able to become all tissues in the body and the extraembryonic placenta.
A: Stem cell colonies that are not yet differentiated.
cells, an example of a cell type
Potency specifies the differentiation potential (the potential to differentiate into different cell types) of the stem cell.
- Totipotent (a.k.a. omnipotent) stem cells can differentiate into embryonic and extraembryonic cell types. Such cells can construct a complete, viable organism. These cells are produced from the fusion of an egg and sperm cell. Cells produced by the first few divisions of the fertilized egg are also totipotent.
- Pluripotent stem cells are the descendants of totipotent cells and can differentiate into nearly all cells, i.e. cells derived from any of the three germ layers.
- Multipotent stem cells can differentiate into a number of cell types, but only those of a closely related family of cells.
- Oligopotent stem cells can differentiate into only a few cell types, such as lymphoid or myeloid stem cells.
- Unipotent cells can produce only one cell type, their own, but have the property of self-renewal, which distinguishes them from non-stem cells (e.g. progenitor cells, which cannot self-renew).
In practice, stem cells are identified by whether they can regenerate tissue. For example, the defining test for bone marrow or hematopoietic stem cells (HSCs) is the ability to transplant the cells and save an individual without HSCs. This demonstrates that the cells can produce new blood cells over a long term. It should also be possible to isolate stem cells from the transplanted individual, which can themselves be transplanted into another individual without HSCs, demonstrating that the stem cell was able to self-renew.
Properties of stem cells can be illustrated in vitro, using methods such as clonogenic assays, in which single cells are assessed for their ability to differentiate and self-renew. Stem cells can also be isolated by their possession of a distinctive set of cell surface markers. However, in vitro culture conditions can alter the behavior of cells, making it unclear whether the cells shall behave in a similar manner in vivo. There is considerable debate as to whether some proposed adult cell populations are truly stem cells.