HEMATOLOGY EBOOK

The classical defintions of a stem cell requires that it possess two propeties:

• 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.

Potency defintions

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 sells 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 an of the three germ layers.
• Multipotent stem cells can differentiate into a number of cells, but only those of a closely related family of cells.
• Oligoptent stem cells can differentiate into only a few cells, 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 from non-stem cells (e.g. muscle stem cells).

Identification

The practical definition of a stem cell is the functional definiton -  a cell that has the potentia to regenerate tissue over a lifetime. For example, the gold standard test for a bone marrow or hematopoietic stem cell (HSC) is the ability to transplant one cell and save an individual without HSCs. In this case, a stem cell must be able to produce new blood cells and immune cells over a long term, demonstrating potency. 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 ilustrated in vitro, using methods such as clonogenic assays, where single cells are characterized by their abillity to differentiate and self-renew. As well, stem cells can be isolated based on a distrinctive set of cell surface markers. However, in vitro culture conditions can alter the behavior of cells, making it unclear whether the cells will behave in a similar manner in vivo. Considerable debate exists whether some proposed adult cell populations are truly stem cells.