Identification
What could cause a stem cell to become isolated?
Stem cell
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 will behave in a similar manner "in vivo". There is considerable debate as to whether some proposed adult cell populations are truly stem cells.