Apoptosis differentiated from necrosis

The different degrees of potential for proliferation by stem cells also cause it to be one of the most sensitive to the toxicity of drugs (Harper and Rich, 2013). Most stem ceUs are in a quiescent, nonproliferative state (Rich, 2015b) however, this does not mean that they are immune to potential toxicity. Small molecules may modulate quiescent cells, and for stem cells, this effect may not be evident until the cells reenter the cell cycle. Attenuations may also appear during cell development or differentiation as increased apoptosis or necrosis or other events that inhibit the system from establishing and/or maintaining homeostasis. 

In more complex systems such as tissues and ttimours not all cells will be involved in an active cell cycle. This introduces two further concepts of growth fraction and cell loss. The term GO has been adopted to refer to those cells that are out of cycle. However, this rather nondescript term is used to describe cells that may be out of cycle for vastly different reasons. Cells may be out of cycle because they have started the process of differentiation and maturation or they may have reached their life span and are undergoing cell death. They may be in a poorly vascularized area of the tissue and cannot continue their cycle due to nutrient deprivation this may leave them in a state of temporary hypoxia in which they may be able to re-errter the cycle if the microenvironment improves or they may die if the situation becomes chronic. Cell loss also describes a diversity of underl3dng processes from exfoliation in mucosal tissues to prograrrrmed cell death or apoptosis, necrosis, or even metastatic loss in tumoirrs. 

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