Cells have intrinsic signalling mechanisms that are capable of sensing various deleterious conditions, both normal and pathological, and respond by mounting a variety of stress responses. Examples of normal signals are the cytokines that induce inflammatory responses in cells. Pathological signals include UV and X-ray irradiation, hydrogen peroxide (H2O2), abrupt anoxia and physicochemical injury through heat or noxious chemicals. A process of wound healing can function to repair the damage caused by such injuries. In many cases, especially if the stress signal is not too severe, the cell can survive and can even become tolerant to further insults. If cells are growing, such sub-lethal insults can either cause the cell to stop growing temporarily to allow adequate time to repair the damage, or the process of cell proliferation can be stopped more permanently and the cell will enter a state of senescence. Another example of an evolutionarily conserved survival mechanism is autophagy, which enables cells to cope with periods of starvation. If such stresses become too severe, however, the cell dies, either through a process of necrosis, which is rapid and catastrophic, or through a slower and more controlled process that is carried out by a highly regulated process of programmed cell death known as apoptosis.
Although the morphological characteristics of necrosis and apoptosis are clearly distinct, they do share some similarities in that they are induced by similar stimuli and often employ the same signalling mechanism. Necrosis occurs when the cell is overwhelmed by the insult and rapidly disintegrates. The cell volume expands rapidly, the mitochondria become swollen, and the plasma membrane suddenly ruptures, causing release of the cellular contents into the intercellular spaces where they can elicit an inflammatory response. By contrast, apoptosis is a much more orderly affair in that proteases and nucleases within the confines of an intact plasma membrane disassemble the cell that gradually shrinks in size and is then engulfed by neighbouring cells, thus avoiding any inflammatory reactions.
- © 2014 Portland Press Limited