“ In vitro” means outside a living organism-for example, in a test tube or petri dish. The study of survival curves revealed many aspects of radiation damage long before the structure of DNA was elucidated. These changes in condition include using different types of radiation, changing the environment, adding or subtracting oxygen, or delivering the radiation at different times or during different parts of the cell cycle. The effect of radiation on cells can be observed by noting how the cell survival curve changes under different conditions. Mammalian cell survival curves generally show an exponential response to high doses of radiation with a “shoulder” of varying widths in the low-dose range. Usually, the log of the surviving fraction is plotted on the vertical axis versus dose on the horizontal axis. A cell survival curve is a plot of the fraction of cells that survive (normalized by the fraction of cells that survive with no radiation exposure) versus the radiation dose each point on a cell survival curve corresponds to a single dose of radiation. Each colony is assumed to be derived from a single surviving cell. The number of cell colonies that subsequently grow is determined. In a cell survival experiment, cells are seeded onto petri dishes and exposed to various doses of radiation. Some damaged cells may continue to function for a time, but if they do not reproduce, they are not counted as survivors. Thus, cell survival curves measure reproductive cell death. A cell survival curve is a plot of the number of cells that survive to form colonies as a function of radiation dose.
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