In each case, it is the dose required to reduce the fraction of surviving cells to 37% of its previous value. The quantities D 1 and D 0 are the reciprocals of the initial and final slopes. In this model, the survival curve is described in terms of an initial slope, D 1, resulting from single-event killing a final slope, D 0, resulting from multiple-event killing and some quantity (either n or D q) to represent the size or width of the shoulder of the curve. First, the multitarget model that was widely used for many years still has some merit ( Fig. Two descriptions of the shape of survival curves are discussed briefly with a minimum of mathematics (see Fig. The plating efficiency is given by the formula The term plating efficiency indicates the percentage of cells seeded that grow into colonies. Ideally, it should be 100, but it seldom is for various reasons, including suboptimal growth medium, errors and uncertainties in counting the cell suspension, and the trauma of trypsinization and handling.
For a nominal 100 cells seeded into the dish, the number of colonies counted may be expected to be in the range of 50 to 90. All cells making up each colony are the progeny of a single ancestor. In this way, for example, 100 individual cells may be seeded into a dish if this dish is incubated for 1 to 2 weeks, each single cell divides many times and forms a colony that is easily visible with the naked eye, especially if it is fixed and stained ( Fig. The number of cells per unit volume of this suspension is counted in a hemocytometer or with an electronic counter.
Cells from an actively growing stock culture are prepared into a suspension by the use of trypsin, which causes the cells to round up and detach from the surface of the culture vessel.