On the Use of an Arbitrary Minimum Clone Size to Define Survival of Cell Reproductive Capacity: An Analysis of Clone Size Distribution in HeLa Cells X-Irradiated after Attachment to a Polystyrene Surface

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Abstract
Clonal growth experiments with polystyrene-attached HeLa cells indicate that analysis of clone size distribution in mammalian cells grown in vitro must be carried out within one plating experiment. The pattern of control growth differs greatly between experiments, depending on the plating efficiency, and effects of irradiation are merely superimposed upon the control growth pattern in any given experiment. Unirradiated cells remained attached to the polystyrene petri dishes for the full 14-day incubation period, whether or not they were destined to grow into "viable" clones. After as little as 400 rads of X-irradiation, there was marked loss of small clones from the petri dishes so that the final distribution of clone sizes represented the descendants of only a small proportion of the cells originally plated. The use of an arbitrary criterion of minimum "viable" clone size did not affect the surviving fraction determined in experiments with widely differing control plating efficiencies. However, in HeLa cells the proportion of slowly growing "small colonies" among X-ray survivors was not related to radiation dose in any simple mathematical form, so that clone size distribution patterns must be known in any experiment in which different survival levels are compared before useful deductions can be made as to the shape of the X-ray survival curve.

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