A method allowing cloning of monolayer cultured cells with a low plating efficiency was developed. Cells were grown in several small palladium squares to obtain a high cell density. These squares were surrounded by non-adhesive agarose to prevent large distance migration and thereby mixing of the clones. By using easily-cloned hamster cells for comparison it was found that the survival curves were similar to the curves obtained with conventional cloning. The new method was used to compare the radiosensitivity of cultured human glia and glioma cells which both have a low plating efficiency (less than 5 per cent) when seeded sparsely. The survival curves for the glioma cells had high Do-values (1.5--2.5 Gy) and large shoulders (extrapolation numbers around 5) indicating that they were rather resistant and had a high capacity for accumulation of sublethal damage. The survival curves for glia cells had lower Do-values (1.3--1.5 Gy) and no shoulders at all, indicating that they were more sensitive than the glioma cells.
The detachment, attachment and growth were studied in malignant glioma and normal glia cells in culture, using the palladium‐agarose cloning method. The number of cells in each individual clone was repeatedly counted throughout 10 days. All four glioma cell‐lines studied detached and attached themselves at higher rates than the normal, diploid, glia cells. This dynamic behaviour seemed to be a general property of cultured, malignant, glioma cells. All the cell‐lines contained clones with different properties. Some clones were growing, others were nearly constant in cell number and yet others decreased in cell number as a function of time. The differences between these three types of clones were surprisingly sharp.
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