Cellular interaction within irradiated colonies

HeLa S3 cells were allowed to grow logarithmically on a plate. When a colony had more than 50 cells, it was irradiated with 180 kVp X-rays and each cell in the colony was followed by time-lapse cinemicrography in situ. Pedigrees were classified into 3 classes: A. all cells were viable, B. at least 1 cell was lethal, C. all cells were lethal. Sister cells were identified immediately before irradiation and viability of each sister was tested from 50 to 200 hrs after irradiation. On the assumption of independent extinction, the combined frequencies of A x A, A x B etc above were calculated. Observation supported this assumption when a colony was irradiated with 2000 rads. In a colony irradiated with 1000 rads, however, observed frequency was significantly lower than expected. Number of cells in contact during irradiation did not affect the time of cellular extinction. These experiments showed that extinction in situ was independent on neighbouring cells but was dependent on sister cells when a dose was low. (author)