Survival characteristics of normal differentiated rat thyroid cells maintained in vitro.

The radiation response of a cultured line of differentiated rat thyroid epithelial cells (FRTL-5) was determined by clonogenic assay in vitro and compared to the previously reported response curve for primary rat thyroid cells assayed by transplantation in vivo. The beta components (or D0 estimates) for the two sets of data were not significantly different although the response of the FRTL-5 cells was displaced to the right as a consequence of differences in the shoulder of the survival curves. These results indicate that this in vitro model can be used to study the radiation response of thyroid epithelial cells and by extrapolation perhaps other hormonally responsive, slowly proliferative parenchyma. The cell age response for FRTL-5 cells was determined after separation of the cells into various phases of the cell cycle by centrifugal elutriation. Resistant peaks in mid-G1 and G2M phases of the cell cycle were detected. The cell age response data for other portions of the cell cycle should be interpreted cautiously because less than satisfactory enrichments were achieved.