Changes in normal and abnormal colony formation of thyroid cells in vivo, when transplanted at 1 day or 6 weeks after X-irradiation in situ.

Thyroid cells were given 5.5 Gy X-rays in situ and transplanted after 1 day or after 6 weeks to form colonies in fat pads. The colony-forming efficiency unexpectedly decreased by a factor of about 2 with this increase in delay time before transplantation. In addition, there was a concomitant marked increase above the control levels in the proportion of new structures of thyroid origin containing abnormal follicles. These quantitative and qualitative increases in injury may be related to the gradual expression of thyroid abnormalities following irradiation in situ, and they may have implications for the recovery of functional tissue subunits in other organs.