Clonogen number and radiosensitivity in rat thyroid follicles.

A technique has been developed for transplanting whole thyroid follicles into the fat pads of recipient thyroidectomized rats to assess the ability of the follicular cells to proliferate and form colonies, i.e., clusters of new follicles. Of the transplanted follicles, 80-90% formed follicle clusters, indicative of the presence of transplanted follicles not containing colony-forming cells (clonogens) or some reproducible degree of transplantation trauma. The initial number of clonogens per regenerative follicle was calculated from data from split-dose experiments to be 3.3 +/- 1.5, and their sensitivity was characterized by a D0 value of 350 +/- 95 cGy. Three clonogens among about 80 epithelial cells per regenerative follicle and the 10-20% of nonregenerative follicles represent an overall colony-forming efficiency of about 3%. This is similar to the value of 2-3% reported by others using single-cell transplantation techniques.