Measurement of Radiation Effects on Thyroid Cell DNA Synthesis using Tritiated Thymidine

INTRODUCTION The rat thyroid is a suitable model with which to study radiation effects on thyroid cell proliferation in vivo. The model has the advantage of a relatively simple cell population; about70% of the cells are follicular and about 30% are stromal.' There is no migra tion of cells to or from the gland either normally or when growth is artificially promoted by a goitrogenic challenge. The tissue is thus normally closed and non-dividing, or when goitrogen stimulated which induces cell hypertrophy and hyperplasia it is closed and dividing.2 lonising irradiations impair the capacity of the rat thyroid to undergo its normal 2 to 3 fold increase in weight in response to a continuous goitrogenic growth stimulus in vivo. The degree of impairment of weight response has been employed as an approximate index of irradiation effects on the reproductive potential of thyroid cells..3'5 In these latter studies an assessment was made of changes during growth with and without irradiation due to cell size and number using histological measurements. In the current investigation these studies are complemented by biochemical measurements of nucleic acid synthesis. In addition irradiation on the follicular and stromal cells have been distinguished in the present study. A detailed study of the effects of different doses of X-irradiation on the thyroid cell population of the normal rat and on the cell population during goitrogenic growth was carried out. The techniques employed included sequential measurement of total thyroid weight, cell density, cell composition, RNA and DNA synthesis using chemical and cell labelling methods. The data are critically discussed with emphasis on how to use the rat thyroid as a radiobiological model with which to study radiation effects on proliferation of differentiated thyroid cells in vivo and DNA synthesis too. X-irradiation was used as a precisely dosed and homogeneous radiation.