Hormonal Effects on the Quantitative Transplantation of Monodispersed Rat Thyroid Cells*

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Abstract
Functional thyroid follicles develop at the site of inoculation of adequate numbers of isologous monodispersed rat thyroid cells. Under fixed hormonal conditions, the proportion of graft sites in which follicular units (FU) develop is quantitatively related to the number of monodispersed cells inoculated. The current studies were designed to assess the effect of various hormonal conditions on FU formation from grafted thyroid cells. Approximately 9 times as many cells were required to produce FU in 50% of the graft sites in intact animals as were required in thyroidectomized recipients. The slope of the cell dose-FU response curves in the two types of recipients were not significantly different, however. The hormone-dependent decrease in transplantation efficiency in the nonthyroidectomized recipients was not the result of cell loss from the transplant site or death of potential clonogens. Rather, it was demonstrated that potential thyroid clonogens remain quiescent at the transplant site in intact animals for at least 4 weeks after grafting. At that time, they can become committed to follicle formation, presumably as a result of stimulation by elevated TSH levels after thyroidectomy. Iodide was found to be necessary for the most efficient FU formation in the presence of elevated TSH levels. These results combined with previous morphological studies of FU development indicate that the efficiency of FU formation is TSH dependent. However, once a given clonogen is committed to FU formation, the sequence of differentiation proceeds independently of the presence or absence of elevated TSH levels. Finally, our results indicate that in the presence of iodine, FU synthesize and secrete, T4 and are subject to normal biofeedback regulation. (Endocrinology106: 1769, 1980).