Naturally occurring clones of cells with high intrinsic proliferation potential within the follicular epithelium of mouse thyroids.

The proliferation pattern of some scattered clones of naturally occurring follicular cells with an exceedingly high intrinsic growth potential was investigated in the mouse thyroid gland. In particular, evidence was sought to demonstrate that the high propensity to replicate is a stable trait transmitted from the progenitor cells to their offspring. We hypothesize that these cell clones are at the origin of the multiple adenomas that invariably arise in chronically stimulated thyroid. Growth stimulation was induced either by hemithyroidectomy or by methimazole feeding. In a first series of experiments, involving hemithyroidectomized animals, [3H]thymidine was administered continuously for 3 weeks by means of osmotic minipumps, so that all cells entering the mitotic cycle during that time were labeled. Hemithyroidectomy led to a 3-fold increase of the fraction of labeled cells in the remaining lobe. The increase was prevented by thyroxine treatment in thyroid-stimulating hormone-suppressing doses. Autoradiographs of contiguous serial sections across whole follicles showed that roughly 75% of the labeled cells were clustered in groups of 3 or more, rather than being randomly distributed. In a second set of experiments, glands stimulated by methimazole-induced thyroid-stimulating hormone hypersecretion were pulse-labeled by a single i.p. injection of [3H]thymidine. Animals were sacrificed either 2 h or 3 weeks after the administration of the label. The thyroids were excised and the fate of labeled thyroid cells was analyzed autoradiographically. In the 2-h exposure, about 95% of all labeled follicular cells were single and the remaining 5% were in pairs. In contrast, about 50% of all labeled cells were clustered in groups of 3 to 12 cells 3 weeks after the pulse labeling. The number of silver grains per nucleus was compared to that of the identically exposed controls. The intensity of label per cell appeared to be decreased in proportion to the size of the labeled clusters, indicating that clusters had generated several subsequent generations of cells. The results support previously produced evidence that highly growth-prone cells naturally occur within the normal thyroid and demonstrate, in addition, that their high intrinsic growth rate is a stable, inheritable trait. Cells which replicate at a rate faster than that of the average epithelial cell have a tendency to overgrow during goitrogenesis. They may be at the very origin of the nodules and adenomas commonly found in experimentally produced and naturally occurring goiters.