The effects of goitrogenesis, involution, and goitrogenic rechallenge on the clonogenic cell content of the rat thyroid.

A fraction of enzymatically monodispersed rat thyrocytes from untreated animals clonally proliferate into thyroid follicular units following transplantation into the subcutaneous fat pads of syngeneic recipients. During the induction of experimental goiters in rats either with 3-amino-1,2,4-triazole/iodine sufficient diet or KClO4/Remington low iodine diet, the clonogenic fractions of cells from aminotriazole goiters decreased to 1.9 x 10(-4) and KClO4 goiters to 9.8 x 10(-5) as compared to 5.8 x 10(-3) for cells from age-matched controls during the growth phase of goitrogenesis. With continued aminotriazole treatment after thyroid hyperplasia had ceased, the clonogenic fraction increased to 2.0 x 10(-3) while continued KClO4 treatment had little further effect. The changes in the clonogenic fraction induced by both regimens were reversed during involution; goitrogenic rechallenge of involuted glands led to changes in the clonogenic fraction similar to that noted during the initial challenge. The clonogenic fractions of cells from aminotriazole goiters were greater than that of cells from KClO4 goiters at all time points examined despite similar TSH levels in situ. We conclude that the rat thyroid contains a hierarchy of cells with different proliferative capacities and that the clonogenic thyrocytes possess many of the attributes of a stem-cell.