Mitotic Counts in Rat Adenohypophysial Thyrotrophs and Somatotrophs: Effects of Short-Term Thyroidectomy, Thyroxine, and Thyrotropin-Releasing Hormone.

The question of whether thyroxine (T(4)) and thyrotropin-releasing hormone (TRH) affect mitoses in pituitary thyrotrophs (Tt) and somatotrophs (St) of hypothyroid rats was investigated. Fifteen day thyroidectomized (Tx) rats were used. Groups of Tx animals received T(4) or TRH or both. Except 6 and 24 h TRH groups, the animals were sacrificed 12 h after injections. Unoperated euthyroid rats served as controls. In Tx group adenohypophysial mitoses were significantly increased. T(4) diminished mitoses in Tx rats. Mitotic counts were decreased in 6 and 24 h Tx groups, but increased in 12 h TRH group. TRH plus T(4) in Tx animals had a synergistic effect on adenohypophysial mitoses. In unoperated controls few mitoses were observed in Tt and more mitoses in St. In Tx rats more mitoses were seen in Tt than in St. T(4) alone failed to reduce mitoses in Tt but increased them in St. We concluded that T(4) affects Tt and St replication. In normal rats mitoses occur mainly in St. In Tx rats mitotic activity increased in Tt. TRH plus T(4) have a synergistic mitogenic effect on St. T(4) but not TRH affects St replication. It appears that the presence of T(4) is necessary for St multiplication.