Ultrastructure of fetal pars-tuberalis-specific secretory cells under the influence of propylthiouracil and thyroxine.

The rat pituitary pars tuberalis (pt) is a functional and morphological unique component of the adenohypophysis. It mainly consists of specific secretory cells whose morphological and functional characterization is far from being complete. In this study the ultrastructure of fetal secretory pt cells developing under hypo- and hyperthyroid conditions was examined. Besides controls, young mature pregnant Wistar rats were treated with propylthiouracil (PTU) or thyroxine (T4). Areas of pt cells and of their nuclei were measured. Different cell organelles per cell were counted, and area densities of these organelles were determined. While the number of dictyosomes per cell area did not change, the area densities of long ribbons increased significantly in the experimental groups. Lysosomes and secretory granules, however, were found to be significantly diminished in pt cells of T4- and PTU-treated animals. The latter finding corresponds to earlier investigations when changes of the thyroid-stimulating-hormone-like immunoreactivity of fetal pt-specific cells were observed under the same experimental conditions. Results indicate that fetal pt-specific cells respond to changes of thyroid status in a manner different from pars distalis thyrotrophs. An interaction of pt-specific cells in thyroid regulation mechanisms is assumed.