Ultrastructural and morphometric studies on the rat pituitary thyrotrophs and thyroid follicular cells following administration of thyrotropin releasing hormone.

The pituitary thyrotrophs and thyroid follicular cells of rats were studied by electron microscopy and morphometric analysis at 5, 10, 20, 30 and 60 min after intravenous injection of 200 microgram of thyrotropin releasing hormone (TRH) and at 10 min after injection of 10 microgram TRH. Multiple granule extrusions in a group were often observed around the pituitary thyrotrophs at 5, 10 and 20 min after administration of 200 microgram TRH, as well as at 10 min after injection of 10 microgram TRH. The number of released granules reached its maximum at 10 min after 200 microgram TRH injection, but the total number of secretory granules in the cytoplasm of thyrotrophs did not show any significant variation throughout the experimental period. Percent area of the rough endoplasmic reticulum indicated the maximum value at 20 min and that of the Golgi apparatus at 10 min. These findings suggest that TRH stimulates the thyrotroph and accelerates synchronously the secretion as well as the synthesis of TSH. In the thyroid follicular cells after 200 microgram TRH injection, numerous small vesicles which might be secretory granules were found in the apical cytoplasm at 5 and 10 min. Pseudopod formation on the luminal surface and accumulation of colloid droplets and lysosome-like granules in the apical cytoplasm were most frequently observed at 20 and 30 min. The value of the mean ratio of the diameter of the colloid lumen to the cell height of follicular cells in the thyroid follicles of all experimental animals stimulated with TRH decreased much more than that of the control animals. This value may indicate the increased activity of follicular cells to reabsorb the colloid.