Relationship between the Thyroid Parafollicular Cells and Pituitary Gland

The interrelation of the thyroid parafollicular cells and pituitary gland was studied in young dogs.1. After hypophysectomy, the parafollicular cells were essentially unchanged both light and electron microscopically.2. Following subacutely and chronically induced hypercalcemia by CaCl2 and vitamin D3, not only the parafollicular cells but also the growth hormone (GH) secreting cells in the pituitary underwent hypertrophy and degranulation. Degranulation in both cells was in proportion to the duration of hypercalcemia. In chronically hypercalcemic dogs there were increased GH secreting cells which lost almost all of their secretory granules and contained increased and enlarged elements of endoplasmic reticulum and Golgi complexes, suggesting an elevated secretion.3. Hypophysectomy before the administration of calcium salt resulted in a marked reduction of the parafollicular cell reaction to hypercalcemia. The mitotic cell proliferation causing huge parafollicular cell groups, which occurs in hypercalcemized and nonhypophysectomized animals, was completely inhibited and degranulation was remarkably reduced. Nuclear and cytoplasmic volumes were not enlarged and the increase in the elements of endoplasmic reticulum was only slight.Based on these results, it can be assumed that the GH cells exert a synergistic and stimulatory effect on the reaction of the parafollicular cells to induced hypercalcemia, although the relationship between both glands does not represent the typical hypophysistarget gland feedback system.