Experimental morphological studies on the parafollicular cells of the rat thyroid gland, with special reference to the source of thyrocalcitonin.

The term "parafollicular cell" was named by NONIDEZ (1932) for a special cell type different from the ordinary follicular cells in the dog thyroid gland. The parafollicular cells occur along the basal surface of the follicular epithelium in most mammalian thyroids, and never face the colloid-containing follicular lumen. Morphological studies have been carried out by many investigators, but the functional significance of this cell has not been clearly interpreted. On the other hand, a serum-calcium lowering factor which was found by COPP et al. (1962) and was recently confirmed to be contained in the thyroid gland by HIRSCH et al. (1963). This new thyroid hormone is called calcitonin (COPP et al. 1962) or thyrocalcitonin (HIRSCH et al. 1963). On the basis of the data concerning enzymatic activities as demonstrated by histochemical examinations, FOSTER et al. (1964) suggested that the thyrocalcitonin might be secreted from the so-called mitochondrion-rich cells of the thyroid. But the identification of these cells with the parafollicular cells could not be established. In order to elucidate a possible relationship between the parafollicular cell and the hormone concerning the calcium metabolism, light and electron microscopic examinations were performed on the thyroid glands of normal and calcium-injected rats. The thyroids of hypophysectomized rats were also observed for the clarification of the hormonal dependency of the parafollicular cells.