Abstract
The parafollicular cells of the thyroid glands in the normal, calcium-injected and hypophysectomized rats were observed with the electron microscope, with special reference to their secretory granules. The general structures in normal and experimental conditions of the parafollicular cells were described in detail. Two hours after calcium injection, the number of secretory granules showed a slight decrease, but no prominent morphological changes related to the other cell organelles could be found. Four hours after the injection a certain number of extremely degranulated parafollicular cells appeared, being intermingled with parafollicular cells which contain various numbers of secretory granules. In hypophysectomized rats, no morphological changes could be found and the secretory granules were still as abundant as in the normal control. The distribution of population density of secretory granules of the parafollicular cells in each group of the normal, the calcium-injected and the hypophysectomized rats was measured. Following the administration of calcium, the number of granules clearly decreased. The functional significance of the parafollicular cells was not totally clarified, but it is most likely that the parafollicular cells secrete thyrocalcitonin.
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