Parafollicular cells of the rat thyroid gland after treatment with vitamin D.

Ultrastructural studies on rat thyroid demonstrated two types of secretory granules in parafollicular cells. In control animals were significantly more frequent strong osmophilic granules of approximately 130 nm in diameter than weak osmophilic granules of approximately 200 nm in diameter. Number and ratio of granules of the two types varied greatly in parafollicular cells. Administration of vitamin D3 induced a depletion of strong osmophilic granule number while the larger, weak osmophilic secretory granules continued to be stored. Immunocytochemical tests detected calcitonin in the small, strong osmophilic granules and showed that calcitonin content decreased after vitamin D3 administration. The obtained results suggest that the smaller, strong osmophilic granules contain calcitonin only, while the larger, weak osmophilic ones may contain somatostatin only.