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Ultrastructural observation was performed in C cells of sheep injected intramuscularly with a dose of 2 million IU of vitamin D3 daily for 10 days, 20 days and 30 days, respectively. After treatment with vitamin D3, hyperplasia and hypertrophy of C cells were noticed mainly at the periphery of the thyroid follicles. Most of hypertrophied C cells degranulated conspicuously and contained many prosecretory granules near the well developed Golgi apparatus which were in the actively secreting and packaging phase of their secretory cycle. The other C cells had prominent lamellar arrays of rough-endoplasmic reticulum and aggregations of free ribosomes in the cytoplasm which were interpreted to be in the actively synthesizing phase of their secretory cycle. Atrophic C cells which contained degenerative organelles in the cytoplasm were occasionally observed among the hypertrophied C cells. The present ultrastructural findings clarified that C cells synthesize and secrete calcitonin continuously due to prolonged hypercalcemia induced by long-term administration of excessive doses of vitamin D3 in sheep.

Ultrastructure of Thyroid C Cells in Sheep Treated with Vitamin D3.