Degranulation and appearance of vesicular inclusions in canine C cells after administration of antithyroid drug.

When stimulated by the feeding of ethylenethiourea for a period of 6 months, dog thyroid glands increased to approximately 30 times their normal size. Not only follicular cells but also C cells had hyperplastic and hypertrophic features, compared with normal controls there was in the C cells a marked decrease in secretory granules immunoreactive for calcitonin. Furthermore, vesicular inclusions of various sizes and dilated nuclear enveloped, which showed a positive immunoreaction for calcitonin, were observed in the C cells. These findings indicate that the antithyroid drug interferes with synthesis of calcitonin by C cells probably by inhibiting the conversion of a larger precursor to calcitonin by C cells, probably by inhibiting the conversion of a larger precursor to calcitonin.