Light and Electron Microscopic Alterations of the Dog Parafollicular Cells Induced by Antithyroid Drug

Following continuous adminstration of thiourea as an antithyroid drug for one to four months, not only the follicular cells but also the parafollicular cells and parafollicular cell complexes of the dog thyroid showed conspicuous changes.By selective staining methods for the parafollicular cells, i. e., silver impregnation, pseudoisocyanin and lead-hematoxylin, it was found that the affected parafollicular cells revealed sparse dispersion of the secretory granules, enlarged cell bodies and distribution pattern surrounding the follicular cell masses and follicular lumens, in contrast to the normal cells filled with stained granules and in para- and interfollicular positions.Electron microscopic examination confirmed these alterations observed in light microscopy and revealed the most pronounced alteration to be a dilation of cisternae of rough-surfaced endoplasmic reticulum. The percentage of the cells with vacuolated cisternae of endoplasmic reticulum among all the parafollicular cells rose to 65% from 17% in normal glands. Furthermore, the parafollicular cells displaying ultrastructural features of hypertrophic follicular cells except for a few characteristic secretory granules were often observed.The parafollicular cell complexes became larger and the parafollicular cells and follicular cells in complexes displayed the same reactions as those in the thyroid. The undifferentiated epithelial cells occurring both in the parafollicular cell complexes and sometimes in large parafollicular cell groups also showed an appearance similar to the follicular cells.On the basis of these results, the relationship between the parafollicular cells and the follicular cells was discussed.