Abstract
Electron microscopic studies were made on the thyroid parafollicular cell and the parathyroid cell of normal and vitamin D2 and CaCl2 treated rats.1. The parafollicular cells of the normal rat thyroid were characterized by numerous dense granules, 100-240mμ in diameter, and occasionally displayed cytoplasmic filaments and autophagic vacuoles containing many secretory granules. Desmosomes between the parafollicular cell and the follicle epithelial cell were rarely encountered.2. The secretory granules of the parafollicular cell are markedly reduced in number in animals exposed to hypercalcemia for 2-4 weeks. In these animals the Golgi field was more extensive with numerous small vesicles and prosecretory granules and the rough endoplasmic reticulum was better developed than in control animals. This suggests an activated granular formation. The secretory granules are thought to be formed in the rough endoplasmic reticulum and the Golgi apparatus system as in other protein secreting cells. Though features suggesting reverse pinocytosis were seen in a few cells, it is difficult to determine whether or not the granule is released only by this type.3. The parathyroid cell markedly reacts to long-term hypercalcemia as the parafollicular cell of the thyroid gland does. In most parathyroid cells of hypercalcemic animals, the secretory granules are increased in number, the rough endoplasmic reticulum and the Golgi apparatus are reduced in quantity, the tortuosity of the cell membrane is decreased and the lipid droplets are increased in number. These changes are considered to be related to the long-term suppression of the synthesis and release of parathormone. Some of the cells in the experimental animals have characteristically large mitochondria associated with endoplasmic reticulum showing markedly enlarged cisternae. This type cell is considered to be degenerated.
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