Title: Electron Microscopic Studies on the Parafollicular Cells and Parafollicular Cell Complexes in the Dog
Abstract: The parafollicular cells and parafollicular cell complexes of the dog under normal and experimental conditions were investigated by electron microscopy.1. The parafollicular cells are characterized by numerous secretory granules believed to contain calcitonin. In addition to the typical parafollicular cells, “ER vacuolated parafollicular cells, ” containing markedly enlarged cisterns of rough endoplasmic reticulum occurred in 17% of all parafollicular cells. They seem to be inactive cells on the basis of their ultrastructural characteristics. Some of the large parafollicular cell groups included undifferentiated cells corresponding to those in parafollicular cell complexes (vide infra).2. The parafollicular cell complexes consisted of numerous parafollicular cells, some undifferentiated epithelial cells and sometimes follicular-like cells. Immature parafollicular cells of variable developmental stages were also found. The undifferentiated cells were characterized by small cytoplasm and by chromatin dense nucleus, though there were variable cell patterns. Their transitional forms to the parafollicular cells were often recognized. Conspicuous aggregation of microfilaments and variable numbers of glycogen particles sometimes occurred in the undifferentiated cells. The follicular-like cells were structurally identical with thyroid follicular cells, and immature follicular cells indicated their developmental patterns. It was difficult to clearly differentiate between some immature follicular cells and undifferentiated cells. It remains undecided whether the undifferentiated cells give rise to both parafollicular cells and follicular cells or these represent two distinct cell lines.3. Experimentally induced hypercalcemia caused hypertrophy and hyperplasia of the parafollicular cells, accompanied by degranulation and increase of endoplasmic reticulum. After parathyroidectomy, on the contrary, the parafollicular cells accumulated larger and denser secretory granules. These subtle responses to the altered level of calcium in the blood were shared also by the parafollicular cells enclosed in the parafollicular cell complexes. Undifferentiated epithelial cells, after induced hypercalcemia, also revealed enlarged cytoplasm with increased endoplasmic reticulum.