Title: The Occurrence and Distribution of the Parafollicular Cells in the Thyroid, Parathyroid IV and Thymus IV in Some Mammals
Abstract: The thyroid glands of eight rabbits, fifteen cats and eight mice of various ages and both sexes were made into serial sections, one lobe from each individual, and investigated by means of silver impregnation, pseudoisocyanin and lead-hematoxylin stainings. It was recognized that the parafollicular cells stained by these techniques were not restricted to the thyroid gland but were also distributed in the parathyroid IV and thymus IV.1. In rabbits the parafollicular cells occurred in the parathyroid IV in six out of seven cases. Although the proportion of parafollicular cells to parathyroid cells differed from individual to individual, it reached 10% in the most numerous case.2. In cats the parafollicular cells constantly occurred in the parathyroid IV. They were found also in the thymus IV of nine out of eleven cases. From forty to fifty parafollicular cells were generally counted in cat parathyroid IV. In two cases with numerous parafollicular cells, they were estimated as 1-3% of the parathyroid cells. In the thymus tissue their population was lower than in the parathyroid gland.3. No parafollicular cells were found in the parathyroid III and the thymus III.4. The distribution of parafollicular cells in the thyroid of rabbits and mice was confined in a more or less restricted region of the gland around the parathyroid IV and thymus IV, or around ciliated cysts (vide infra).5. In all three species a few ciliated cysts containing a foamy colloid-like substance constantly occurred in the thyroid grand.6. These observations were discussed in relation to the ultimobranchial origin of the parafollicular cells.