Anomalous occurrence of immunoreactive calcitonin cells in the thymus of the rat.

In a study of the effect of pinealectomy on thyroid C-cell number, 8 animals out of 66 were found to have thymic tissue in close association with the thyroid. Cells containing immunoreactive calcitonin were found in all of the thyroids but in only one of the 8 pieces of thymus. These cells found in a piece of thymic tissue associated with the right thyroid lobe were located immediately under the capsule and did not form or associate with follicles. Unlike the other animals the rat with thymic calcitonin cells had an unequal distribution of C-cells between the left and right thyroid lobes, but the total number of thyroidal C-cells was the same as that of the other rats. Since the thymus proper was not examined in these 66 animals, ten additional rats were taken for such a study. Thyroid-associated thymic tissue was found in three of these, but none of these thymi showed any immunoreactive cells.