The ultimobranchial remnant and its hyperplasia or adenoma in equine thyroid gland.

Abstract

Cell aggregations were detected in thyroid glands of 26 of 97 horses, using calcitonin and thyroglobulin immunohistochemistry and electron microscopy. Cell aggregations were classified into 3 types according to histologic changes, 8 horses had ultimobranchial remnant merely embedded in thyroid tissue, without compression of adjacent tissue, 13 horses had nodular hyperplasia of the ultimobranchial remnant that compressed adjacent follicular structures, and 5 horses had adenoma of the ultimobranchial remnant surrounded by a fibrous capsule. These 3 types of cell aggregations consisted of 1 or more of 3 cell types: undifferentiated and parafollicular cells forming solid sheets and follicular epithelial cells forming follicular structures. One horse had papillary hyperplasia of follicular epithelial cells with an absence of undifferentiated and parafollicular cells. Tumors consisting purely of parafollicular cells, like those in human medullary carcinoma, were not observed among the 97 horses. The possibility of an uncommitted status of undifferentiated cells from which they can differentiate into either follicular cells or parafollicular cells, was discussed.