Intermediate thyroid carcinoma in humans and ultimobranchial tumors in bulls: A comparative morphological and immunohistochemical study.

The intermediate type of thyroid carcinoma in humans has been defined as having characteristics of both follicular and parafollicular cell carcinoma. The ultimobranchial (UB) body in mammals is believed to harbor stem cells capable of developing both follicular and parafollicular cells. Hyperplastic and neoplastic lesions of the UB remnants and of the parafollicular cell system frequently occur in bulls. Such lesions, found in 64 individuals (6%) derived from an autopsy material of 1,101 bulls, have been compared structurally and immunohistochemically with 18 human cases of thyroid carcinoma of the intermediate type, in order to define their possible biological relationship. UB changes in bulls formed a continuum ranging from hyperplastic nodules to gross tumors. They contained all epithelial components present in the normal UB remnants in cattle: UB cysts and tubules and solid nests of small basophilic immature cells, which were immunocytochemically indifferent, as wellas mature follicular and neuroendocrine cells. The indifferent cell component dominated in most bull tumors; a minority were mainly formed by mature follicular or parafollicular cells. Human tumors resembled the bull UB tumors structurally and immunohistochemically, although generally the degree of maturation was higher in human tumors. A few were mainly formed by indifferent immature cells and contained typical UB cysts and tubules. One bull tumor and one human tumor contained amyloid. UB changes in bulls were invariably associated with a marked hyperplasia of the parafollicular cell system, in some cases even with tumor development. A similar hyperplasia, but without neoplastic change, was found in 4 of 11 human cases in which nontumorous thyroid parenchyma was available for examination.The findings suggest that intermediate thyroid carcinoma rather than medullary carcinoma is the human equivalent to the bull UB tumors. It is concluded that although both the medullary and the intermediate type of carcinoma appear to be histogenetically related to the UB body, the former shows evidence of a pure parafollicular cell differentiation, whereas the latter develops both follicular and parafollicular, as well as intermediate, cell forms and sometimes also immature structures of the type seen in UB remnants of the adult human thyroid gland.