A study on the relationship between solid cell nests and mucoepidermoid carcinoma of the thyroid

Abstract

Mucoepidermoid carcinoma of the thyroid (MECT) is recognized as a pathological entity. The origin of MECT is unknown but the morphology of this tumor closely resembles features seen in the ultimobranchial body (UB) vestiges. The so-called solid cell nests (SCN) of the thyroid may correspond to the human UB vestiges. To investigate whether these vestiges are the site of origin of this tumor a comparative study of SCN and MECT was undertaken. One hundred autopsied thyroids cut at 2-3 mm intervals were studied for the presence of SCN. Histochemical (H and E [hematoxylin and eosin], Alcian blue-PAS [periodic-acid Schiff], Mayer mucicarmine) and immunohistochemical studies (calcitonin, epidermal keratin) were performed in SCN and 4 cases of MECT. Of the thyroids, 6% had SCN. They were mainly composed of epidermoid-like cells arranged in solid structures or lining cystic cavities, tubular and follicular structures. Solid clusters usually showed lumina containing PAS-positive and mucin-positive cell debris. Mucin stains also revealed mucinous cells placed around lumina filled by mucosubstances. Characteristic PAS-positive rounded bodies were found filling lumina as well as within some apical epidermoid-like cells, mucinous cells and cell debris. An obvious transition between these cells, cell debris and mucosubstances filling the lumina was noticed, suggesting degenerative changes undergone by the epidermoid-like cell. MECT basically presented all histological and histochemical features shown by SCN. Calcitonin-containing cells were observed in 54% of SCN, while a metastatic MECT also showed scattered C cells within solid islands. Evidently, MECT could originate in the SCN or human UB vestiges.