S-100 protein is a differentiation marker in thyroid carcinoma of follicular cell origin: an immunohistochemical...

S-100 protein, a dimer of S-100 alpha and S-100 beta subunits (S-100 alpha and S-100 beta), is widely distributed in human tissue, and several papers describing S-100 protein expression in follicular cells of the thyroid have been published. In the present study, 105 cases of thyroid carcinoma (of which 96 were papillary, four follicular, two undifferentiated, and three medullary) were analyzed immunohistochemically for the expression of S-100 protein, S-100 alpha, S-100 beta, and thyroglobulin. In papillary carcinoma, 188 lesions were studied and classified into well differentiated types (56 papillary, 45 follicular) and poorly differentiated types (41 trabecular, four solid, eight squamoid, three tall, and one insular), because the histological structure of each tumor was heterogeneous. The percentage of lesions which expressed positively for S-100 protein and S-100 alpha, respectively, according to type were: papillary, 96 and 99%; follicular, 96 and 100%; trabecular, 95 and 100%; solid, 50 and 50%; squamoid, 50 and 75%; and tall, 33 and 100%. The insular type was negative for both. For papillary carcinoma, well differentiated lesions showed stronger S-100 alpha expression than poorly differentiated lesions. S-100 alpha expression was weaker in follicular and undifferentiated carcinoma than in papillary carcinoma. Medullary carcinoma also expressed S-100 alpha. S-100 beta was positive in lesions that expressed S-100 alpha strongly. Expression of S-100 protein and S-100 alpha protein correlated with thyroglobulin synthesis in the follicular cells. It was concluded that S-100 protein, mainly S-100 alpha, exists in thyroid follicular cells, that it exists in higher quantity in most of the well differentiated lesions but in lower quantity in poorly differentiated or undifferentiated lesions, and that S-100 protein, especially S-100 alpha, is a differentiation marker in carcinoma of thyroid follicular cell origin.