Solid cell nests (SCN) in Hashimoto's thyroiditis

In Hashimoto's thyroiditis squamous metaplasia has been described by several authors. Such foci resemble the so-called solid cell nests (SCN) of the thyroid, epidermoid structures thought to be remnants of the fourth endodermal pouch. These cell nests can be identified by their particular histological appearances and by their positive reaction with polyclonal anti-CEA. In order to study this phenomenon more closely we examined the H & E-stained histological sections of 79 cases of Hashimoto's thyroiditis systematically. In 39 cases cell nests of three different types could be demonstrated: Small groups of elongated cells organized into solid epidermoid clusters, larger epithelial cells forming solid nests or similar epithelial but cystic structures. 29 of these 39 cases were further investigated immunohistochemically for the presence of thyroglobulin, CEA (polyclonal antiserum) and calcitonin. The first type of cell nest did not show any CEA-positivity, whereas the second and third type contained CEA-positive cells in 73% of the cases. In no cases were thyroglobulin- or calcitonin-positive cells identified in these epidermoid foci. Slide series of 25 of the 39 cases have further been investigated immunohistochemically for the presence of CEA (monoclonal antiserum), chromogranin, keratin and the epitope for Lu-5. In these additional series foci of epidermoid cells could be demonstrated in up to 15 of the 25 cases. They showed a positive reaction for the monoclonal CEA antiserum in only 3 of 15 cases, for anti-keratin in 5 of 14 cases and for Lu-5 in 13 of 15 cases. Immunoreactions for chromogranin were negative in all cases. Our findings suggest that epidermoid cell nests in Hashimoto's thyroiditis more closely resemble SCN than foci of follicular cell squamous metaplasia.