Squamous cells in the human thyroid gland.

Squamous epithelium in the thyroid, which may be basaloid, epidermoid, or truly squamous, may be derived from several sources. In the normal thyroid, usually rests of one type or another are responsible for these cell nests. Thus, thymic or ultimobranchial remanants may be found in the lateral lobes of normal thyroid glands. Maldevelopment or excessive descent of thyro-glossal duct remnants may be responsible for squamous epithelial cysts noted in the central portions of the thymus, especially in the isthmus. In diseased thyroids, the most likely derivation of squamous or epidermoid epithelium is through metaplasia of follicular epithelial cells. Thus, in adenomatous goiter and various types of thyroiditis as well as follicular derived neoplasms, such a derivation is most likely. In tumors containing malignant squamous epithelium (adenoacanthoma, adenosquamous carcinoma, or pure squamous carcinoma) there is a range of such lesions, and since many of them appear to be derived from follicular epithelium, it is most likely that squamous metaplasia which has undergone malignant transformation has occurred in these neoplasms.