Papillary carcinoma of the thyroid, oxyphil cell type, "clear cell" variant: a light- and electron-microscopic study.

A unique case of papillary carcinoma of the thyroid having the combined features of three morphological subtypes--tall cell, clear cell and Hürthle cell--has been studied by light and electron microscopy. The distinctive neoplastic cells had an oxyphilic basal zone, a mid-placed nucleus and a clear apical region. Ultrastructurally, the cytoplasm was virtually filled with mitochondria, characteristic of Hürthle cells, but the unusual finding was the marked distention and emptiness of those mitochondria located in the apical zone, accounting for the clear phenomenon noted by light microscopy. In contrast, the mitochondria in the basal, or oxyphilic, part of the cells were intact. While a few cases of clear cell and papillary clear-cell carcinoma of the thyroid have been studied by electron microscopy previously, the clear change has never been attributed to dilated mitochondria, but rather to the presence of glycogen. The reason for the mitochondrial swelling is not answered, but it is probably an in vivo effect. Some of the other characteristics of papillary carcinoma of the thyroid described previously, such as microvilli, absence of colloid, infolded nuclei and nuclear bodies, were also present in this case. However, ground-glass nuclei, a frequently reported feature, were not found.