[Undifferentiated clear cell thyroid carcinoma. Diagnostic interest of ultrastructural study (author's transl)]

Optical and electron microscopy features of an undifferentiated clear cell carcinoma of the thyroid are presented. Tumor cells have irregular, often multiple nuclei, showing numerous mitosis. Diastase-digested P.A.S. positive material is demonstrated in their cytoplasm. Numerous lymphocytes and plasma cells are present in the stroma. Electron microscopy showed large cytoplasmic areas of monoparticular free glycogen, few lipid droplets, numerous desmosomes and some intercellular microvilli. Most cells contained varying amounts of tonofilament bundles suggesting a partial epidermoid differentiation. This atypical appearance has not been described in metastases from clear cel carcinoma of kidney and may assist the primary nature of undifferentiated clear cell thyroid carcinoma.