Fine structure of a diffuse undifferentiated small-cell carcinoma of the thyroid.

Differentiating diffuse undifferentiated small-cell carcinoma of the thyroid from malignant lymphoma is difficult. A case of small-cell carcinoma was studied by electron microscopy and is reported here to illustrate the value of electron microscopy in separating epithelial thyroid tumor from lymphoma. Ultrastructurally, the tumor was composed of cells possessing occasional intercellularjunctional complexes and spaces with microvilli. There were abundant free ribosomes and well-developed rough endoplasmic reticulum in the cytoplasm. Golgi apparatus, smooth endoplasmic reticulum, membrane-bound cytosomes, and centrioles were also evident. Numerous swollen mitochondria were seen in some tumor cells. A few large tumor cells characterized by abundant cytoplasm containing few mitochondria, endoplasmic reticulum and numerous microfibrils were also observed. A few collagen fibers, lymphocytes and plasma cells were present among tumor cells. No basement membranes nor virus-like particles were seen.