Electron microscopy of clear cell thyroid carcinoma.

Three thyroid carcinomas composed in part or entirely of clear cells were studied by light and electron microscopy, and thyroglobulin content was determined by biochemical methods. Clear cells have been found in follicular and papillary thyroid carcinomas and in undifferentiated carcinomas. The clear (wasserhelle) appearance of the cytoplasm was due to the accumulation of glycogen. The major ultramicroscopic features of the clear cells were the presence of glycogen granules, the decreased amounts of rough endoplasmic reticulum, the increased amounts of free ribosomes arranged in polysomes, a hypertrophic Golgi apparatus, and a sparsity of dense bodies of lysosomal character. It is hypothesized that the intracellular accumulation of glycogen may be a result of a selective loss of the peptide portion of thyroglobulin, and may influence the degree of biochemical differentiation and the natural course of the tumors.