Ultrastructure of anaplastic (spindle and giant cell) carcinoma of the thyroid.

Three anaplastic (spindle and giant cell) carcinomas of the thyroid were studied by light and electron microscopy; two of the tumors also included foci of recognizable follicular carcinoma. The follicular carcinoma cells desplayed prominent mitochondria and rough endoplasmic reticulum, and showed evidence of secretory activity. Desmosomes and complex cellular interdigitations were evident. Basal laminae were present, with conspicuous reduplication in the well-differentiated foci. However, some epithelial clusters were surrounded by basal lamina, showing focal discontinuities through which epithelial cells protruded into the stroma. The pleomorphic spindle and giant cells showed cytoplasmic and nuclear characteristics similar to the better differentiated carcinomatous follicular elements, but showed rare desmosomes and no basal laminae. The basic ultrastructural similarity between follicular and anaplastic tumor cells confirms their common epithelial origin. However, while partially retaining their secretory capability, the anaplastic cells progressively lose their capacity to synthetize basal lamina and develop complex cellular attachments.