The ultrastructure of the thyroid medullary carcinoma

The ultrastructure and the morphometrical pattern of secretory granules were studied in six cases of thyroid medullary carcinoma. The tumor cells were fusiform or polyhedral with irregular, mostly elongated nuclei. Phagolysosomes containing a crystalloid material, probably degraded lipoprotein complexes, degeneratively changed mitochondria, moderately developed rough endoplasmic reticulum and Golgi complexes were commonly found. Amyloid occurred as small fibrils in intercellular spaces. Marked dystrophic lesions of tumor cells surrounding amyloid fibrils were found. Numerous roundshaped electron-dense secretory granules were noticed in tumor cell cytoplasms. The morphometrical analysis showed that the size of granules oscillated between 60 and 450 nm with mean values ranging from 171.4 +/- 31.8 to 227.7 +/- 28.1 nm. Frequency distribution curves showed at least two peaks varying with the investigated case at different intervals. In two cases two distinct groups of granules were found within the same cells: one group of electron-dense, compact, smaller sized granules and another group of larger, finely granulated, less dense granules. In the other four cases the granule sizes were more homogeneous. These results might indicate that the granule size depends on the maturation degree and functional activity or that there are several kinds of granules specialized in the secretion of various substances.