Granules and Thyrocalcitonin-like Activity in Medullary Carcinoma of the Thyroid Gland

Abstract

A medullary carcinoma of the thyroid gland was examined by electron microscopy, histochemistry and bioassay for thyrocalcitonin. Numerous secretory granules were present in the cytoplasm of the neoplastic cells, which closely resembled thyroid parafollicular cells. Thyrocalcitonin-like activity was demonstrated in the neoplasm at a level of at least 100 times that normally found in the human thyroid gland. Tests for catecholamines and serotonin were negative. Acid-phosphatase and nonspecific esterase were present in lysosomes and autophagic vacuoles, but not in the secretory granules. Neoplastic cells maintained in tissue culture for four months showed diminished granulation. The observations are consistent with the hypothesis that medullary carcinoma is a thyrocalcitonin-producing neoplasm of parafollicular cells.