Electron and immunoelectron microscopic study of thyroidal medullary carcinoma.

A correlated light, immunofluorescent, electron and immunoelectron microscopic study on the histogenesis and immunochemical nature of the tumor amyloid in medullary carcinoma of thyroid (MCT) was performed. The light microscopic finding of cytoplasmic inclusions in tumor cells was related ultrastructurally to a unique form of cellular degeneration composed of particulate and fibrillar matters, which could be the precursor of amyloid deposits. The immunofluorescent staining revealed calcitonin in tumor cells and in portions of amyloid. Ultrastructurally, the storage granules were morphologically heterogeneous and contained variable amount of immunoreactive calcitonin. Immunoelectron microscopy demonstrated disintegration of the granules with apparent discharge of calcitonin into the matrix of the above-mentioned particulate and fibrillary mass. Immunoreactive calcitonin was not shown on the mature amyloid fibrils nor in most of the granular matters closely associated with the amyloid. The results indicate that the amyloid in MCT, though related to the secretory products of the tumor cells, may not represent the hormonally active calcitonin molecule.