Calcitonin-like immunoreactivity of amyloid fibrils in medullary thyroid carcinomas: an immunoelectron microscope study

Using 3 polyclonal antisera directed against synthetic human calcitonin, we investigated at the electron microscope level the intra-or-extra-cellular fibrillar/filamentous aggregates found in 4 amyloid-rich medullary thyroid carcinomas (MTC) and in a number of other endocrine polypeptide tumours with or without demonstrable amyloid deposition. The antisera were applied by the immunogold procedure on ultrathin sections of glutaraldehyde-fixed, usually osmium-postfixed, tissues. In MTC cases, a strong labelling was present over two types of aggregates: one composed of rigid, criss-crossing fibrils 7-10 nm in diameter, suggestive of amyloid, and the other consisting of loosely arranged fibrils, 4-7 nm in width, often wavy or poorly defined. In both cases, the labelling was closely associated with that part of the sectioned fibril exposed to the antiserum. Amorphous material was sometimes present adjacent to the later aggregates, but did not bind the calcitonin antibodies. In contrast, no labelling occurred over the amyloid deposits found in two non-calcitonin-producing endocrine tumours of the pancreas, nor over the cytoskeletal filaments stored in various endocrine polypeptide tumours. The specific value of the labelling for calcitonin-like immunoreactivity was assessed by control tests, such as absorption of the antiserum by excess calcitonin and comparative use of normal serum and antisera directed against human IgG and P component. No immunoreactivity of the MTC amyloid fibrils was found using antibodies directed against katacalcin and human prealbumin. We conclude that in tumour tissues conventionally processed for electron microscopy, MTC amyloid fibrils of varying morphology can be selectively and specifically labelled for calcitonin-like immunoreactivity.