THYROID MEDULLARY CARCINOMA WITH THYROGLOBULIN IMMUNOREACTIVITY IN SPORADIC MULTIPLE ENDOCRINE NEOPLASIA TYPE 2-B

Background. Thyroid carcinomas historically have been divided into two groups according to their presumedly separate embryonic origins: thos e of neuroectodermal derivation (parafollicular or medullary carcinoma [MCT]) and those of foregut endodermal origin (follicular and papilla ry carcinomas). The validity of this concept has been questioned by th e recognition that some MCT may show immunocytochemical and ultrastruc tural evidence of follicular components, and display features of folli cular function (e.g., organification of iodine, immunoreactivity for t hyroglobulin). Methods. A 14-year-old boy presented with the physical features of multiple endocrine neoplasia type 2-B (MEN 2B) and a thyro id mass. His thyroid lesion was studied by light microscopy; electron microscopy; immunohistochemistry using antisera to calcitonin, thyrogl obulin, and other peptides; and in situ hybridization. Results. The tu mor was identified as an MCT by light microscopy. It stained positivel y with calcitonin, thyroglobulin, chromogranin, neuron-specific enolas e, and serotonin. At the ultrastructural level, the tumor cells contai ned numerous neurosecretory granules and showed evidence of follicular differentiation (luminal microvilli, follicle formation, and tight ju nctions), suggesting a dual neuroendocrine and follicular differentiat ion. Conclusions. The morphologic findings suggest that a small number of MCTs arise from a common stem cell (possibly the ultimobranchial b ody) that may give rise to both MCT and follicular carcinoma. This pat ient and patients in similar cases documented in the literature challe nge the classic concept of separate pathways of embryogenesis for thes e two cell types.