Compact buds with biphasic differentiation and calcitonin-expressing neuroendocrine cells—previously unrecognized structures of thyroglossal duct unveiled by immunohistochemistry

Immunophenotype of thyroglossal duct (TGD) cysts, including lining epithelium and thyroid remnants, is scarcely addressed in the literature. There is indirect evidence that C cells may be derived from progenitor cells of the midline thyroid primordium. This is supported by the recent concept of the endodermal origin of lateral thyroid anlagen and several case reports. We aimed to search for neuroendocrine cells in TGD cysts and to characterize immunophenotype of the thyroid follicles and epithelial lining of TGD. Out of 98 TGD cysts, 70% contained both cyst-lining epithelium and thyroid follicles, whereas 30% possessed only cyst-lining epithelium. Specimens eligible for immunohistochemistry (n = 61) were stained for thyroid-specific and neuroendocrine markers. Thyroid remnants were positive for thyroid transcription factor 1 (TTF-1) and other thyroid tissue-specific markers and negative for calcitonin. TGD epithelium showed strong p63 positivity. We found that respiratory epithelium in 9.8% of TGDs contained neuroendocrine cells positive for calcitonin, chromogranin A, and synaptophysin but negative for carcinoembryonic antigen. In 44.2% of the cases, we detected compact buds, microscopic structures appearing as nests of epithelial cells with a biphasic population of basal (p63+) and central (TTF-1+) cells. Thyroid remnants in TGD expressed full spectrum of thyroid-specific markers and contained no C cells. Instead, calcitonin-expressing neuroendocrine cells were found among the respiratory epithelium of TGD. These cells can be a potential source of neuroendocrine tumors mimicking medullary carcinoma in median anlage derivatives. We also discovered precursor compact buds with dual immunophenotype and proposed a concept of their morphogenesis.