The ultrastructural features of solid cell nests (SCN), made of squamous cells, and associated calcitonin cells (C cells), of the thyroid gland were studied in only a few cases in humans. A study was performed on 8 paraffin-embedded SCN, postembedded in Epon, to look for their ultrastructural features. Immunohistochemical analysis using calcitonin antibody was performed on semithin sections of SCN to explore the presence of C cells. Three cases (37.5%) of SCN were positive for calcitonin, and electron-dense secretory granules were observed in the cytoplasm. In two of these cases, an increased number of C cells in the adjacent thyroid parenchyma was observed. The presence of ciliated and lymphoid cells, in addition to intracytoplasmic microvacuolar and microfollicular (microglandular) structures, was noticed. Ciliated cells have already been reported in embryonic rests of human and animals, but ultrastructurally for the first time in human SCN. The presence of microfollicular structures, intracytoplasmic microvacuolar, secretory granules features, and ciliated cells, in addition to lymphoid cell, suggests the existence of a common ultimobranchial stem cell for C cells or for one or more cell types of the thyroid gland.
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