Ultrastructural features of "solid cell nest" of the human thyroid gland: A study of 8 cases

The ultrastructural features of solid cell nests (SCN), made of squamous ce lls, and associated calcitonin cells (C cells), of the thyroid gland were s tudied in only a few cases in humans. A study was performed on 8 paraffin-e mbedded SCN, postembedded in Epon, to look for their ultastructural feature s. Immunohistochemical analysis using calcitonin antibody was performed on semithin sections of SCN to explore the presence of C cells. Three cases (3 7.5%) of SCN were positive for calcitonin, and electron-dense secretory gra nules were observed in the cytoplasm. In two of these cases, an increased n umber of C cells in the adjacent thyroid parenchyma was observed. The prese nce of ciliated and lymphoid cells, in addition to intracytoplasmic microva cuolar and microfollicular (microglandular) structures, was noticed. Ciliat ed cells have already been reported in embryonic rests of human and animals , but ultrastructurally for the first time in human SCN. The presence of mi crofollicular structures, intracytoplasmic microvacuolar, secretory granule s 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 m ore cell types of the thyroid gland.