A systematic anatomical study of 100 adult human thyroids from autopsies was undertaken for the presence of solid cell nests (SCN). SCN were mainly located in the middle third, with a slight tendency to the upper third, of the lateral thyroid lobes, and placed along a central to paracentral and slightly dorsal longitudinal axis. Immunohistochemical studies for thyroglobulin revealed positive staining in foUicular cells connected to SCN and, occasionally, in isolated cells lying within solid clusters from SCN. The anatomical position SCN showed in the present survey is comparable to that shown by the ultimobranchial body (UB) vestiges of human fetuses. The presence of thyroglobulin-positive cells within solid clusters, together with the existence of foUicular cells connected to SCN, suggest that SCN may also be a probable source of foUicular epithelium as occurs with the UB of some mammals.

The ultimobranchial thyroid solid cell nests (SCN), irregular structures of about 1 mm in maximal diameter, are usually found in the middle third of the thyroid lateral lobes. SCN are basically composed of non-keratinizing epidermoid cells which lack intercellular bridges and are immunohistochemically positive for a panel of high and low molecular weight keratin proteins, as well as for carcinoembryonic antigen. In addition, SCN display isolated or grouped peripheral calcitonin-immunoreactive 'clear' (C) cells in up to 54 per cent of cases. The SCN central lumen, when present, is usually surrounded by mucinous cells; in addition, it may contain desquamated cells, cell debris, acid mucosubstances, characteristic PAS-positive granular material after diastase treatment, and colloid-like material. The so-called mixed follicles, structures lined by epidermoid cells of SCN and follicular epithelium, are often found as an additional component of the ultimobranchial remnants. The relationship of SCN to thyroid parenchymal cells and the probable implications of the thyroid 'ultimobranchial system' to tumour histogenesis are analysed. Pitfalls that may emerge with regard to SCN in practical pathological approaches are emphasized.

Human thyroid follicles containing acid mucins have been regarded as a very rare finding and their significance has not yet been clarified. Therefore, a systematic anatomical, histochemical and immunohistochemical survey for the presence of such follicles in human thyroids was undertaken. Follicles with Alcian blue-positive acid mucins were practically confined to the 18% of sections that also contained ultimobranchial solid cell nests. Immunohistochemistry revealed that these follicles were mostly composed of and/or related to the presence of numerous calcitonin-immunoreactive cells, sometimes intermixed with occasional alcianophilic mucinous cells. These findings, with histometrical studies, demonstrate that there exists a relationship between mucinous C cell complexes and mucin/C cell-containing solid cell nests. The finding of calcitonin immunoreactivity in very occasional groups of cells with mucinous changes further suggests that at least some human follicular cells originate in ultimobranchial tissue.