Thyroid follicles with acid mucins in man: a second kind of follicles?

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.