The mammalian thyroid gland contains a secondary structure, the ultimobranchial body remnants (UBBRs), which is incorporated into thyroid parenchyma and remains there as scattered parafollicular and interfollicular calcitonin secreting cells or gathered into follicle-like structures and cellular clusters, termed solid cell nests (SCNs) and cystic cell nests (CCNs). The UBBRs of present study have been revealed as ectopic structures of various forms and sizes alongside with the usual thyroid follicles which demonstrated in form of large follicles or cysts, large ducts or branching tubules, irregular or labyrinth-like follicles locating under the capsule or deeply situated in the interstitial connective tissue of donkey's thyroid gland. In addition, several small secondary or differentiating follicles, epithelial outgrowths, as well as various cellular aggregates that described in form of SCNs or CCNs were also demonstrated in close vicinity with the aforementioned structures. Mixed ultimobranchial follicles (UBFs) might be found to be admixed with SCNs which composed of solid cell nests cells and follicular cells, forming lumen-like pattern were also demonstrated. The lining epithelium of UBFs was mostly cuboidal in appearance, but sometimes varied from squamous to pseudostratified or stratified in form. The cellular components of UBFs were consisted of usual follicular cells and clear or calcitonin cells. Meanwhile, the SCNs were comprised of main polygonal cells with eosinophilic cytoplasm and large sized calcitonin cells (C-cells) with pale staining cytoplasm which accounts a minor cellular proportion. Surface of most ultimobranchial (UB) follicular cells was almost hexagonal in shape, studded with sparse or dense pleomorphic microvilli, which were denser at the cellular borders. Each follicular cell bore single cilium that projecting over the cell surface. Interestingly, bleb-like apocrine protrusion of the apical cytoplasm of some UB follicular cells was frequently demonstrated. These apocrine blebs (aposomes) were varied in shape, size and showed smooth or irregular surface. In addition to such aposomes, deep to shallow circumscribed or eroded areas of the apical cell surface was frequently demonstrated. These features indicate that the UB follicular cells in donkeys are similar to those of buffaloes and exhibit an apocrine activity alongside with the usual merocrine mode of secretion. In conclusion, although minor in mass, UBB contribution to the thyroid is important because this structure has been cited as the source of C-cells in mammals including human. Despite the biologic significance of UBBRs remains disputable, the heterogenous expression of antigens in SCNs has indicated that the cells comprising these structures have different biologic functions; at least some C-cells and follicular cells would originate in SCNs. Furthermore, it has been suggested that these remnants might constitute the origin of some ectopic structures described in the thyroid gland including some types of thyroid neoplasms.
Abbreviations used in this study: APUD cells, amine precursor uptake and decarboxylation cells; C-cells, Calcitonin cells; CCN(s), cystic cell nest(s); SCN(s), Solid cell nest(s); UB, ultimobranchial; UBB, ultimobranchial body; UBF(s), ultimobranchial follicle(s); UBBR(s), ultimobranchial body remnant(s).