The remnants of ultimobranchial body (UBB) in the thyroid gland of adult buffalo were studied. This remnant occupied a peripheral location beneath the capsule, as well as being embedded mostly in the vascular connective tissue of the thyroid. It demonstrated as microscopic nodules (ultimobranchial nodules, UB nodules), in the vicinity of thyroid capsule, septa or frequently embedded within the thyroid tissue proper. The UB nodules were consisted of various aggregations of striking follicles (ultimobranchial follicles, UBFs), exhibiting unusual shape and size and sometimes exhibited unusual lining epithelium. The UBFs had cystic-, elongated-, tubular-, duct- or labyrinth-like formations of various sizes with irregular outline and more or less folded lumen. This lumen might contain debris of disintegrating cells and/or colloid. Apart from the aggregated UB structures, occasional solitary UBFs or other related derivatives were rarely distributed within the thyroid tissue proper. The UBFs were lined with a single layer of cubical, columnar, flattened or sometimes pseudostratified epithelial formations. Solid cell nests (SCNs), as well as some cystic follicles exhibiting both simple and stratified epithelial variety were also demonstrated. In addition, mixed UB follicles that lined with both simple and stratified epithelium were also revealed. Thyrocalcitonin cells (C-cells), with various cytoplasmic intensities of immunoreactive products against anti-calcitonin human immune serum, were predominantly localized in the wall of UBFs. These cells distributed singly or in groups between the component cellular elements lining of UB structures. They characterized by the presence of fine electron dense membrane-bound granules (150-200 nm) scattered in the cytoplasm together with several mitochondria, small profiles of rough endoplasmic reticulum, as well as few small-flattened cisternae of Golgi apparatus. Scanning electron microscopy revealed that the surface of UB follicular cells was almost flat and polyhedral or hexagonal in shape. Other follicles possessed some cells with dome shaped apical surface demarcated by a shallow intercellular depression. The luminal surface of follicular cell lining was studded with pleomorphic microvilli which were numerous at the intercellular borders. Interestingly, most of UB follicular cells were provided with single cilium, projecting over the cell surface. Notably, some UBFs presented various stages of apocrine activities.
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).