C‐cell differentiation in the wall of an aberrant ultimobranchial sinus in the thyroid gland of an old rat

BACKGROUND: In mammals, the thyroid gland possesses two types of endocrine cells, follicular cells and C cells, which have different functions but share a similar endodermal origin (although from different regions of the primitive pharynx). Specifically, follicular cells derive from the ventral pharyngeal floor, while C cells derive from the fourth pair of pharyngeal pouches through the ultimobranchial bodies (UBBs). Disruptions to human midline thyroid morphogenesis are relatively frequent and known as thyroid dysgenesis, which is the leading cause of congenital hypothyroidism. In contrast, fourth branchial apparatus anomalies are very rare clinical entities. OBJECTIVES: The aim of this study was to analyze the morphological features and the immunohistochemical pattern of an aberrant ultimobranchial remnant, align with its persistent contribution to the formation of new C cells. METHODS: The thyroid gland of an old rat was serially sectioned and immunostained for the following markers: calcitonin, thyroglobulin, cytokeratins, PCNA, P63(,) E‐cadherin, beta‐tubulin and CD3. RESULTS: We detected a spontaneous congenital defect in the organogenesis of the UBB in an old rat, giving rise to an ‘ultimobranchial sinus’, which was accompanied by thymic tissue and an abscess. The epithelium contained basal/stem cells and contributed to the formation of abundant C cells and scarce follicular cells. CONCLUSIONS: The ultimobranchial sinus is an exceptional finding for representing the first spontaneous abnormality in the development of UBB reported in rats, and the opportunity to observe sustained C‐cell differentiation from stem cells in an old rat. These findings are consistent with a common origin of both C cells and follicular cells from UBB.