Characterization of primary cilia in different epithelial cells of thyroid gland.

The primary cilium (PC) is a biosensor with diverse functions, depending on cellular type. In the thyroid, where it was first described, PCs are located at the apical pole of the follicular epithelium, sensing the lumen's environment. They probably contribute to follicular homeostasis, although their presence in other thyroid epithelial cells remains unclear. Thyroglobulin, stored in the lumen as colloid, is the primary regulator of thyroid-specific gene expression under constant TSH levels. The mechanism by which thyroglobulin signal is transduced remains unresolved. This study investigates the evolution of PCs in different types of thyroid follicles, based on their functional activity, using both normal human thyroids and functional thyroid pathologies as models. It also compares PC morphology between human and rat thyrocytes and explores their presence in other thyroid epithelial components such as C cells and ultimobranchial remnants. Human and Wistar rat thyroid tissues were analyzed using histological, immunohistochemical, immunofluorescence, and electron microscopy techniques. Morphometric analyses quantified PC changes (frequency and length) in various follicular patterns, and statistical analyses were performed. Four types of thyroid follicles were identified: active, hyperactive, hypoactive, and empty follicles. PCs were most abundant and longest in active and significantly reduced in empty follicles. PCs were more prominent in human than in rat thyrocytes, present in both normal and neoplastic C cells, but sporadic in ultimobranchial remnants. PCs vary according to follicular activity and likely act as mechanosensors in thyroid hormone regulation, detecting colloid density and contributing to the regulation of thyroid hormone biosynthesis.