Structure and motility of primary cilia in the follicular epithelium of the human thyroid.

In order to clarify contradictory reports concerning ciliary structure and function, follicular epithelium from macroscopically normal portions of 37 surgical specimens of human thyroid were processed for video-microscopy and/or transmission electron microscopy. The cilia of living cells were immotile. In transverse sections the cilia revealed a 9 + 0 pattern at the base of the shaft, whereas towards the distal end the number of microtubular doublets diminished. Dynein arms, radial spokes and central microtubules were absent. The immotility and structure of these primary cilia implies that their function is not related to motility. The phylogenetic and ontogenetic development of the thyroid suggests that tumor cells of follicular origin displaying abnormal secondary cilia may represent a pathological variant of differentiation.