The primary cilium, a solitary non-motile structure projecting from the centriole of mammalian cells, has been a subject of interest of anatomists since 1898. Surprisingly, the function of the primary cilium in mammalian cells is completely unknown. Virtually all of the epithelial cells of the mammalian kidney, with the exception of the intercalated cells of the collecting duct, express a single primary cilium on their apical (lumenal) surface. These structures were ignored by physiologists until recently when the mechanical properties of the primary cilia of cultured cells of renal origin were measured and it was proposed that they could serve as flow sensors. The proposal stemmed from the observation that flow rates comparable to those observed in renal tubules resulted in a deflection of the cilium of a few microns and that the stiffness of the cilium was commensurate with mechanosensing.

The primary cilia of renal tubular cells are generally about 2 - 3 μm long whereas those of cultured renal cells become much longer as the cells age and may extend 50 μm or more into the medium. MDCK cells, a cultured cell line derived from the collecting duct of the canine kidney, exhibit two cell types analogous to the principal and intercalated cells of the collecting duct. MDCK principal cells express a primary cilium that is visible by high-resolution DIC microscopy. About one week after splitting, the primary cilium of MDCK principal cells is about 8 μm long and is well suited for testing the hypothesis that the cilium serves as a mechanosensor of fluid flow.