The pipe and the pinwheel: is pressure an effective stimulus for the 9+0 primary cilium?

Almost universally, the effective stimulus for mammalian 9+0 primary cilia has been taken to be bending. In this article I point out that in several physiological contexts there is great advantage in detecting pressure differences across the cell wall, i.e. axially directed forces pushing fluid to and fro through the hollow cilium and its basal body beneath. The form of the cilium--a fluid-filled pipe that connects to an intricate pinwheel-shaped basal body--is well configured for detecting fluid flow. Pressure-detection calls for compressible elements within the cell, but it effectively matches form and function in a range of cases. The "pipe and pinwheel" scheme suggests that the bulbous swellings commonly found near the tip of some primary cilia are compliant, pressure-sensitive elements which act like the bulb of an eye-dropper. In looking exclusively at the bending of cilia, we might be missing aspects of a dual-stimulus system.