Primary cilium--is it an osteocyte's strain-sensing flowmeter?

With few exceptions, the non-cycling cells in a vast range of animals including humans have a non-motile primary cilium that extends from the mother centriole of the pair of centrioles in their centrosomes located between their Golgi apparatuses and nuclei. It has very recently been shown that the primary cilium of a dog or a mouse embryonic kidney cell is a fluid flowmeter studded with heterodimeric complexes of mechanoreceptors linked to Ca(2+)-permeable cation channels that when the cilium is bent can send Ca(2+) signals into the cell and beyond to neighboring cells through gap junctions. More than 30 years ago, osteocytes were reported also to have primary cilia, but this was promptly ignored or forgotten. Osteocytes are the bones' strain sensors, which measure skeletal activity from the effects of currents of extracellular fluid caused by their bones being bent and squeezed during various activities such as walking and running. Since bending a kidney cell's primary cilium can send a Ca(2+) wave surging through itself and its neighbors, the bending of an osteocyte's primary cilium by sloshing extracellular fluid is likely to do the same thing and thus be involved in measuring and responding to bone strain. Copyright 2003 Wiley-Liss, Inc.