Most cells, osteoblasts, and osteocytes included (Ilaria del Pra, Ubaldo Armato and your author, as yet unpublished observations) appear to have a strain-sensing device known as the primary or (solitary) cilium that extends into the extracellular fluid and matrix from the mother-daughter pair of centrioles in a cell's microtubule-organizing centrosome lying beside the nucleus. A striking example of one of these cilia is illustrated here. This one, which belongs to a Ptk kangaroo rat's kidney epithelial cell, looks a lot like a car aerial and is at least the cell's fluid flowmeter, but it might also be equipped with receptors to detect various hormones and other things flowing by in the fluid. The bending back and forth of such a cilium by sloshing extracellular fluid, for example, perhaps in an osteocyte's lacuna (Whitfield, 2003a), sends a stream of signals into and through gap junctionally connected cells like flashes of light through a fiberoptic network (Nauli et al., 2003; Praetorius and Spring, 2001, 2003; see also (fig. 6). The frequency and intensity of the signal pulses depend on the frequency and extent of bending of the cilium. This photograph was generously sent to me by Dr. Sam Bowser of the Wadsworth Center, Albany, NY.