The chondrocyte primary cilium

The presence and role of primary, or non-motile, cilia on chondrocytes has confused cartilage researchers for decades. Initial explanations attributed a vestigial nature to chondrocyte cilia. Evidence is now emerging that supports the role of the chondrocyte primary cilium as a sensory organelle, in particular, in mechanotransduction and as a compartment for signaling pathways. Early electron microscopy images depicted bent cilia aligned with the extracellular matrix (ECM) in a manner that suggested a response to mechanical forces. Molecules known to be mechanotransducers in other cell types, including integrins and proteoglycans, are present on chondrocyte cilia. Further, chondrocytes which lack cilia fail to respond to mechanical forces in the same manner that chondrocytes with intact cilia respond. From a clinical perspective, chondrocytes from osteoarthritic (OA) cartilage have cilia with different characteristics than cilia found on chondrocytes from healthy cartilage.This review examines the evidence supporting the function of chondrocyte cilia and briefly speculates on the involvement of intraflagellar transport (IFT) in the signaling pathway of mechanotransduction through the cilium.Emerging evidence suggests cilia may be a promising target for preventing and treating OA.