CoachA eukaryotic cilium or flagellum is a membrane-covered extension built on the axoneme: nine doublet microtubules in a ring around a central pair, held by nexin links and radial spokes, and grown from a basal body, a modified centriole. Axonemal dyneins attached to each doublet walk along the neighbouring doublet toward its minus end at the base; since the doublets are tied together they cannot slide freely, and the sliding is converted into bending, propagated along the axoneme as a wave by switching the active dyneins from one side of the ring to the other. A cilium is long and beats times a second; a sperm flagellum is and undulates. The proteins of a cilium are carried up from the cell body by kinesin-2 and back by dynein-2 along the doublets, intraflagellar transport, which is also how a non-motile primary cilium — present, one per cell, on most vertebrate cells — is built; that cilium is a sensory antenna, housing receptors for light (the rod outer segment), odours, flow and developmental signals, and its defects cause the ciliopathies: polycystic kidneys, retinal degeneration, obesity, extra digits.
