Classic transmission electron micrograph of mouse oviduct cilia. Cross-sections show the 9 + 2 axoneme of motile cilia (asterisk). The axoneme grows from a basal body, with a basal foot (arrowhead) pointing in the direction of the effective stroke. The transition zone between basal body and axoneme contains the ciliary necklace (arrow). (From Dirksen and Satir 1972, unpublished, with permission)

Primary cilia of human embryonic stem cells. Immunofluoresence microscopy using acetylated α tubulin antibody (tb) reveals the presence of primary cilia (arrows) on human embryonic stem cells. In the absence of stimulation, the hedgehog receptor ‘patched’ (Ptc) colocalizes with the acetylated α tubulin all along the ciliary membrane. Red and green channels are displaced in the images to define colocalization more clearly. Nuclei are stained with DAPI (blue). Upon stimulation, as part of the signaling cascade, Ptc leaves the cilium and the smoothened receptor (Smo) enters to activate the hedgehog signaling cascade. Asterisk marks the ciliary base. (From Kiprilov et al. 2008, with permission, courtesy of The Journal of Cell Biology)