The contrasting roles of primary cilia and cytonemes in Hh signaling.

Hedgehog (Hh) is a paracrine signaling protein with major roles in development and disease. In vertebrates and invertebrates, Hh signal transduction is carried out almost entirely by evolutionarily conserved components, and in both, intercellular movement of Hh is mediated by cytonemes - specialized filopodia that serve as bridges that bring distant cells into contact. A significant difference is the role of the primary cilium, a slender, tubulin-based protuberance of many vertebrate cells. Although the primary cilium is essential for Hh signaling in cells that have one, most Drosophila cells lack a primary cilium. This perspective addresses the roles of primary cilia and cytonemes, and proposes that for Hh signaling, the role of primary cilia is to provide a specialized hydrophobic environment that hosts lipid-modified Hh and other components of Hh signal transduction after Hh has traveled from elsewhere in the cell. Implicit in this model is the idea that initial binding and uptake of Hh is independent of and segregated from the processes of signal transduction and activation.