Hedgehog signalling in development of the secondary palate.

The Sonic hedgehog (Shh) gene encodes a secreted signalling molecule that plays an important role during numerous aspects of vertebrate development. In the developing palate, Shh is strongly expressed in the epithelium on the oral surface, in a series of stripes corresponding to the future rugae palatini. There is now good evidence that Shh is involved in a number of signalling interactions that take place between the epithelium and mesenchyme during normal palatogenesis. In particular, being able to induce Fgf10 in mesenchyme of the anterior palate which, via Fgfr2, is able to induce Shh in the epithelium. These interactions are essential for normal growth and development of this region; in the absence of normal Shh signalling, mice develop a cleft of the secondary palate. Growth and patterning of the secondary palate are closely linked, with successive rugae forming within a mid-palatal growth zone. Shh also plays a key role during this early patterning process, along the anteroposterior axis of the secondary palate. Specifically, acting as an inhibitor within a reaction-diffusion mechanism that is responsible for establishing primary architecture of the rugae.