Model of major postulated interactions in the patterning of the somite. A combination of Wnts (probably Wnt1 and Wnt3a) are induced by BMP4 in the dorsal neural tube. These Wnt proteins, in combination with low concentrations of Sonic hedgehog from the notochord and floor plate, induce the epaxial myotome, which synthesizes the myogenic transcription factor Myf5. High concentrations of Sonic hedgehog induce Pax1 expression in those cells fated to become the sclerotome. Certain concentrations of neurotrophin-3 (NT-3) from the dorsal neural tube appear to specify the dermatome, while Wnt proteins from the epidermis, in conjunction with BMP4 and FGF5 from the lateral plate mesoderm, are thought to induce the hypaxial myotome. (After Cossu et al. 1996b.)