Fig. 2.

Specification of axial MNs in tetrapods and fish. a Specification of early axial MN identities. Graded sonic hedgehog (Shh) acts along the dorso (d)-ventral (v) axis to specify MN progenitors (pMN) and ventral interneuron fates. Graded Wnt signaling promotes sustained expression of Lhx3 in MMC neurons, while Hox signaling specifies segmentally-restricted MN columnar fates, including limb-innervating lateral motor column (LMC) neurons. b Axial MNs in tetrapods can be defined by expression of specific transcription factors. MMC neurons express Fgr1 and are attracted to mesodermally-derived FGF signaling. c Primary MNs in zebrafish. Four distinct axial MN types can be defined by their rostrocaudal position and muscle target specificity. dRoP, dorsal rostral primary; vRoP, ventral rostral primary; CaP, caudal primary; MiP, middle primary MN