Figure 2.

Molecular specification of dorso-ventral neuronal cell fate in the hindbrain. (A) Schematic diagram showing the DV organization of a rhombomere from dorsal to ventral: roof plate (RP), dorsal (alar) and ventral (basal) subdomains, floor plate (FP) and notochord (NC). Each domain is formed by progenitors in the ventricular zone and by postmitotic neurons in the mantle zone. (B) Two opposite morphogen gradients of Wnt and BMP (dorsalizing signals) produced by the overlying ectoderm and roof plate, and Shh (ventralizing signal) produced by the notochord and floor plate control the expression of bHLH and HD proteins along the DV axis establishing distinct progenitor domains and directing the differentiation of neuronal subtypes. In the alar plate, dA1-dB4 classes of dorsal neurons are subdivided into class A and class B neurons, respectively repressed or activated by Lbx1 (in red). In the basal plate, HD class I genes are repressed and HD class II genes are activated by Shh signaling, and cross-repressive interactions between class I and class II proteins refine and maintain progenitor domains and determine interneuron (V0-V2), somatic (MNs), and visceral (MNv) motor neuron differentiation. Modified from Takahashi and Osumi (2002), Melton et al. (2004), and Storm et al. (2009). (C) Rhombomeric mapping of hindbrain DV domains, modified from Gray (2008). The DV domains extend differently across rhombomeres along the AP axis.