Fig. 3
From: Development, functional organization, and evolution of vertebrate axial motor circuits
Diversification of axial MN subtypes in tetrapods. a At rostral cervical levels, HMC-like precursors give rise to phrenic motor column (PMC) neurons through the actions of Hoxa5 and Hoxc5 proteins. The activities of Hox5 proteins are inhibited by Lhx3 in MMC neurons, and Foxp1 in LMC neurons. Hox5 proteins work in conjunction with the Pou domain protein Scip to promote PMC-restricted gene expression. b In the absence of Hox5 genes, PMC neurons are disorganized and revert to an HMC-like state. c Pbx genes are required for the columnar organization of axial MNs. In the absence of Pbx genes, Hox-dependent MN subtypes (LMC and PGC neurons) are lost, and acquire an HMC fate. The remaining HMC and MMC subtypes are disorganized at all spinal levels. d Pbx proteins act in conjunction with other MMC-restricted factors such as Lhx3 to promote MMC specific gene expression