Figure 1. Profiles of Hox Gene Expression, dMNs, and vMNs in the Hindbrain and Spinal Cord .

(A) Schematic figure of the caudal embryonic neural tube divided into four distinct regions along the rostrocaudal axis: rHB; r7 and r8 defined as cHB; the region of the spinal cord located at the level of somite 6–19 defined as the rSC; and the region located caudal to somite 19 defined as cSC.

(B) In a stage 17 (30-somite) chick embryo, the rostral borders of Hoxb4, Hoxb8, and Hoxc9 expression are located just caudal to the otic vesicle (OV) (at the level of the r6/7 border), at the level of somite 5/6, and at the level of somite 19/20, respectively. Hoxb4 expression in the absence of Hoxb8 and Hoxc9 is characteristic of cells in the cHB (r7 and r8). Expression of Hoxb4 and Hoxb8 in the absence of Hoxc9 is characteristic of the rSC domain, and expression of Hoxb4, Hoxb8, and Hoxc9 is characteristic of cells in the cSC domain.

(C) In stage 20 (42-somite) chick embryos, the rostral boundaries of expression of Hoxb4, Hoxb8, and Hoxc9 in the neural tube are maintained as in a stage 17 embryo (± 1-somite). Tbx20 ⁺/Isl ⁺ dMNs are present at high numbers in the cHB and at lower numbers in the rSC. No Tbx20 ⁺/Isl ⁺ dMNs are found in the cSC. In contrast, Hb9 ⁺/Isl ⁺ vMNs are present at high numbers in both rSC and cSC, and at lower numbers in r8 of the cHB. Hoxc9 protein is expressed in a subset of Hb9 ⁺/Isl ⁺ vMNs in the cSC and thus, distinguishes vMNs in the cSC from vMNs in the rSC.

(D) Horizontal bars represent rostrocaudal restrictions (applied to Figure 1A) of marker genes expressed by neural progenitor cells in the stage 17 neural tube, and by MNs in stage 20 embryos.