Profiles of <i>Hox</i> Gene Expression, dMNs, and vMNs in the Hindbrain and Spinal Cord

<div><p>(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.</p> <p>(B) In a stage 17 (30-somite) chick embryo, the rostral borders of <i>Hoxb4, Hoxb8,</i> and <i>Hoxc9</i> 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. <i>Hoxb4</i> expression in the absence of <i>Hoxb8</i> and <i>Hoxc9</i> is characteristic of cells in the cHB (r7 and r8). Expression of <i>Hoxb4</i> and <i>Hoxb8</i> in the absence of <i>Hoxc9</i> is characteristic of the rSC domain, and expression of <i>Hoxb4, Hoxb8, and Hoxc9</i> is characteristic of cells in the cSC domain. </p> <p>(C) In stage 20 (42-somite) chick embryos, the rostral boundaries of expression of <i>Hoxb4, Hoxb8,</i> and <i>Hoxc9</i> in the neural tube are maintained as in a stage 17 embryo (± 1-somite). <i>Tbx20⁺</i>/Isl⁺ dMNs are present at high numbers in the cHB and at lower numbers in the rSC. No <i>Tbx20⁺</i>/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. </p> <p>(D) Horizontal bars represent rostrocaudal restrictions (applied to <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.0040252#pbio-0040252-g001" target="_blank">Figure 1</a>A) of marker genes expressed by neural progenitor cells in the stage 17 neural tube, and by MNs in stage 20 embryos. </p>