Topographic organization of myogenesis and nerve pattern in the CM muscle.

<p><b>(A)</b> Scheme representing the shape of the CM muscle seen from the side of an embryo, featuring the area covered by <i>Gdnf</i>^<i>LacZ+</i> muscle progenitors in purple, <i>MLC3F-2E</i>⁺ muscle fibers represented in blue, and CM-innervating axons represented in green, indicating (vertical lines) the level corresponding to sections shown in <b>(B)</b> and <b>(C)</b>. <b>(B)</b> Cross section of an E12.5 <i>Gdnf</i>^{<i>LacZ/+</i>} embryo, at the middle CM level, immunostained with antibodies to Pax7 (red) and β-galactosidase (green), showing that <i>Gdnf</i>^<i>LacZ</i> is expressed in Pax7⁺ progenitors. <b>(C)</b> Cross sections of an E12.5 <i>Gdnf</i>^{<i>LacZ/+</i>}<i>; Etv4-GFP</i>⁺ embryo at the anterior CM (top pictures), middle CM (middle row), and posterior CM (bottom row) levels and immunostained with antibodies to Pax7 (red), β-galactosidase (red), Myh1 (green), and GFP (green) and with DAPI (blue). At each level, three neighboring sections of the same embryo were used with the indicated antibody combinations. In <b>(C)</b>, inserts show high magnifications of the area highlighted with the yellow dotted square. Scale bars: low-magnification pictures in C: 200 μm; inserts in C: 40 μm; high-magnification (right) pictures in B: 40 μm; low-magnification pictures (left) in B: 200 μm. ant, anterior CM level; β-Gal, β-galactosidase; CM, cutaneous maximus; C5 to C8, Cervical levels 5 to 8; <i>Etv4</i>, Ets variant gene 4; <i>Etv4-GFP</i>, transgenic line in which expression of an Etv4-GFP fusion protein is driven by the <i>Etv4</i> locus (<a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.2004734#pbio.2004734.s002" target="_blank">S1 Table</a>); GFP, green fluorescent protein; mid, middle CM level; Myh1, myosin heavy chain 1; Pax7, paired box 7; post, posterior CM level; T1 to T3, Thoracic levels 1 to 3.