Arch Pou3f3 expression correlates with gill cover formation. (A) Vertebrate phylogeny highlighting the gill cover type of each lineage. (B–F and B′–F′) In situ hybridizations at two stages for Pou3f3 homologs in lamprey, little skate, elephant fish, zebrafish, and mouse show conserved expression in the CNS but divergent patterns in the pharyngeal arches. Note the later suppression of Pou3f3 in the posterior arches of the elephant fish. The dotted lines in D′ outline the first and second arches. (Scale bars, 250 μm.) (G) Schematized frontal sections through the pharyngeal arches (numbered) at early (Top) and late (Bottom) stages, with arch Pou3f3 expression coded in purple, PEMs in red, neural crest-derived mesenchyme in gray, endoderm in yellow, and ectoderm in blue. E, embryonic day; st, stage.
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