33-hpf wild-type sibling (A) and vgo mutant (B) stained with the antibody Zn-5. Zn-5 stains the endoderm of the pharyngeal pouches (prospective gill slits), as well as sensory neurons and ganglia of the nervous system. In the wild-type embryo five pouches are visible; the most posterior pouches have not formed yet (A). In vgo mutant embryos only the first endodermal pouch is visible and dispersed staining can also be observed ventral to the ear (B, arrow). However, these Zn-5-positive cells do not show any sign of segmental organization. Other endodermal pouch derivatives, such as the thymus, are absent in vgo mutants, as revealed by in situ hybridization with rag-1 (C, D) of 4-dpf embryos. The lack of organization of the endodermal pouches can also be observed in 2-μm-thick horizontal plastic sections of 2-dpf wild-type (F) and vgo (G) embryos. (E) A schematic drawing of a section to show the location of the developing pharyngeal arches that are magnified in (F) and (G). In the wild-type sibling (F) the pharyngeal arches p3–p7 are separated from one another by two rows of endodermal cells (arrows). At this stage, only the endodermal pouch between the second and the third arches has pierced through the ectoderm and thus has formed a gill slit (arrowhead). In the vgo larvae (G) the second arch has formed properly but no organization of the posterior arches can be observed.
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