Fig. 5. The developing ventral organ expresses placode-marking gene orthologs. In early hatchlings, a ring of ectodermal cells concentric with the mouth primordium expresses eya (A) and six1/2 (B). (B) Expression of six1/2 in the ventral ectoderm reveals bilaterally paired domains (red asterisk) where the ventral organ develops (insets dorsal view, and ventral superficial view). The epidermis of most the trunk also diffusely expresses six1/2, but this expression is excluded from cells just posterior to the rostral ring. (C) A ring of ectoderm around the mouth also expresses pitx in animals just hatched (inset, superficial ventral view). Cells of the developing mouth itself and the pharynx also express pitx. (D) In mid-hatchling stages, pitx expression continues in the ring, and expands in the anterior pharynx endoderm. Other expression domains include the dorsal brain, the migrating buccal glands, the left side of the stomach, and epidermis of rostral tail fin and trunk/tail junction. Red asterisks in C,D indicate ventral organ primordium. (E,F) In late hatchlings, cells along the rostral nerve trunks express pitx (E, dorsal view; F, left lateral view). A cell (bc) bulging from the left side of rostral-most brain expresses pitx, as do the cells of the rostral pharynx. (G) In late hatchlings, the cells lining the rostral nerves also express six3/6a. The anterior brain expresses six3/6a, as does the pharyngeal roof (see Fig. 4), and the ascending esophageal wall. bg, buccal gland; bc, brain cell; br, brain; cf, ciliary funnel; e, endostyle; ep, epidermal expression; gi, gill pouches; li, lip; ph, pharynx; rn, rostral nerve; st, stomach. Scale bars: 20 µm.