Fig. 4. The FEZ induces outgrowth of an ectopic beak that exhibits dorsoventral polarity. (A) Schematic view of a stage 25 host FNP, indicating that the intersection of the dorsal (green) and ventral (yellow) compartments was the site of outgrowth (red arrow). (B) By stage 36, an egg tooth (*) was evident on the dorsal upper beak, while the ventral surface has no ectodermal elaborations. (C) Sagittal trichrome-stained section through the embryo shown in B demonstrates the dorsoventral relationships among structures in the upper beak. The egg tooth (asterisk) is located on the dorsal surface; beneath it is the premaxillary bone (pmx), and below this is the prenasal process (pnp). The ventral surface of the upper beak (V) lacks any elaborated ectodermal appendages. (D) Schematic view of a stage 25 host after transplantation of quail stage 25 FNP ectoderm (red DV). (E,F) This isotopic transplantation had no effect on facial morphology, despite the fact that the graft was derived from quail. (G) Schematic view of stage 25 host after FEZ transplantation. The graft is positioned in the normal, dorsoventral orientation. The expected outgrowths (arrowheads), with their predicted dorsoventral polarity are indicated. (H) This grafting scenario resulted in an outgrowth from the proximal region of the upper beak and contained two egg teeth. The autochthonous egg tooth (arrowhead) appears normally positioned. (I) The ectopic beak comprised two egg teeth (red asterisk and arrowhead) located above two premaxillae (red pmx) each surrounding a prenasal process (red pnp). These relationships indicated that the ectopic beak had dorsal-ventral/ventral-dorsal polarity (DVVD). The autochthonous beak contained an egg tooth (black asterisk), the premaxilla (black pmx), and the prenasal process (black pnp, dorsal ventral indicated by D and V). (J) Schematic view of stage A 25 host after transplantation of a rotated FEZ. The expected outgrowths (arrowheads), with their predicted dorsoventral polarity are indicated. (K) The ectopic upper beak had a single egg tooth (red asterisk). (L) The presence of a single egg tooth (red asterisk), a single premaxilla (red pmx), and a single prenasal process (red pnp) indicated the ectopic structure had ventral/dorsal polarity. (M) Schematic view of a stage 25 host after isotopic transplantation of a rotated FEZ. The expected outgrowths (arrows) and their predicted dorsoventral polarity are indicated. (N) An ectopic structure was evident and, (O) histological analysis indicated the presence of two egg teeth, a dorsal premaxilla and two prenasal processes. The relative positions of these structures indicated a DV/VDV polarity to the beaks. Scale bars: 1 mm.

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