Fig. 1. Defining morphogenetic movements and molecular boundaries in the upper beak. (A) A stage 20 embryo illustrating four primary sites of retroviral application. Broken red lines trace the border of the FNP and maxillary processes, and colored dots correspond to injections sites shown in B-E. (B) Infected cells were visualized after histochemical detection of alkaline phosphatase activity (blue precipitate). A frontal view of a stage 30 embryo showed that injections at site B labeled a patch of ectodermal cells that extended along the dorsal aspect of the FNP. The egg tooth is indicated by an asterisk. (C) Frontal view of a stage 28 embryo showed that injections at site C labeled ectodermal cells on the dorsal margin of the upper beak. (D) Frontal view of a stage 30 embryo showed that injections at site D produced labeled cells that resided along the dorsal margin of the upper beak. (E) Ventral view of a stage 30 upper beak, which demonstrated that injections at site E resulted in labeled cells along the midline of the ventral upper beak. The broken red line indicates the dorsoventral boundary, (F) which was demarcated by the Shh expression domain. (G) Whole-mount in situ hybridization demonstrated that by stage 20, Shh was expressed in ventral FNP ectoderm (V); transcripts were also evident in hyoid arch (hy) ectoderm. (H) Also at stage 20, Fgf8 was expressed in dorsal FNP ectoderm (D); transcripts were detectable around the nasal pit (np), in the maxilla (mx) and mandible (mn), and on the posterior aspect of the hyoid arch. (I) The juxtaposed (red arrowhead), but not overlapping boundary of Fgf8 (transcripts pseudocolored green) in dorsal FNP (D) and Shh (yellow) in ventral ectoderm (V) was confirmed by section in situ hybridization. ps, palatal shelf; pe, pharyngeal endoderm; rp, Rathke's pouch; di, diencephalon; or, optic recess; t, telencephalon. Scale bar: in B, 1 mm for B-F; in I, 0.5 mm.