Fig. 2.

Fgf10 and Fgf3/SOX3 expression in the forming infundibulum and collar zone. (A-C) Anterior fp subpopulations (A and P) used in targeted fate mapping (A) were explanted and cultured in vitro (B) for 30 hours (E2.5 equivalent). The anterior (prosencephalic neck)-derived population expresses Fgf3 but not Fgf10, whereas the adjacent posterior population-derived cells express Fgf10 but not Fgf3 (C). (D-W) Analysis of infundibulum and collar zone markers in transverse section (D-F,H-L,O-S,V,W), wholemount (G,M,N) or ventral (T,U) views (section levels at specific stages are shown in X, with planes labelled in accordance with panels). (D-F) At E2.5, prosencephalic neck descendants (Dil, red arrows) express Fgf3 (E,F), whereas posterior fp descendants (DiO, green) express Fgf10 (D,F). (G) At E4.5, a semi-ellipse of Fgf3⁺ cells lies between the Fgf10 and Shh domains. (H-L) At E4.5-5, posterior fp descendants (green cells in L and in I inset) express Fgf10, strongly labelled prosencephalic neck descendants (arrows in L and I inset) are located in Fgf3⁺ Sox3/SOX3⁺ regions (arrows, I,J). (M-O) Fgf10 marks the infundibulum at E7, its dorsal boundary at the collar zone (arrows, O). (P) Double labelling shows that expression of Fgf3 and of Fgf10 abut. Arrow indicates collar zone. (Q) Double labelling shows that expression of Sox3 and of Fgf10 abut (single-channel view of Sox3 shown in the right-hand panel). Sox3 is expressed in the collar zone (arrows) and ventral hypothalamus. (R) SOX3 is detected only in the collar zone and immediately adjacent cells (arrow). (S) Comparative analysis shows that strongly labelled prosencephalic neck descendants (white arrow) are located in the SOX3⁺ Fgf3⁺ collar zone and weakly labelled descendants are in adjacent cells in the Fgf10⁺ infundibulum. Coloured arrows in S indicate viewing axes. Comparison of O and S shows that Fgf10⁺ infundibular cells have a dual fp origin. (T) SOX3 in collar zone cells (arrowheads) at E7. (U-W) Co-analysis of SOX3 and TUJ1 in double-labelled wholemount view (U) or serial adjacent sections (V,W). Dotted lines in V and W outline the infundibulum, arrows in V indicate collar zone. TUJ1⁺ nascent neurons lie dorsal to SOX3⁺ collar cells. (X) Schematics (anterior to left) showing the expression domains of Fgf3, SOX3 and Fgf10 at E2.5, E4.5 and E7. Light green represents Fgf10⁺ prosencephalic neck descendants in the a/d infundibulum. D-V, dorsoventral. Scale bars: 50 μm in C-F; 250 μm in G; 60 μm in H-J; 80 μm in K,L; 200 μm in N; 100 μm in O-W.