(A, F) SEM images, dorsal view of bichir embryos with developing external gills (exg), showing the level of virtual sections in B and G. Notice the correspondence of the hyoid pharyngeal endoderm (B, G) and the external gills (A, F). (B–D, G–H) 3D models of pharyngeal endoderm (yellow) from dorsal (C, H), and lateral (D, I) view, respectively. (E, J) Transversal sections show prominent lateral expansion of hyoid pharyngeal endoderm (white arrow). (K–M) Transversal sections show wild-type expression of Fgf8, Pea3, and Dusp6 (black arrow) in the external gills primordium. (N) Immunostaining of anti-activated MAP kinase antibody on transversal section of the external gills primordium. (O) Scheme summarizing Fgf8, Dusp6, and Pea3 (K–M) expression patterns in the external gills formation at stage 26. Violet indicates Fgf8 expression; blue marks Dusp6 expression in the endoderm and adjacent mesenchyme of the external gills; yellow depicts expression of Pea3 in the mesechyme of the external gills. (P–U) Inhibition of pouch-like endodermal outpocketings (P, R, T–U), dorsal view. (P–Q) Control larvae treated with DMSO develop normal pouch-like endodermal outgrowths (white arrow). (R) Larvae exposed to SU5402 from stage 20 till stage 26. (S) Transversal section shows loss of external gill anlagen. (T–U) SU5402 treated larvae fixed at stage 26 and probed for Pea3 (T) and Dusp6 (U). Nuclei are stained with DAPI (blue), basal laminae with anti-fibronectin (red). White arrowheads mark spiraculum (hyomandibular cleft) and black arrowhead marks hyo-branchial pouch. Asterisk, cement gland; b, brain; green, otic vesicle; e, eye primordium; nt, notochord; ot, otic vesicle; ph, pharynx.