The pharyngeal pouch/cleft interface of transgenic Sox17 zebrafish and mice. (A-D) Confocal images of sox17:gfp zebrafish coronal sections (anterior to the top). (A) The endoderm of all the pharyngeal pouches has pushed out toward the external surface of the embryo. The posterior pharynx is blocked from view because of the still enlarged yolk (orange asterisk). The operculum (pink asterisk) covers the first two posterior arches and its inner lining expresses Sox17. The boxed region is magnified in (C) showing that the pouch (pp5; white arrow) has grown outward past the adjacent pharyngeal arches. (B) Confocal section of a 72 hpf zebrafish reveals the pharyngeal arches (white asterisks) with endoderm contributing to their external lining. The boxed region is magnified in (D) showing the anterior half of the pouch extends over the posterior portion of the anterior adjacent arch. (E-J) Sox17iCre;R26R mice (anterior to the left). (E) A coronal section through the pharyngeal arches at E9.5 reveals the morphology of the pharyngeal pouches. The first pouch (pp1) is separated from the overlying ectoderm by mesenchyme. The second pouch (pp2) interface has thinned so only a narrow band of endoderm connects the second and third arches. The third pouch (pp3) is in contact with the overlying ectoderm and appears to bulge through it, with the ectoderm much thinner than the endoderm. The black box is magnified in (G) and the red box in (I). (F) A coronal section through the arches at E10.5 reveals the morphology of the pouches at this later stage. The first pouch (pp1) is still separate from the cleft. The second pouch (pp2) has now broken through, with endoderm from the anterior border of the pouch beginning to extend posteriorly over the anterior surface of the third arch as the second arch expands caudally. The region inside the black box is magnified in (H). The third pouch (pp3) endoderm has broken through and is now in contact with the external environment. The region in the red box is magnified in (J).