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Fig. 5 Germ layer contribution to oropharyngeal epithelium and teeth in zebrafish, Danio rerio. (A–C) Sagittal section of a double-transgenic (krt4:tomatoCAAX;sox17:egfp) 55 h post-fertilisation (hpf) embryo showing periderm having partially invaded pouch 2 (intense orange), and connecting to periderm-like cells investing the oropharynx (red, indicated by white arrowheads, shown also in E); boundary indicated by a small white arrow; both populations do not show further mixing. Micrographs are shown in the red (A) and green channel (B), and as overlay (C). Dotted line in C marks the outline of pouch 2. Scale bar for A–C = 20 μm. (D) Magnification of the pharynx lining shown in C; periderm-like cells (arrowheads) express both krt4 and sox17 and cover endoderm, in which sox17 is downregulated (asterisks). Scale bar = 20 μm. (E) Sagittal section of a transgenic (krt4:tomatoCAAX) 55 hpf embryo showing periderm-like layer (arrowhead). A lumen has appeared rostrally between the two periderm-like layers (asterisk). Scale bar = 20 μm. (F) Cross section of a 60 hpf Tg(krt4:gfp) embryo at the level of the forming pharyngeal dentition; periderm-like layer indicated by an arrowhead. Scale bar = 100 μm. (G) Magnification of F, showing the placode of the first pharyngeal tooth (arrow). This develops from the unlabelled basal (endodermal) layer (large asterisk), while being covered by a krt4-positive superficial layer (arrowhead). A dotted line delimits the pharyngeal epithelium; a lumen starts to appear between the two layers of periderm-like cells (small asterisk). Scale bar = 10 μm. (H) Toluidine blue-stained semithin cross section, showing placode stage of first pharyngeal tooth formation corresponding to G (tooth placode indicated by a dotted outline). Note superficial periderm-like layer (arrowhead) with forming pharyngeal lumen (asterisk). Laterally, the pouch (pouch 6, P6) is still bilayered. Scale bar = 10 μm.