Fig. 1

Pharyngeal teeth develop from the endoderm lining pouch 6 after pouch 6 has made contact with the ectoderm. (A) Development of the placodes of teeth 4V1 (thick arrows) from the endoderm at 48 hpf; on each body side, the contact point of pouch 6 (P6, thin arrows) with the ectoderm is indicated by asterisks. Posterior to this cross-sectional level, there is no longer contact of endoderm with ectoderm. No lumen is present in the pharynx yet. Arrowheads point to flattened midline cells (encircled by yellow dotted line) squeezed between endodermal layers (surrounded by black dotted line). (B) At 96 hpf, more tooth germs have developed. Pouch 6 has opened into a gill slit anterior to this cross-sectional level (visible on Fig. 1J) and the pharynx is now wide open. (C and D) Tooth germs (4V1, thick arrows) at 56 hpf are derived from sox17-positive endoderm (outlined by a black and white dotted line, respectively). Periderm-like cells are outlined by a yellow dotted line. (E) Magnification of the tooth germ shown in D, indicating that folding into the prospective enamel organ occurs by the basal, sox17-positive, endodermal layer only. White and yellow dotted lines surround endoderm and periderm-like cells, respectively. (F?H) Successive stages in the formation of contact of pouch 6 (outlined by a dotted line) with the ectoderm. (F??H?) Corresponding stages in Tg(sox17:egfp) embryos, immunostained for laminin. Depending on the angle of sectioning, pouch 5 can still be visible in the section along with pouch 6. Note breaking up of the basal lamina at 38 hpf (G?) and basal lamina of pouch 6 continuous with that underlying the ectoderm (H?). Arrowheads point to basal lamina of pouch 6 at its (prospective) contact place with skin ectoderm. (I) Cross-section of 72 hpf embryo at the level of P6 showing ongoing lumen formation (asterisks) (endoderm surrounded by dotted line). (J) At 96 hpf, pouch 6 has opened on either side into a gill slit. Cartoons show representative embryonic and postembryonic developmental stages and level of transverse sections shown in the different figures (boxed areas). Dark gray: cartilage; medium gray: endoderm; light gray: other tissues. Abbreviations: 3V1, 4V1, 5V1: first-generation teeth in positions 3, 4, and 5 of the ventral tooth row; 4V2: second-generation tooth in position 4V (compare ref. 24); b: brain; CB5: ceratobranchial 5; e: eye; nt: notochord; ov: otic vesicle; P6: pouch 6; WT: wild type; y: yolk. (Scale bars in A?D and F?J, 50 µm.), (Scale bar in E, 20 µm.)