Fig. 5

Periderm partly covers odontogenic epithelia in sterlet sturgeon, but not in axolotl. Sagittal and cross sections (histology and lineage tracing with CDCFDA, resp.) of the four first tooth fields that develop in sturgeon: dermopalatine (A–A‴), dentary (B–B‴), palatopterygoid (C–C‴) and hypobranchial 1 (D–D‴) tooth fields. Arrows indicate tooth germs in the respective fields. Note that the epithelium of the oral (i.e., dermopalatine and dentary) teeth is covered with labeled periderm, but not the epithelium covering the palatopterygoid or hypobranchial 1 tooth germs, which are positioned deeper within the oropharynx. The low magnification of the hypobranchial tooth germs in cross sections (D″, D‴) clearly shows the difference between unlabeled odontogenic epithelium and strong labeling of the periderm in the skin (double arrow), 7 days after labeling. In axolotl embryos, teeth are restricted to oral and palatal fields (there are no pharyngeal teeth). Oral teeth (arrows) develop in the absence of periderm, which is still covering the body surface (E–E‴). Labeled periderm is present around the outgrowing external gills and in the distalmost part of the pouches (F–F‴). asterisks oral cavity, b brain, en endoderm, g external gill, ov otic vesicle, P2 pouch 2. In all sagittal sections, anterior is to the left. Scale bars in A–D′ = 100 µm; in E–F′ = 500 µm; in D″, D‴, E″, E‴, F″, F‴ = 200 µm