Schematic of successional dental epithelia in different tooth regeneration systems. Each of the four columns of illustrations represents an idealized transverse cross-section of a tooth position from each of the polyphydont vertebrate groups indicated at the top. Each row of illustrations shows a different phase of the regeneration process as indicated on the left. Mineralized tissues are shown in red, and epithelia are shown in yellow. The continuous basalmost layer of epithelium that abuts the basement membrane is additionally delineated with a dotted line. Arrows indicate tooth germs, arrowheads indicate the successional dental epithelium (SDE) in each species as suggested by our model. The epithelia are depicted slightly off-plane (shallower on the z-plane) with respect to the erupted tooth (analogous to viewing a thick section). The left two columns (stickleback and salmonids) illustrate species that lack a successional dental lamina (SDL), whereas the right two columns illustrate species that do have an SDL; the zebrafish, which exhibits a transient SDL (note lack of SDL in the bottom row) vs. salamanders, which maintain a permanent SDL (arrowhead in bottom row). The stickleback drawings are representative of both oral and pharyngeal tooth regeneration and based on our own observations. The zebrafish pharyngeal tooth drawings are based on our own observations in conjunction with previous literature [42, 69]. The salmonid drawings are based on data from rainbow trout [47, 49] and Atlantic salmon [48, 136], and are based primarily on oral tooth histology (though pharyngeal teeth are described as behaving similarly in rainbow trout ). The salamander oral tooth drawings are based on data from the Iberian ribbed newt [137, 138] and axolotl .