Getting out of an egg: Merging of tooth germs to create an egg tooth in the snake

Abstract

Background:The egg tooth is a vital structure allowing hatchlings to escape from the egg. In squamates (snakes and lizards), the egg tooth is a real tooth that develops within the oral cavity at the top of the upper jaw. Most squamates have a single large midline egg tooth at hatching, but a few families, such as Gekkonidae, have two egg teeth. In snakes the egg tooth is significantly larger than the rest of the dentition and is one of the first teeth to develop. Results: We follow the development of the egg tooth in four snake species and show that the single egg tooth is formed by two tooth germs. These two tooth germs are united at the midline and grow together to produce a single tooth. In culture, this merging can be perturbed to give rise to separate smaller teeth, confirming the potential of the developing egg tooth to form two teeth. Conclusions: Our data agrees with previous hypotheses that during evolution one potential mechanism to generate a large tooth is through congrescence of multiple tooth germs and suggests that the ancestors of snakes could have had two egg teeth. K E Y W O R D Scongrescence theory, dentition, egg tooth, fusion, snake Juan M. Fons and Marcia Gaete contributed equally.

Do all geckos hatch in the same way? Histological and 3D studies of egg tooth morphogenesis in the geckos Eublepharis macularius Blyth 1854 and Lepidodactylus lugubris Duméril & Bibron 1836

“…Scale bars indicate 100 (a), 50 (b), 100 (c), and 50 μm (d). AB, ameloblasts; BV, blood vessel; D, dentin; DP, dental pulp; EO, enamel organ; ET, egg tooth; Mes, mesenchyme; NC, nasal capsule; PMX, premaxilla; white arrow, melanocyte stages, supporting the concrescence theory (integrated development; Fons et al, 2019). This theory assumes that rudiments of reduced teeth can contribute to development of functional dentition, and, in squamates, result in formation of the egg tooth which is considerably bigger than marginal teeth.…”

“…This theory assumes that rudiments of reduced teeth can contribute to development of functional dentition, and, in squamates, result in formation of the egg tooth which is considerably bigger than marginal teeth. (Fons et al, 2019;Peterková, Peterka, Viriot, & Lesot, 2002). Previously, rudimentary premaxillary teeth were observed developing alongside the egg tooth in the grass snake.…”

“…Most squamates have a single medial egg tooth, while paired egg teeth are only characteristic for Gekkota (Fioroni, 1962). Typically, egg teeth are the first teeth to begin their development, as in other squamates (Bandali, Buchtová, Richman, & Tucker, 2007; Buchtová, Zahradníček, Balková, & Tucker, 2013; Fons et al, 2019; Hermyt, Janiszewska, & Rupik, 2020; Hermyt, Kaczmarek, Kowalska, & Rupik, 2017; Zahradnicek, Buchtová, Dosedelova, & Tucker, 2014), and they follow a similar developmental process to all teeth of vertebrates (Kerr, 1960; Luckett, 1993; McCollum & Sharpe, 2001).…”

Embryonic skull development in the gecko,Tarentola annularis(Squamata: Gekkota: Phyllodactylidae)

“…Other squamates have a single median egg tooth (Edmund, 1969; Fons et al . 2019; Hermyt et al . 2020), but this can arise in different ways.…”

“…In snakes, a single egg tooth can arise from the fusion of paired adjacent tooth germs during early embryonic stages (Fons et al . 2019), but in lizards, a median egg tooth can result from the degeneration of one of an original pair of egg teeth or from a single median tooth germ (De Beer, 1949; Hermyt et al . 2020).…”