Development of the egg tooth - The tool facilitating hatching of squamates: Lessons from the grass snake Natrix natrix

Most embryos of squamates use their egg tooth to facilitate hatching when their development is completed. After they are out of the shell, this tooth is shed and, in the case, of the grass snake (Natrix natrix), not replaced by a successor teeth. The structure of this transient tooth resembles the development and histology of the regular teeth of vertebrates. Morphological, histological and scanning electron microscopic observations indicated that the egg tooth of the grass snake has four developmental phases. Like the teeth of other vertebrate species, it undergoes oral epithelium thickening as well as the bud, cap and bell phases. However, due to the specialised function it performs, the egg tooth differs significantly from the other teeth both in its morphology and development. The egg tooth of Natrix natrix embryos is an unpaired true tooth, as in most squamates. Our study indicated that the egg tooth started its development in the rostral part of the snout by the thickening of the oral epithelium and there was a condensation of mesenchyme underneath it. It formed very early, around developmental stage III, at approximately the same time as the null-generation teeth. After the thickening of the oral epithelium, only one tooth germ is formed, in contrast to lizards in which two germs can be observed during their embryonic life; however, in the course of development, one regressed and the other shifted into the midline position and developed into the functional egg tooth. The next step in the egg tooth development was the differentiation of the enamel organ and the dental papilla. Three layers of the enamel organ developed - the inner enamel epithelium, the stellate reticulum and the outer enamel epithelium, while a superficial layer of the dental papilla differentiated into the odontoblasts. The egg tooth was ready to erupt when its development ended at developmental stage XII, after the hard tissues developed.