Tooth development in a model reptile: functional and null generation teeth in the gecko Paroedura picta

Abstract

This paper describes tooth development in a basal squamate, Paroedura picta. Due to its reproductive strategy, mode of development and position within the reptiles, this gecko represents an excellent model organism for the study of reptile development. Here we document the dental pattern and development of non-functional (null generation) and functional generations of teeth during embryonic development. Tooth development is followed from initiation to cytodifferentiation and ankylosis, as the tooth germs develop from bud, through cap to bell stages. The fate of the single generation of non-functional (null generation) teeth is shown to be variable, with some teeth being expelled from the oral cavity, while others are incorporated into the functional bone and teeth, or are absorbed. Fate appears to depend on the initiation site within the oral cavity, with the first null generation teeth forming before formation of the dental lamina. We show evidence for a stratum intermedium layer in the enamel epithelium of functional teeth and show that the bicuspid shape of the teeth is created by asymmetrical deposition of enamel, and not by folding of the inner dental epithelium as observed in mammals.

Evolution and developmental diversity of tooth regeneration

“…A similar mirror image development of palatal teeth is also found in Sphenodon [45]. In most toothed reptiles a non-functional set of teeth is observed developing early on in the embryo, these teeth forming superficially on the oral surface rather than being associated with a dental lamina [13,48,52,54]. These non-functional tooth germs have been proposed to be remnants of a larval dentition, that became redundant with the extension of the embryonic period [52].…”

“…Similar to the chondrichthyan and amphibian species studied, the replacing series of tooth germs in polyphyodont squamates are united by a permanent dental lamina linking the developing teeth in a chain to the functional dentition, and the oral surface [10,12,47,48]. At birth 3-4 developing generations of teeth are found linked to the dental lamina, making a developmental series ready to replace the functional tooth.…”

“…These non-functional tooth germs have been proposed to be remnants of a larval dentition, that became redundant with the extension of the embryonic period [52]. The first developing teeth are not physically connected to the functional teeth and are not part of a replacement series, however these first rudimentary tooth germs develop in positions near to where the dental lamina is due to develop and may play a role in patterning of the later functional dentition [48]. All functional teeth, however, develop from the dental lamina.…”

Dinosaur incubation periods directly determined from growth-line counts in embryonic teeth show reptilian-grade development

“…The timing for the establishment of amniote hatchling functional dentitions is well established. In crocodilians (extant sister taxon to Dinosauria) it typically occurs between 42% and 52% of the total incubation period (3) and at >51% in squamates (64,65,(67)(68)(69). In chicken (Gallus gallus domesticus), living dinosaurs, teeth primordia-none of which become functional-appear at 66% through incubation (98).…”

“…Almost all toothed reptiles produce several generations of teeth before establishing the functional compliment at hatching (66). The primordial null-generation teeth are resorbed, incorporated into the jaws, or shed in ovo (3,66,67). For example, American alligators (Alligator mississippiensis) go through two to four tooth-replacement cycles before hatching (95)(96)(97).…”