Initiation and patterning of the snake dentition are dependent on Sonic Hedgehog signaling

Abstract

Here we take the first look at cellular dynamics and molecular signaling in the developing snake dentition. We found that tooth formation differs from rodents in several respects. The majority of snake teeth bud off of a deep, ribbon-like dental lamina rather than as separate tooth germs. Prior to and after dental lamina ingrowth, we observe asymmetries in cell proliferation and extracellular matrix distribution suggesting that localized signaling by a secreted protein is involved. We cloned Sonic hedgehog from the African rock python Python sebae and traced its expression in the species as well as in two other snakes, the closely-related Python regius and the more derived corn snake Elaphe guttata (Colubridae). We found that expression of Shh is first confined to the odontogenic band and defines the position of the future dental lamina. Shh transcripts in pythons are progressively restricted to the oral epithelium on one side of the dental lamina and remain in this position throughout the prehatching period. Shh is expressed in the inner enamel epithelium and the stellate reticulum of the tooth anlagen, but is absent from the outer enamel epithelium and its derivative, the successional lamina. This suggests that signals other than Shh are responsible for replacement tooth formation. Functional studies using cyclopamine to block Hh signaling during odontogenesis prevented initiation and extension of the dental lamina into the mesenchyme, and also affected the directionality of this process. Further, blocking Hh signaling led to disruptions of the inner enamel epithelium. To explore the role of Shh in lamina extension, we looked at its expression in the premaxillary teeth, which form closer to the oral surface than elsewhere in the mouth. Oral ectodermal Shh expression in premaxillary teeth is lost soon after the teeth form reinforcing the idea that Shh is controlling the depth of the dental lamina. In summary, we have found diverse roles for Shh in patterning the snake dentition but, have excluded the participation of this signal in replacement tooth formation.

Replacing the first-generation dentition in pufferfish with a unique beak

“…3) appear to develop entirely within the surface epithelial layer rather than through invagination of the entire tooth germ. This invaginated dental epithelium is considered essential for tooth replacement in all other known vertebrate dentitions (17,21,32,33). The first-generation parasymphyseal tooth germs in Monotrete spp., which also begin their development in a superficial position, later invaginate deep into the underlying mesenchyme.…”

Evolution and developmental diversity of tooth regeneration

“…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.…”

“…This bulge has been found during development in all toothed reptile species examined, even those with only one set of functional teeth [44,46,55]. The squamate successional lamina is a region of high proliferation and low apoptosis (programmed cell death) [47,56]. Fate mapping of the successional lamina in the snake has shown that the cells in the successional lamina contribute both to the new generations of teeth and are retained at the tip of the lamina [10].…”

Evolution and development of the mammalian dentition: Insights from the marsupial Monodelphis domestica

“…Shh and Fgf8 are expressed in the dental lamina in mouse (Bitgood and McMahon, 1995;Kettunen and Thesleff, 1998). Shh is also expressed in the dental lamina of snakes (Buchtova et al, 2008), shrews (Yamanaka et al, 2007), and catsharks (Smith et al, 2009), suggesting a strong conservation at this early stage of tooth development and a role in the generation of replacement teeth.…”