Abstract
BackgroundThe mouse embryonic mandible comprises two types of tooth primordia in the cheek region: progressive tooth primordia of prospective functional teeth and rudimentary tooth primordia in premolar region – MS and R2. Mice lacking Sprouty genes develop supernumerary tooth in front of the lower M1 (first molar) primordium during embryogenesis. We focused on temporal-spatial dynamics of Sonic Hedgehog expression as a marker of early odontogenesis during supernumerary tooth development.ResultsUsing mouse embryos with different dosages of Spry2 and Spry4 genes, we showed that during the normal development of M1 in the mandible the sooner appearing Shh signaling domain of the R2 bud transiently coexisted with the later appearing Shh expression domain in the early M1 primordium. Both domains subsequently fused together to form the typical signaling center representing primary enamel knot (pEK) of M1 germ at embryonic day (E) 14.5. However, in embryos with lower Spry2;Spry4 gene dosages, we observed a non-fusion of original R2 and M1 Shh signaling domains with consequent formation of a supernumerary tooth primordium from the isolated R2 bud.ConclusionsOur results bring new insight to the development of the first lower molar of mouse embryos and define simple tooth unit capable of individual development, as well as determine its influence on normal and abnormal development of the tooth row which reflect evolutionarily conserved tooth pattern. Our findings contribute significantly to existing knowledge about supernumerary tooth formation.
Highlights
The mouse embryonic mandible comprises two types of tooth primordia in the cheek region: progressive tooth primordia of prospective functional teeth and rudimentary tooth primordia in premolar region – Mesial segment (MS) and Tooth rudiment (R2)
We showed that First molar (M1) primary enamel knot (pEK) in its accepted meaning arises as late as after the fusion of the original R2 and early M1 Sonic Hedgehog (Shh) signaling domains
Dynamics of Shh expression influenced by Sprouty gene dosages For all nine possible genotypes (Table 1), the shape of dental epithelium and a presence and pattern of Shh expression domains were evaluated during the tooth development in the lower jaw from E11.5 till 16.5
Summary
The mouse embryonic mandible comprises two types of tooth primordia in the cheek region: progressive tooth primordia of prospective functional teeth and rudimentary tooth primordia in premolar region – MS and R2. Adult mouse diastema does not contain teeth, there are tooth rudiments transiently apparent in the upper and lower embryonic diastema [1]. Enamel knots (EKs) are transient signaling centers comprising non-dividing cell population in dental epithelium [5,6,7]. EKs are important structures initiating and regulating tooth shape and determining the number of tooth cusps [5,6,7] They were considered to be the main regulator of tooth development [7]. It has been discovered that the primary enamel knot (pEK) controls morphogenesis in the first molar germ in mouse [6] and that Sonic Hedgehog (Shh) is expressed there [8]. We showed that M1 pEK in its accepted meaning (as a signaling center of a tooth) arises as late as after the fusion of the original R2 and early M1 Shh signaling domains
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