Browsing by Subject "tooth development"
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(2020)Mammalian dentitions exhibit extraordinary diversity in morphology and function. Yet,the mechanisms governing dental development are considered highly conserved across Mammalia. The inhibitory cascade (IC) model is a developmental model explaining variation in molar size proportions observed in mammals. The IC model predicts a range of dental phenotypes based on a dynamic of cumulative inhibition and activation in the sequential development of molars. Whereas most mammals fit these predictions well, bears(Ursidae) are a known exception.Here I employ dental topographic analysis to examine the developmental basis of ursine molar dentitions defying the IC model.I quantified two aspects of tooth shape:molar complexity using orientation patch count (OPC), and tooth size as the area of occlusal surface. As the complexity and size of a tooth mainly emerge based on two different developmental processes –patterning and growth, respectively –these measurements were used to decompose the two phases of tooth development producing the final phenotype. To this end, also an estimate of feature density was calculated.As in previous studies, the molar size proportions of bears were highly incongruent with the IC model. However, complexity along the molar row followed a trend more closely matching the model. Feature density was highest in the third molar. Altogether, these observations suggest an early arrest in the growth of the third molar as the principal cause for bears falling outside the predictions of the IC model –consequently supporting the idea of the inhibitory cascade as a plesiomorphy of Mammalia.As an auxiliary part of this project, I assessed the functionality of Morphoviewer, a new piece of software for measuring complexity. Morphoviewer was successfully applied to infer diet from tooth complexity in a limited sample of carnivorans; and was thus used for all further OPC analyses.
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(2019)Lack of Ectodysplasin (EDA), caused by a mutated Eda gene, leads to a syndrome called hypohidrotic ectodermal dysplasia (XLHED) with defects in ectodermal organs such as teeth, hair and sweat glands. The molar teeth of Eda knock out (Eda KO) mice are absolutely and relatively smaller and have fewer cusps than the wild type (WT) molar teeth. In the absence of the EDA protein, the receptor of the EDA signalling pathway (EDAR) remains functional, and therefore EDA-protein therapy can rescue the development of ectodermal organs. The aim of this study was to determine EDA sensitivity windows and to describe the Edar expression pattern in developing mouse lower molars. Eda KO mouse skulls treated with EDA for 24 hours at different stages of development were imaged using x-ray microtomography. The response was studied by analysing the cusp patterns and size proportions of lower molars. In situ hybridisation was used to detect the Edar expression in the developing Eda KO and WT molars at different stages. The results show that molars are sensitive to EDA at the early stages of crown patterning, at the time when Edar is expressed in the primary enamel knot and the secondary enamel knots. The Edar expression pattern suggests that EDA signalling regulates molar size and cusp development through these signalling centres. EDA-treatment during a sensitivity window enhances the growth of the EDA sensitive molar, thereby breaking the previously reported inhibitory cascade –rule. The results of this study provide information for optimising the EDA therapy for XLHED patients.
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