Chapter category: Development
The Teeth as Models for Studies on the Molecular Basis of the Development and Evolution of Organs
Molecular Basis of Epithelial Appendage Morphogenesis
Edited by: Cheng-Ming ChuongISBN: 1-57059-490-2
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Chapter authors:
Irma Thesleff and Johanna Pispa
Teeth develop from stomodeal or pharyngeal epithelium and the underlying neural crest-derived mesenchymal cells in a way very similar to skin derivatives such as hair, feathers and scales (Fig.1). The first morphological sign of tooth development is a thickening of the oral epithelium, which subsequently buds into the underlying mesenchyme. The mesenchymal cells condense around the bud, and during the following cap and bell stages the epithelium undergoes folding morphogenesis, resulting in the establishment of the tooth crown form. The mesenchyme, which becomes surrounded by the dental epithelium, forms the dental papilla, giving rise to tooth pulp and the odontoblasts. The more peripheral cells of the condensed dental mesenchyme extend around the epithelial dental component, the enamel organ, forming the dental follicle. The follicle subsequently gives rise to the cementoblasts depositing dental cementum, as well as to the fibrous periodontal membrane connecting the roots of the teeth to the alveolar bone. It should be noted that teeth always develop separately from bones, although in fish, amphibians and some reptiles they become united to bone during development. In many vertebrates, notably in mammals, the periodontal membrane separates the teeth from the bones, and the teeth have a central role in the regulation of the growth of the alveolar bone.
The terminal differentiation of the dentine-forming odontoblasts and the enamel-forming ameloblasts takes place during the bell stage at the interface of the epithelium and mesenchyme. The preodontoblast cells align under the epithelial basement membrane, polarize and start to secrete the collagenous dentin matrix, which subsequently mineralizes into a bone-like hard tissue. The preameloblast cells opposite the odontoblasts become columnar, their nuclei polarize and they start the deposition of the enamel matrix. This matrix directs the mineralization of the enamel into the hardest tissue in the body, and during this process the enamel proteins are almost entirely degraded. After crown morphogenesis, the roots of the teeth develop and subsequently the teeth erupt into the oral cavity (Fig. 1, for more details see ref. 1).
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