The Enamel-Dentine Junction (EDJ) Morphology
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The junction between enamel and dentine defines the shape and size of the crown and is first mapped out as a sheet of cells (the inner enamel epithelium) before the onset of tooth mineralization. Once this begins, enamel is secreted on its outer aspect and dentine on the inner aspect. This halts all further cell proliferation and any change in size and shape at the EDJ. The EDJ is thereafter defined in hard tissues. While the tooth germ is still growing a cluster of non-dividing epithelial cells that express a wealth of signalling factors controls tooth cusp formation along the inner enamel epithelium (the future EDJ). This cluster of cells is called the enamel knot. In particular, Fgf-4 from the enamel knot, stimulates proliferation of both dental epithelial and mesenchymal cells, but not those of the knot itself. The knot may control tooth morphogenesis by stimulating cusp growth at the same time as directing the inner enamel epithelium to fold around itself. The inner enamel epithelium is like a diaphragm within the tooth germ separating the future dentine forming cells and central pulp chamber beneath from the future enamel forming cells above and a cap of cells (the stellate reticulum) beyond them. Cell division at any point along the inner enamel epithelium causes it to buckle into the stellate reticulum so mapping out the shape of a cusp tip. Human molar crowns, like those of Gorilla, have tall cusps while Pan and Pongo molars have more rounded cusps and squat crowns. The morphology of the EDJ in fully formed teeth preserves every detail originally mapped out along the inner enamel epithelium of the tooth germ and it can be retrieved with 3D high resolution imaging techniques. Surface enamel smothers some of this detail but adds more of its own.
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