Enamel Structure and Enamel Thickness
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Enamel is the hardest biological tissue known. It covers the dentine cap of a tooth and defines the limits of the crown of a tooth. Enamel is made up of minute crystallites that are organised into bundles known as prisms or rods. Prisms are approximately 6 micrometers in diameter and radiate out from the enamel dentine junction (EDJ) to the enamel surface. Crystalite orientation changes within prisms in a way determined by the shape of the cell processes of the secretory ameloblasts - the so-called Tomes’ process. Where crystallites of differing orientation meet, they create boundaries between adjacent prisms, which differ in their outline shape when prisms are viewed end on. Circular prism boundaries (Pattern 1) are more common in great ape enamel than in modern human enamel. Key-hole-shaped (Pattern 3) prism boundaries predominate in modern human enamel. The thickness of enamel can be measured in several ways. A linear thickness of enamel, from any defined point on the EDJ, can be measured from either naturally fractured surfaces of teeth that have cracked along the lengths of prisms, from specially prepared histological sections of teeth, or from high resolution CT scans of teeth. The area (or volume) of the enamel cap relative to that of the dentine cap can also be calculated from high resolution CT scans. Gorilla and Pan have thinner enamel than modern humans but enamel thickness increases through the molar row towards the back of the mouth in all Hominids. Thin enamel may be an advantage for more folivorous primates where a thin blade of enamel becomes quickly exposed. Thick enamel is advantageous when the diet is abrasive and also lasts longer over an extended modern human lifespan. Enamel prisms that interweave more (or decussate) resist crack-propagation, an additional advantage for hard object feeders.
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