Dentin is a vital tissue, harder than bone but weaker than enamel with a physiological elasticity preventing tooth fracture. Dentin is approx. 74% mineral; the organic phase being approx. 26% type-1 collagen with small amounts of proteins and water. The collagen matrix is secreted as a biologically "plastic" material, which hardens by a variety of mechanisms. Normal dentin is composed of millions of tubules or canals, running from the pulpal wall to the Dentin-Enamel Junction (DEJ). The dimensions of these tubules or canals varies depending on the position within the dentin structure. For instance, tubule diameter at the DEJ is 0.06 µm, and 3.0 µm at the pulpal wall (approx. 60,000 tubules / mm2). Most tubules are filled with fluid, an odontoblast cell process, collagen, and occasional non-myelinated pulpal nerves coursing approx. 150 µm towards the DEJ (Ten Cate 1998).