applied fracture mechanics

112Applied Fracture MechanicsFigure 2. The hard tissues are formed by the odontoblasts (dentin) and ameloblasts (enamel). Duringthe development of the tooth, epithelial cells have formed a bell shaped enamel organ. Inside that bell isconnective tissue that shows active budding of capillaries. At a certain stage, the fibroblasts in contactwith the epithelium bell become highly differentiated and develop into odontoblasts and form the firstlayer of dentin. That layer stimulates the epithelium cells in contact with the dentin at the DEJ to differentiateinto ameloblasts and form enamel. As a consequence, the two cell types move in opposite directionas they form dentin (blue arrows) and enamel (red arrows). When the ameloblasts reach the outercells of the enamel organ they start degrading and lose vitality. At the same time, the dentin has increasedin thickness and the epithelial diaphragm with odontoblasts have grown downwards anddeveloped the root and the pulp chamber (see Figure 1).Figure 3. The top drawing represents a cross-section of dentin, perpendicular to the peritubular dentin(A). The lower drawing represents a plane parallel to the odontoblastic processes (C) and cut along a-a.In the peritubular dentin, collagen fibers represented by pink lines are present parallel to the odontoblasticprocesses. Hydroxyapatite precipitate along these fibers, and together they form the so calledperitubular dentin (A). Collagen precipitates perpendicular to the odontoblastic processes too, andwhen hydroxyapatite precipitate in that matrix, the intertubular dentin (B) is formed.