Oral Histology Enamel 2 PDF

Enamel 2 ENAMEL ENAMEL 2 1  Enamel 2 Incremental lines of enamel: Cross striation: Each enamel rod demonstrates closely positioned striations along its length known as cross-...

Enamel 2 ENAMEL ENAMEL 2 1  Enamel 2 Incremental lines of enamel: Cross striation: Each enamel rod demonstrates closely positioned striations along its length known as cross- striations or short increments. These are thought to be formed by the daily rhythm of the ameloblast laying down more and less mineralized enamel. The striations are approximately 3-4 µm apart. This distance represents one day of enamel deposition. Scanning electron microscopy reveals alternating constrictions and expansions. Incremental lines of Retzius: During development of enamel, variations in the metabolism cause variations in the amount of organic material deposited in the enamel. This causes changes in the coloration of the enamel that is laid down at that time so that alternating dark (higher organic material) and light (less organic material) banding occurs. These bands are called Stria of Retzius. In Cross Section Seen as concentric rings. Seen as a series of dark bands reflecting successive enamel-forming fronts. At the middle and cervical parts: they run obliquely and deviate occlusally reach In Longitudinal Section the enamel surface and become represented as a series of transverse depressions (perikymata). At cusp tips and incisal ridges: these bands form semicircle as they do not reach the surface (surround the tip of dentin). Neonatal line: The neonatal line is a dark stria of Retzius that occurs at the time of birth. It is due to the stress of birth. The neonatal line is usually the darkest and thickest stria of Retzius. Present only in deciduous teeth and first permanent molars (enamel develops partly before and partly after birth). Separate enamel formed before birth from enamel formed after birth. The quality of prenatal enamel is better than the postnatal enamel (more protected condition and constant nutrition of the fetus). 2  Enamel 2 Hypocalcified structures of enamel Amelodentinal junction: The junction between enamel and dentin is seen as a scalloped profile in cross section with the convexities of the scallops directed towards the dentin, these scallops provide undercuts that was thought at one time to be the cause of the firm attachment between enamel and dentin. However, the elevations of the scallops occasionally seem to be absent and in this case the ADJ appears smooth and still enamel and dentin are firmly attached. It seems that the cause of the firm attachment between enamel and dentin is due to the interdigitation at the ADJ between the fibrils of the first formed layer of dentin and the fibrils of the organic matrix of the first formed layer of enamel, and consequently the interdigitation between the hydroxyapatite crystals that are deposited during the mineralization of both structures. ] Enamel tufts: Enamel tufts are less mineralized areas of enamel in the inner third of enamel adjacent to the ADJ. They resemble tufts of grass. Occur developmentally because of abrupt changes in the direction of groups of rods that arise from different regions of the scalloped ADJ. Project from ADJ for a short distance into 1/5-1/3 enamel thickness, they appear to be branched and contain greater concentration of enamel proteins than the rest of enamel. Best seen in transverse thick sections of enamel. 3  Enamel 2 Enamel spindles: The enamel spindles appear as short, straight, thin, darkstructures which extend for only short distances into enamel. They are residual tubules formed when odontoblastic processes extend across the ADJ during odontogenesis before enamel forms. In ground sections, the odontoblastic processes disintegrate and are replaced by air which appears dark in transmitted light. Enamel lamellae: Enamel lamellae are fissure-like defects and extend for varying depth from the surface of enamel and consist of linear longitudinal oriented defects. True lamellae may contain enamel proteins or organic debris from oral cavity. Types: Developmental (true Lamellae) Non Developmental (False Lamellae) Or Cracks Type B Type A Type C Accidental crack 4  Enamel 2 Surface structures in Enamel Outer Structureless enamel ( prismless enamel ) In the surface layer of enamel no prism are visible and the apatite crystals are arranged parallel to one another and perpendicular to the incremental lines of retzius. The high degree of mineralization of the outer 15 µm of the enamel probably reflects events occurring during the late maturation stage as well as post-eruptive mineral accumulation from saliva. Commonly occur at the cervical area more than the cusp tip or incisal edge. Found in all deciduous teeth and 70% of permanent teeth. Perikymata It is the external manifestation of the incremental lines of retzius. Represented as shallow furrows where the striae of retzius end in the outer surface of enamel. Continuous around the tooth and parallel to the CEJ. There are 30 perikymata/mm in cervical area and decrease toward the occlusal surface. Rod end They are concave depression vary in depth and shape. They are shallow at the cervical region and deepen near the incisal or occlusal surface. Cracks They are narrow fissure like structure found on almost all enamel surfaces and extend for varying distance. They are actually the outer edge of enamel lamellae. Completely disappear by careful decalcification. 5