Oral Histology PDF

Enamel ‫زيادات‬ ENAMEL ‫زيادات‬ ENAMEL 1  Enamel ‫زيادات‬ Origin: -Enamel has evolved as an epithelially derived (ectodermal) protective covering for teeth. -The cells that resp...

Enamel ‫زيادات‬ ENAMEL ‫زيادات‬ ENAMEL 1  Enamel ‫زيادات‬ Origin: -Enamel has evolved as an epithelially derived (ectodermal) protective covering for teeth. -The cells that responsible for formation of enamel, are lost as the tooth erupts into the oral cavity, & hence enamel cannot renew itself Physical characteristics Thickness *2 – 2.5 mm. at the cusps of the molars and premolars. -Thinning down to almost knife edge at the cervical margin of the tooth HARDNESS -Is the hardest calcified tissue in the body due to 1- its high mineral content and 2-its crystallite arrangement 3-Enamel of the permanent teeth is harder than that of the deciduous one. 4-Hardness increase with fluride application BRITTLNESS -Enamel is brittle especially when it looses its foundation of elastic dentin. Britleness increase with age , Undermining and if Non vital PERMEABILITY -Permiabilty is mainly from saliva to the outer layer of enamel, -but less from the pulp to the inner enamel layer across the dentin 2  Enamel ‫زيادات‬ Chemical characteristics: inorganic content organic material 96% 4% a crystalline calcium phosphate (non-collagenous proteins” hydroxyapatite and various ions, Amelogenins and enamilin”) and water. strontium, magnesium, lead and organic substance present between the crystals as a fine network fluoride. Methods of studying hard tissue GROUND SECTION DECALCIFIED SECTION Enamel is retained in prepared Because of high crystalline ground sections, so it can be nature the structure of enamel examined under the light is extremely difficult to study microscope either in: in decalcified sections, Longitudinal sections. because the minerals have been dissolved and the organic Cross sections material washed away. 3  Enamel ‫زيادات‬ The structural unite of enamel is the enamel rod Number of enamel rod 5 million (lower central) 12 million (upper first molar) Course of enamel rod -HUNTER-SCHREGER BANDS are Abscent in the region of the Gnarled enamel 4  Enamel ‫زيادات‬ SUBMICROSCOPIC STRUCTURE OF ENAMEL ROD Key hole-fish scales- oval rounded- hexagonal Cylindrical separated by less calcified dark lines (cross striations) 5  Enamel ‫زيادات‬ Key-hole Pattern Enamel prism Interprismatic region Prism sheath keyhole , or fish scale highly calcified as the make an incomplete envelop around the highly calcified prism prism less calcified than the prism By electron microscope The cylindrically shaped enamel prism is made up of crystals with their long axes: -parallel to the long axis of the prism… at its central part. -Then flare laterally to an increasing degree as they approach the prism boundary. -The difference in the angulation of the crystals is the function of toms process Prism sheath: The crystals in the prism sheath are not so tightly packed, allowing more spaces between them for more organic material to be present. Less calcified than prism. 6  Enamel ‫زيادات‬ Remember NUMBER 5 Million in lower lateral to 12 million in upper 6 DIRECTION perpendicular to DEJ then oblique occ then slopes root wise or horizontal in dec COURSE wavy course from th ADJ and become st. at the surface & twisted under cusp tip and incisal edge “ gnarled enamel” DIAMETER increases towards the surface ratio: 1 : 2 Longitudinal Cylindrical , showing cross striations section Cross Section Hexagonal, fish scales, keyhole pattern Brown Striae -It denotes the rhythmic deposition of enamel and are corresponding to the of Retzius weekly rest period of ameloblasts. -Broadening of these lines….. Means prolongation of rest periods………. occur in metabolic disturbances. Cross formed by the daily rhythm of the ameloblast laying down more and straition less mineralized enamel. The striations are approximately 3-4 μm apart. This distance represents one day of enamel deposition. Enamel lamella Type A Type B Type C Stimulus Mild stimulus Sever stimulus Sever stimulus Time of During enamel formation but before After enamel formation After eruption but before eruption incidence calcification Extension Enamel May cross ADJ to dentine May cross ADJ to dentine Content Enamel matrix (organic content) Cells of enamel organ (organic content) Organic materials from saliva Tooth Unerupted Unerupted Erupted Location Restricted to the E Reach into the D Reach into the D Occurrence Less common Less common More common 7  Enamel ‫زيادات‬ AGE CHANGE IN ENAMEL 1. Attrition 2. Color changes 3. Modification in the surface layer (ionic exchange) 4. Decreased Permeability 8