Enamel Histological Structure PDF

ENAMEL Dr. Aya Adel Mekky Lecturer of oral biology department  ENAMEL COMPOSITION Enamel is the most highly calcified and hardest tissue in the human body. It covers the anatomical crown of the teeth Enamel is produced by cells of ectodermal origin...

ENAMEL Dr. Aya Adel Mekky Lecturer of oral biology department  ENAMEL COMPOSITION Enamel is the most highly calcified and hardest tissue in the human body. It covers the anatomical crown of the teeth Enamel is produced by cells of ectodermal origin ??  PHYSICAL PROPERTIES Enamel thickness On the cusps of human molars and premolars, the enamel shows a maximum thickness of about 2-2.5 mm.; While at the bottom of sulcus it is minimum, thinning down to almost a knife edge at the cervical margin of the tooth.  Hardeness Enamel is the hardest calcified tissue in human body due to its high mineral salts and its crystalline arrangement Permanent teeth is hardest than deciduous teeth Brittleness The enamel is brittle tissue and liable to fracture if not supported by dentin  Color The color of the enamel ranges from yellow to shades of gray or gray-blue. This depend on the degree of translucency of the enamel, which in turn depend on the degree of the calcification. The yellow color of dentin is visible through thin translucent enamel Enamel of deciduous teeth is slightly less mineralized and whiter than that of the permanent teeth.  Enamel Permeability Enamel is semi-permeable, permitting partial passage of certain saliva ions and dye stuffs. The main pathway is from the saliva into the upper part of the enamel.  CHEMICAL PROPERTIES Enamel is composed of inorganic crystals (97%by weight) with only a small amount of water and organic matrix Amelogenins & enamelin (3%). The enamel crystals in the form of some hydroxyapatite crystals (Ca10 (PO4)6(OH)2)  AMELOGENESIS It is a process of enamel formation The cell responsible for amelogenesis is ameloblast which secrete both organic and inorganic parts of enamel Stages of ameloblast 1. presecretory stage 2. secretory stage 3. maturation stage 4. Protective stage.  1- PRE SECRETORY STAGE Morphogenetic Differentiation phases phases  MORPHOGENETIC PHASES It is found in early bell stage The cells are short columnar, Proximal end with large oval centrally infranuclear region located nuclei. The synthetic organelles are few and scattered nucleus They are separated from dental papillae by basement Distal end membrane and cell free zone. supranuclear region  DIFFERENTIATION PHASES Proximal end It is found in late bell stage infranuclear The cells are tall columnar, and polarized. region The nucleus shifting to the proximal end of the cell Mitochondria are present in infranuclear region Migration and increase in synthetic organelles supranuclear region (RER ,Golgi apparatus and centrioles) in the distal ends of the cell Distal end  The clear cell-free zone between the inner dental epithelium and the dental papilla disappears, probably due to the elongation of the cells of inner dental epithelium towards the dental papilla inducing them to be differentiated into odontoblasts.  The formation of dentin by the odontoblasts begins the end of the differentiation phase. The basement membrane supporting the preameloblasts disintegrates  2- SECRETORY STAGE After the first layer of dentin has been formed. The distal membrane of ameloblast is flat and Ameloblast produce the first layer of enamel matrix and deposit it against the newly formed dentin. Enamel The hydroxyapatite crystals are parallel to each Dentin other and interdigitate with the crystals of Tome’s process dentin in this initial layers(AMELODENTINAL Odontoblast JUCTION) Initial enamel (rodless enamel) After the first layer formed, the distal ends of ameloblasts extend blunt cytoplasmic process called tome’s process.  The tomes processes have numerous secretory vesicles containing enamel matrix with no cell organelles  A distal terminal web (distal junctional complex) appears as a dark staining line encircling all secretory ameloblasts separating the cell bodies of ameloblasts from tomes process  m e l oblast e cr e t ory a S e l m atrix Enam  Secretory ameloblast Tomes’ process  When the tomes process is established, the enamel matrix is formed in rods The proximal part of Tome's process produces interrod while the distal part produces rod. The difference between rod and interrod enamel is not chemical, but in the orientation of crystals The recess of Tome's process is removed and replaced by enamel proteins called rod sheath.  Following the deposition of the majority of the enamel matrix, The ameloblasts shorten slightly and loss their Tomes processes in preparation for their next stage. The last layer of enamel is rodless enamel as the first layer. The enamel formed mainly of rod and interrod sandwiched between two layers of rodless enamel.  3- MATURATION STAGE It starts after the entire thickness of the enamel has formed. Ameloblasts become shorter and the number of synthetic organelles are decreased Its function to convert enamel from 30% mineralization into 97% mineralization Ameloblasts are existed into two forms a- Ruffle border ameloblast: the distal membrane is ruffled. Its function is to pump calcium and minerals to the maturation enamel. b- Smooth border ameloblast: the distal membrane is smooth. Its function is to withdraw the destructed enamel proteins, organic elements and excess water by endocytosis through distal and lateral membranes.  4- PROTECTIVE STAGE After complete crown formation, Ameloblasts and other enamel organ layers form reduced enamel epithelium. It protect mature enamel from surrounding tissue until tooth eruption.  ENAMEL STRUCTURE A. ROD (PRISM) interrod Number :The number of enamel rods varies in different teeth. It reaches up rod to12 millions in the upper first molar Direction :The direction of the rods in general is perpendicular to the dentin rod surface. In deciduous and permanent deciduous permanent teeth the enamel rods are horizontal. while in cervical region ,they are oblique in permanent and horizontal in deciduous  Course :the course of rod is wavy and near the enamel surface is staight Diameter : The diameter of the rod increases from amelodentinal junction to the surface of the enamel  In cross section under the light microscope, the enamel rods appear as hexagonal, oval, rounded, and fish scales  B. INTERROD (INTERPRISMIC ) The enamel rods, are cemented together by inter-rod substance, which has different crystal orientation than the enamel rod.  C. ROD SHEATH The peripheral part of each rod (rod sheath), which contains more organic matrix than the other region do. It form an incomplete envelope, and due to its high organic content, it stain darker and resist decalcification than the rod. Bonus question THANK YOU HISTOLOGICAL STRUCTURE OF ENAMEL  A.INCREMENTAL LINES OF ENAMEL 1- CROSS STRIATION Each enamel rod is built up of segments separated by dark lines that give it a striated appearance. It is due rhythmic apposition of the enamel matrix by ameloblasts, which represent the daily rate of matrix formation  2- INCREMENTAL LINES OF RETZIUS (ENAMEL BROWN STRIAE) Due to rhythmic deposition of enamel every 4 days In ground section of enamel, they appear as brownish bands. In longitudinal section at the incisal edges and cusp tips of teeth, these brown striation, surrounding the tip of the dentin, and so they do not reach the outer surface of the enamel.  Sometimes ,they reach the outer surface of the enamel in series of transverse depressions known as perikymata.  3- NEONATAL LINE This line separates the enamel which is formed before birth from that formed after birth. The quality of the prenatal enamel is usually better than that of the postnatal enamel, due to constant nutrition of the foetus This is present only in the deciduous teeth and the first permanent molar.  B.AMELODENTINAL JUNCTION The junction between enamel and dentin is seen as a scalloped , with the convexities directed towards dentine. This arrangement 1. Increase the surface area 2. Adherence between the two tissues  C.GNARLED ENAMEL Found over the cusps and incisal edge Due to twisting of the rods over each other Functionally change in rods direction increase the strength of enamel  D.HUNTER SCHERGER BANDS Its an optical phenomena Its seen by reflected lights Its found in the inner 2/3 of enamel They appear as alternating light and dark bands The band are not seen at the outer part of the enamel as the rods run straight to the surface  E.ENAMEL SPINDLES Before formation of enamel, some developing odontoblastic process extend into the ameloblast layer and become trapped to form enamel spindles They are present in great numbers at region of the cusps In a dried ground section ,the space appears dark in transmitted light.  F.ENAMEL TUFTS They are hypocalcified enamel rods projecting from the dentinoenamel junction. They appear to branched and contain greater concentrations of enamel proteins than the rest the enamel. They are so called since they resemble tufts of grass Tufts are believed to occur developmentally  G.ENAMEL LAMELLAE Lamellae are hypomenerlized area extend from outer sureface toward the ADJ They are three types: 1. Type "A": represent hypomineralized enamel rods occuring developmentally and are filled with enamel proteins. 2. Type "B": represent cracks that are produced during development at planes of tension, and are filled with cellular debris 3. Type "C": are also cracks but they are produced after eruption; these are filled with organic debris from the oral cavity. True lamellae may be confused in ground sections with false ones (cracks) which are produced by grinding and become filled with air.  H.SUREFACE STRUCTURE OF ENAMEL 1-RODLESS ENAMEL It is found at the enamel surface, has a thickness of 30u It also found as a very thin layer adjacent to the DEJ. It occurs because of the absence of Tome's processes on the ameloblasts responsible for the development of enamel rods. It consists of closely packed crystals that are parallel to one another and perpendicular to the incremental lines of Retzius. This layer is more highly mineralized than the bulk of enamel beneath it.  2-PERIKYMATA These are transverse, wave like grooves which are the outer manifestation of the striae of Retzius. Perikymata are continuous around the tooth and usually lie parallel to each other and to the cementoenamel junction. They are prominent in the cervical portion of the crown, where the striae reach the surface. With age, the enamel surface becomes worn and the perikymata becomes less prominent, especially on the facial and lingual surfaces of the crown.  3- ROD ENDS The outer enamel surface of newly erupted teeth shows small indentations Corresponding to Tome's processes. The ameloblasts to which they cease enamel production prematurely before they had retracted  PELLICLE AND PLAQUE Erupted enamel is normally covered by pellicle, which is a precipitate salivary proteins. This pellicle reforms within hours after teeth have been mechanically cleaned. Within a day or two after the pellicle has formed becomes colonized by microorganisms to form a dental plaque.  AGE CHANGES 1. Attrition The most apparent age change in enamel is attrition or wear of the occlusal surfaces and proximal contact areas as a result of mastication. 2- Color changes of the tooth Tooth darken with age due to the addition of organic material to enamel or the environment. It may also be due to attrition exposing the yellow color of dentin through the thinning layer of enamel. 3-Alteration in the chemical composition of enamel After tooth eruption, ionic exchange between enamel and saliva occurs and may lead to localized increase of certain elements (e.g. fluorine). A decrease of the water content of enamel  CLINICAL CONSIDERATION Damaged enamel can only be repaired through operative procedures. Repair is affected by preparing a cavity and filling it with an appropriate restorative material. In preparing a cavity, knowledge of the orientation of the enamel rods is of prime importance, because enamel is not elastic but very brittle. It depends on a cushion base of sound dentin for its elasticity and support.  Fluoridation: If the fluoride ion is incorporated into the hydroxyapatite crystals, the crystals become more resistant to acid dissolution. This reaction explains the role of fluoride in caries prevention, for the caries process is initiated by demineralization of enamel.