Enamel PDF 2024

# Enamel Dr. Hala Zaatar Enamel covers the anatomical crown of human tooth. ## Chemical composition: - 96% inorganic substances - Hydroxy appetite crystals - 3Ca3(PO4)2Ca(OH)2 - 4% organic substances - 1-2% Protein and carbohydrates - 2-3% Water This is by weight but by volume...

# Enamel Dr. Hala Zaatar Enamel covers the anatomical crown of human tooth. ## Chemical composition: - 96% inorganic substances - Hydroxy appetite crystals - 3Ca3(PO4)2Ca(OH)2 - 4% organic substances - 1-2% Protein and carbohydrates - 2-3% Water This is by weight but by volume they are the same. ## Physical properties: 1. **Color:** Varies from yellowish white to grayish white which depends on translucency, degree of calcification and homogeneity of enamel. 2. **Thickness:** It is thick at the incisal edge and cusp tip and ends cervically as a knife edge. 3. **Hardness:** It is the hardest calcified tissue in human body because of its high calcification and crystal orientation. 4. **Brittleness:** Although of its hardness, enamel is brittle especially when looses the underlying elastic healthy dentine. 5. **Permeability:** Enamel acts as a semi permeable membrane. ## Histological structure The unite structure of enamel is formed of: Enamel Rod- Rod Sheath and Inter Rod Substance. ### Enamel Rod (Enamel Prism) 1. The number of the enamel rod varies from 5 millions in lower Central incisor to 12 millions in the upper first permanent molar. 2. **Direction:** The enamel rod is perpendicular to the dentin surface. - In deciduous teeth; the enamel rod is vertical at the cusp tip or incisal edge then become oblique toward the occlusal surface at the middle part and become horizontal at the cervical area ( so enamel ends cervically abruptly). - In permanent teeth; the direction of the enamel rods are similar to that of the deciduous teeth at the occlusal 2/3 but at the cervical region are directed root wise (so the enamel ends cervically as a knife edge). 3. **Course:** the enamel rod starts straight at dentino-enamel junction (D.E.J.) for about 30 µ then has a wavy course till near the outer surface of enamel where it becomes straight once more. - At the incisal edge or cusp tip the enamel rod has a twisted course and is called gnarled enamel. 4. **Diameter:** the diameter of the enamel rod increases from the dentino-enamel junction to the outer enamel surface by a ratio of 1:2. - Because: The surface of the enamel at D.E.J. is smaller than the outer surface. 5. **Cross section:** the enamel rods appear as hexagonal, oval, round or fish scales shape. The amount of the inter rod substance is variable according to the direction of cutting. 6. **Longitudinal section:** the enamel rods appear as cylinders with cross striations. ### Rod Sheath (Prism Sheath) Hypocalcified Each rod has an incomplete envelop which resists the decalcification and stains darker than the enamel rod and is called prism sheath. ### Inter-rod substance (inter-prismatic substance) The enamel rods are cemented together by the inter-rod substance which has higher refractive index than the rod. ## Hunter-Shreger bands: - It is an optical phenomenon caused by changes of rod direction (the wavy course). - They are seen clearly by longitudinal ground section viewed by reflected light at cervical 2/3. - The phenomenon appears as dark and light alternating bands, starting from the D.E.J. and ends shortly from the outer surface of enamel where the enamel rods run straight to the surface. - The dark bands (Diazones) absorb the light where the light bands (Parazones) reflect the light. - If the light passes from the opposite side, the light and dark bands will be reversed. - Hunter-Shreger bands can not be seen under the incisal edge or cusp tip because of the complicated course of enamel rods (gnarled enamel). ## Incremental lines of enamel 1. **Cross striations (short increments):** - The enamel rod is formed in a rhythmic manner, every segment of 4 microns in length and formed in a day. - By mild decalcification there will be transverse striations denoting these segments, the striations appear dark and called cross striations. 2. **Incremental lines of Retzius:** - In longitudinal ground section they appear as dark bands reflecting the rhythmic enamel formation. - At the incisal edge and cusp tip they arise from the D.E.J. then. Go upward and outward surrounding the tip of dentine and come to D.E.J. again so, they do not reach the outer surface of enamel. - Then towards the cervical line the incremental lines reach the outer surface of enamel in a series of transverse depression called perikimata. - In cross section they appear as concentric circles. - The incremental lines denote the periods of rest of ameloblasts and in between there is an area of activity. - The ameloblasts take 4 days to form the enamel between 2 dark lines (so, between 2 dark bands there will be 4 segments of enamel rod of 16 microns length). 3. **Neonatal lines:** - It is one of the brown stria of Retzius which present in all deciduous teeth and 1st permanent molar. - This line separates between enamel formed before birth and enamel formed after birth. - This line is due to the sudden change of nutrition and environment due to birth. - The quality of enamel formed before birth is better than that formed after birth, because of the more protected conditions and constant nutrition of the fetus. ## Primary enamel cuticle. - It is the last product secreted by ameloblasts. - By electron microscope it is a typical basement membrane. ## Enamel lamellae. - It is a hypocalcified rod and inter-rod substance as a result of abnormal tension during enamel formation. - It appears as a thin leaf or sheet-like structure which extends from the outer surface of enamel to a considerable distance of enamel till the D.E.J. and may extend to dentine. - It appears in both longitudinal and transverse ground sections. - If the stimulus is severe; a crack may be formed. ### Types of enamel lamellae: We have three types: type (A) type (B) and type (C) 1. **Enamel lamella type (A) {true lamella}.** - It is due to poorly calcified enamel rod and inter-rod substance. It is limited to the enamel. 2. **Enamel lamella type (B)** - It is a type of crack that occurs before eruption and is filled with the adjacent tissues. - It may be filled with epithelial cells of the enamel organ so, the cells in the depth will degenerate and those on the surface will be changed into hornified cuticle. - It may be filled with connective tissue of the dental sac so; it will be filled with cementum. 3. **Enamel lamella type (C)** - It is also a type of crack which occurs after eruption and contains the organic substance of saliva. - lamellae type (B) and (C) may be limited to the enamel or reach the dentine. - Enamel lamella could act as caries spread way. ## Enamel tuft. - It always appears in transverse ground section and rarely in longitudinal ground section. - It is a hypocalcified prism and inter-prismatic substance. - They arise from D.E.J. to about 1/5 to 1/3 of the enamel thickness as tufts of grass. It takes this shape because of the wavy course of the enamel rod for several layers leading to this tuft form. - The tuft occurs more than lamella. ## Enamel spindle. - It is an odontoblastic process which extends in between the cells of inner dental epithelium before the formation of enamel. - They appear in ground section (L.S. & T.S.) as pointed or rounded process. It is black because the protoplasmic process of the odontoblast is burned during the preparation of the section (so, it becomes an empty space and appears black). - They have right angle to the dentine surface and occur in great number under the tip of the cusps or incisal edge. ## Surface structure of enamel. 1. **structure-less enamel** - This layer is highly mineralized than the rest of enamel and its thickness is 30 microns from the surface. - It is found in all deciduous teeth and 70% of permanent teeth. - This surface layer of enamel has no prism and the appetite crystals are arranged parallel to one another and perpendicular to the incremental lines of Retzius. - Commonly they occur at the cervical area more than the cusp tip or incisal edge. 2. **Perikymata** - It is the external manifestation of the incremental lines of Retzius and represented as transverse wave like grooves on the surface of the enamel. - These grooves are continuous around the tooth and they are parallel to each other and to the cemento-enamel junction (C.E.J.). - There are 30 perikymata / mm in cervical area and decrease towards the occlusal surface. 3. **Rod ends** - They are concave depressions vary in depth and shape, where they are shallow at the cervical region and deepen near the incisal or occlusal surface. 4. **Cracks** - They are the outer edges of the enamel lamellae and occur as fissure like structure founded in all enamel surfaces and extend for varying distance. - By careful decalcification crack completely disappears 5. **Primary enamel cuticle.** - See before. 6. **A fibrilar cementum.** - A type of cementum formed on the cervical part of the enamel for a short distance. - It is formed due to the degeneration of the reduced dental epithelium covering the cervical area of the enamel before eruption. ## Life cycle of ameloblasts 1. **Morphogenic stage:** - The inner enamel epithelium arranges itself on basement membrane determining the D.E.J. - The cells are short and columnar with large oval nuclei the almost fill the cell body. - The I.E.E. is separated from the dental papilla by basement membrane and cell free zone. 2. **Organizing stage:** - The I.E.E. becomes tall (40 micron) in length (4-7 micron) in diameter. - Alternation of functional polarity of the cells i.e. Golgi apparatus and centeriols are distally where nucleus and mitochondria become at the proximal end, the nucleus occupies 1/2 of the cell. - The cell free zone disappears because of the elongation of I.E.E. which will cause differentiation of the odontoblasts by a process called induction. - At the end of this stage dentine will begin to be formed. 3. **Formative stage:** - The stage started after the formation of dentine. - The presence of dentine will cause histodifferentiation of the I.E.E. by reciprocal induction into ameloblasts. - The ameloblasts are long cells with the nucleus situated proximally and the Golgi complex increases in volume and present in the central core of the cell. - The cytoplasmic organelles needed for protein formation will be increased in number. - The ameloblests are closely aligned together by junctional complex proximally & distally (terminal bar). Any protein forming and secreting cell must fulfill the following: 1. Large opened face nucleus. 2. Abundant cytoplasm. 3. Increase in number of mitochondria. 4. Increase in number of Golgi apparatus. 5. Increase in number of rough endoplasmic reticulum (R.E.R.). 6. Presence of secretory granules. ## 4- Maturative stage: - It will start after most of enamel matrix at the cusp tip or incisal edge is formed, where cervical enamel formation is progressing. - The cells of stratum intermedium will change from flat cells into spindle shaped cells. - The ameloblasts during maturation will reduce in length, decrease in its volume and organelle content. - The excess of organelles associated with enamel formation will be enclosed in autophagic vacuoles and digested by lysosomal enzymes. - The distal end of the cell membrane will show folding to form striated border which will increase the surface area (this is for active transport of material across the plasma membrane). - Increased in number of mitochondria which will be shifted to the distal end of the cell. ## 5- Protective stage: - After formation and maturation of the enamel, the ameloblasts will be hardly differentiated from the other layer of enamel organ which will be formed of 3-4 layers of stratified epithelium covering the enamel. - The epithelium is called reduced enamel epithelium which will protect the mature enamel from the surrounding C.T. - If the C.T. comes in contact to the enamel before tooth eruption; the enamel may be resorbed or cementum will be formed on enamel. ## 6- Desmolytic stage: The epithelial cells of reduced enamel epithelium will secrete desmolytic enzymes that cause degeneration of the C.T. separating the developing tooth from the oral epithelium leading to eruption of the tooth. ## Amelogenesis Although the mineralization of enamel does not wait for the completion of matrix formation, the amelogensis is formed of 2 steps: 1. Matrix formation. 2. Mineralization (maturation). ## 1- Formation of enamel matrix ### B- Development of Tomes' process. - After formation of the first layer of enamel, the ameloblasts will move away from the dentine (upward) and develops a short conical projection at its distal end called Tomes' process. - The Tomes' process is delineated from the cell proper by incomplete septa called the distal terminal bar. - The ameloblastic cytoplasm continues to Tomes' process which contains secretory granules and no cell organelles. - The ameloblast contains the protein forming organelles. - The secretory granules in Tomes' process will be secreted with perpendicular angle to the membrane of Tomes' process. - This direction of enamel protein secretion will be responsible for the appearance of enamel prism and inter-prismatic substance. ### C- Distal terminal bar. - By the formation of Tomes' process there will be distal terminal bar at the distal end of the ameloblasts delineating the Tomes' process from the ameloblasts. - The distal terminal bar is a condensation of cytoplasm with thickening of the cell membrane. - Their exact function is not known but may help in the following: 1. Prevent the cell organelles from entering Tomes' process (intracellular function). 2. Prevent leakage of the secretion in between the ameloblasts (extracellular). ## 2- Mineralization and maturation of the enamel matrix. Mineralization occurs in two steps: (A)- Initial (immediate) or partial mineralization which occur in the same time during matrix formation. Its amount is about 25-30% of the total mineral of the mature enamel. (B)- The second stage of mineralization will gradually complete the calcification of enamel to reach the amount of 96% inorganic substance. - After eruption of the tooth, maturation of the enamel continues by deposition of ions from the saliva to reach 98% of its weight inorganic material. ## Age changes of enamel: ### A) Attrition. It is the physiological wearing away of tooth hard substance due to functions (contact between teeth during mastication). 1. Attrition occurs in incisal, occlusal, and proximal surfaces. ### B) Permeability. 1. In newly erupted teeth, the enamel is permeable from the outer surface and from the pulpal side equally. 2. In old enamel the permeability from the oral side will decrease or may disappear; but remain from the dentinal side. ### C) Hardness. Because enamel is a semi permeable membrane, ionic exchange will increase certain elements as nitrogen and fluorine in its superficial layers. The total inorganic content will increase which will leads to raise the resistance to decay. ### D) Color. Enamel becomes dark by age.