Enamel (1) - Oral Biology and Histology 1 - Nile Valley University - PDF

Nile Valley University Faculty of Oral and Dental Medicine Oral Biology Department Oral biology and Histology 1 DOB 111 Enamel (1) Enamel Objectives: By the end of all enamel lectures, the students should be able to: 1-Understand the physical and che...

Nile Valley University Faculty of Oral and Dental Medicine Oral Biology Department Oral biology and Histology 1 DOB 111 Enamel (1) Enamel Objectives: By the end of all enamel lectures, the students should be able to: 1-Understand the physical and chemical characteristic features of enamel. 2-Recognize the surface and histological structures of enamel. 3- Understand the life history of ameloblasts and amelogenesis. 4- Recognize the age changes of enamel.  Enamel 1-Definition. 2- Origin. 3- Physical properties. 4- Chemical composition. 5- Enamel rods. 6- Histological Structure of Enamel. * Enamel rod * Incremental lines of Enamel. * Hunter Schreger bands. * Amelodentinal junction. * Enamel lamellae. * Enamel tufts. * Enamel spindles. * Surface structures of Enamel. 7- Age changes of enamel. 8- Life cycle of ameloblasts and amelogenesis. 1) Definition; It is the hardest mineralized tissue in the human body that covers the anatomical crown of teeth. It forms a protective covering of teeth to resist the stresses and forces of mastication. 2) Origin: ectodermal in origin from the IEE. Formative cells: Ameloblasts Oral ectoderm *Mineralized epithelial tissue that is totally acellular. Dental *Non-vital. lamina *Insensitive. *If wear or Caries, it can’t be replaced Inner or regenerated (can’t renew itself). enamel epithelium *Permeable. *Unique crystalline structure. *Unique matrix protein. (3) Physical properties Color: Yellowish white to grayish white (according to thickness, degree of translucency, degree of calcification & homogeneity of enamel). Thickness: Variable (max. on cusp tips {2-2.5mm.}& Kinfe edge at cervical margin). Hardness: Hardest calcified tissue in the body due to: [↑↑content of mineral salts & its crystalline arrangement]. Hardness of permanent teeth ˃ deciduous. Hardness at the surface ˃ DEJ. Hardness at cusp & incisal edge ˃ hardness at cervical line. Brittleness: Enamel is brittle, especially when looses its elastic foundation of healthy dentin (i.e. undermined enamel). Permeability: Enamel can act as semipermeable membrane, permitting complete or partial slow passage of certain ions & dyes. Mainly from the saliva to the outer layer of enamel, but to a lesser degree from the pulp to the inner enamel layer across the dentin.  (4) Chemical composition: By weight; 4% organic substance and water. -96% inorganic substances 1-2%organic material - hydroxy appetite crystals 2-3 % water 3Ca3(Po4)2.Ca(OH)2. - - Organic material is proteinaceous and - Some molecules may replace the contain some mucopolysaccarides. hydroxyl radical as fluoride, carbonate. - The protein is non-collagenous protein -This is the basis for the national project called enamel protein as enamelin, of fluorination of drinking water, tooth amelogenin and ameloblastin paste, mouth wash and fluoride gel. Enamel structure (5) Histological structure;  The unit structure of enamel is enamel prism which consists of tight packed mass of hydroxy apatite crystals.  Each prism has enveloping prism sheath and interprismatic substances Enamel Rod 1. Number 2. Diameter 3. Direction 4. Course Number: 5 million in lower lateral & 12 million in upper first molars. Diameter: The diameter of the enamel rod increases from the dentino-enamel junction (4μ) to the outer enamel surface (6μ).. The ratio between diameter of the rods at inner & outer enamel surface is (1:2) , this due to difference in the surface area between inner & outer surface of enamel. Direction: perpendicular to the dentin surface Near the cusp tip or the incisal ridge------- vertically. In the middle region of the crown-------oblique toward the occlusal surface. In the cervical region-------horizontal in deciduous teeth. Permanent teeth few rods are tilted apically.  Wavy Course of Enamel Rods Course: Enamel rods follow a wavy course from DEJ outwards, just before outer surface of enamel, they become straight. A more spiral course(complicated or twisted or braided) is noted at cusps & incisal areas Gnarled enamel. to give max. strength against masticatory forces. Length; longer than enamel thickness due to its wavy course. Incremental lines of Enamel 1- Cross striation 2- Brown Striae of Retzius 3- Neonatal line 1) Cross striations or short increments: Represent the daily rate of secretory activity of ameloblasts. Each enamel rod is built up of segments separated by dark lines that gives it a striated appearance. Each segment has a uniform length (4 microns). Cross striation Enamel rods and cross striations 2) Brown striae of Retzius(Incremental lines of Retzius:- They are incremental growth lines indicating the successive rhythmic apposition of enamel layers during formation of enamel. They represent weekly rhythm in enamel production. **In L.S. ground section of enamel : At the incisal ridge & cusp tip, they appear as brownish bands form semicircles do not reach the surface, Arise from DEJ upwards and outwards surrounding the tip of dentin. At the middle & cervical thirds they reach the outer surface of enamel and become represented as series of transverse depressions called (perikymata).  L.S ground section in crown Incremental line of Retizus Enamel spindle DEJ Neonatal line Enamel lamella Dentin Pulp space The relationship between the striae of Retzius and surface perikymata (arrows). 2) Brown striae of Retzius(Incremental lines of Retzius:- **In T.S. : the incremental lines of Retzius appear as concentric rings similar to the growth rings in a cross section of a tree. **Cause: May be due to the rhythmic formation of enamel ( where periods of activity of the ameloblasts alternate with periods of rest.  Brown Striae of Retzius L.S. T.S. Longitudinal ground section showing deposition of the striae of Retzius ( Arrows in the enamel layer). 3) Neonatal line Neonatal line is an accentuated incremental line that apparently reflects the great physiologic changes occurring at birth in enamel. It separates the prenatal formed enamel from the postnatal one, (before and after birth). It is therefore present only in the deciduous teeth as well as the first permanent molars. It is due to abrupt change in nutrition and environmental condition that occur at birth. The prenatal enamel is thought to be more homogenous than the postnatal enamel , due to more protected conditions & constant nutrition of the fetus.  NEONATAL LINE Postnatal Enamel Prenatal Enamel By electron microscope * In cross section (T.S): Enamel rods appeared as hexagonal, oval, round and as fish scales. Enamel rods don’t not have a regular geometry. * In longitudinal section (L.S): The rod appeared as cylinder formed of tightly packed hydroxyapatite crystals ( parallel to the long axis) and less organic content. ** prism sheath : is the peripheral part of enamel rod that forms incomplete envelop around the prism. Less calcified than rod itself. **interprismatic substance: separate the enamel rods from each other. Highly calcified as enamel rods, crystal nearly perpendicular to the rod. **L.S - The cylindrical prism appear to be divided horizontally into equal segments by less calcified dark lines giving:, striated appearance , which are called short increments or cross striation. Cross-striations within longitudinal ground section of enamel (Lines at right angles with long axes of prisms) By electron microscope: Enamel Rod Cross-section As a key hole, the head toward the occlusal or incisal surface where the tail is cervically. Four Ameloblasts are responsible for the formation of one keyhole; one for the head and three for the tail.  Enamel crystals in the center of the head of the keyhole representing the rod as they oriented parallel to the long axis of the rod (cross sectioned). Crystals more far-away from the center of the rod flare laterally to an increasing degree as they approach the rod boundary (tail) until they become nearly perpendicular to the rod in the inter-rod region. The Tomes’ processes responsible for the different crystals orientation and so the rod pattern. Rod Sheath  Enamel 1-Definition. ✓ 2- Origin. ✓ 3- Physical properties. ✓ 4- Chemical composition. ✓ 5- Enamel rods. ✓ 6- Histological Structure of Enamel. * Enamel rod ✓ * Incremental lines of Enamel. ✓ * Hunter Schreger bands. * Amelodentinal junction. * Enamel lamellae. * Enamel tufts. * Enamel spindles. * Surface structures of Enamel. 7- Age changes of enamel. 8- Life cycle of ameloblasts and amelogenesis.