Objectives: 1-Understand the Physical and Chemical characteristic features of enamel 2-Recognize the surface and histological structure of enamel. (1) Definition; Enamel 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 which are lost as the tooth erupts and hence enamel can not renew itself. Oral ectoderm *It is a mineralized epithelial tissue that is totally acellular. Dental *Inert. lamina *Non-vital. *Insensitive. Inner *If wear or Caries, it can’t replaced enamel Or regenerated (can’t renew itself). epithelium *Permeable. *Unique crystalline structure. *Unique matrix protein. (3) Physical properties of enamel: A)Color: Yellowish white to grayish white (according to thickness, degree of translucency, degree of calcification & homogeneity of enamel). B)Thickness: Variable (max. on cusp tips {2-2.5mm.}& kinfe edge at cervical margin). C)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. D)Brittleness: Enamel is brittle, especially when looses its elastic foundation of healthy dentin (i.e. undermind enamel). E)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. GROUND SECTION DECALCIFIED SECTION Methods of Studying ” hard tissues” HISTOLOGICAL PREPARATION OF TEETH GROUND SECTIONS ENAMEL Show enamel, dentin and bone. Do not show cells and soft tissues. PULP CAVITY CEMENTUM DENTIN DEMINERALIZED PULP SECTIONS CAVITY Show cells and organic material. Do not show mineralized components. (4) Chemical composition: By weight; -96% inorganic substances in - 4% organic substance and water. the form of hydroxy appetite (1-2%organic material& 2-3 % water). crystals (0.05-1 mic length& The organic substance present between 300 angstrom thick and 900 the crystals as a fine network or angstrom width) forming a cortex(envelop) or a cover surrounding each crystal. 3Ca3(Po4)2.Ca(OH)2. - - Some molecules may replace - Organic material is proteinaceous and the hydroxyl radical as fluoride, contain some mucopolysaccarides. - The protein is non-collagenous protein Mg, Strontium, carbonate and called enamel protein enamelin(with sulfate. This is the basis for the high molecular weight) or amelogenin , national project of fluorination amino acids( proline, glutamine, Lucien, of drinking water. glycine and histidine.).. **By volume; 50% inorganic substances and 50% organic substances.(1:1) (5) Histological structure; Enamel is difficult to study in decalcified section, due to its high crystalline nature, so it is studied by ground section. ** Enamel rod or prism: is the structural unit of enamel a) Number; 5 million in lower central & 12 million in upper molars. b) Diameter; small at inner side (DEJ) and wide at outer surface. 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. Wavy Course of Enamel Rods c) 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. d) Length; longer than enamel thickness due to its wavy course. e)Direction: perpendicular to the dentin surface In primary teeth, rods are horizontal, at the cervical and central part of the crown, and then change generally to become in oblique direction until they become vertical in the region of incisal ridge or cusp tip. In permanent teeth, the directions of enamel rods are similar to the primary teeth in the occlusal and middle thirds of the crown. In cervical region, the enamel rods deviate from the horizontal to apical direction. By light microscope **T.S. Enamel rods appeared in cross section by light microscope as hexagonal, oval, round and as fish scales. Enamel rods don’t not have a regular geometry. By light microscope, the rod appeared to be surrounded by rod sheath. Rods are separated from each others by interrod region. Enamel rods have clear crystalline appearance, permitting light to pass through them. ** 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, it has different refractive index. **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-striations within longitudinal ground section of enamel (Lines at right angles with long axes of prisms) Electron microscope; - The rod is shaped somewhat like a cylinder and is made up of crystals with long axis that run, for the most part, parallel to the long axis of the rod particularly for crystals along the central axis of the rod. - The interrod region surround each rod, and its crystals are oriented in a direction different from those making up the rod. - The boundary between rod and interred enamel in this region is delimited by a narrow space containing organic materials known as the rod sheath. - This difference in the crystal angulation results in difference in the refractive index of the prism & interprismatic substance ( is a function of Tomes’ processes). ** In the enamel prism & interprismatic region, the crystals are so tightly packed, so no or minute spaces were found between them for the organic material, so organic material only forms an envelope around them. ** While the abrupt change in crystal orientation in the rod & interprismatic region, leads to crystals not tightly packed, allowing more space between them for more organic material to be present , this account for the prism sheath to be visible even in LM. Rod sheath Rod Interrod substance Electron microscope of enamel ** Cross section: (Keyhole appearance) _The head of keyhole represent part of enamel rod with apatite crystals ⁄⁄ to long axis of the rod ( 0-40̊ ), Directed occlusally. _ The tail represents the interprismatic region, directed Cervically (where crystals fan out until become ┴ to the Long axis. **N.B : T.S. of enamel prisms revealed different patterns:- Such as: Hexagonal, round, oval, fish scales and keyhole patterns. Enamel rods Enamel rods Rod Sheath ) َو َسا ِر ُعىاْ إِلَى َم ْغ ِف َر ٍة ِّمن َّربِّ ُك ْم َو َجنَّ ٍة َع ْر ُض َها ال َّس َما َوا ُت َواﻷَ ْر ُض أُ ِع َّد ْت ِل ْل ُمتَّ ِقي َن ( 1- Hunter Schreger bands (optical phenomenon) dark and light bands ** Alternating dark & light bands , start at DEJ & end shortly before outer surface of enamel. They present at middle & cervical thirds of the crown and absent in the region of gnarled enamel. This optical phenomenon is seen when examining LS ground section of enamel under oblique reflected light, this is due to the wavy course of enamel rods. B A Hunter Schreger bands Hunter Schreger bands 2- Incremental lines of Enamel 1- Cross striation 2- Brown Striae of Retzius 3- Neonatal line 1) Cross striations or short increments,: _ The enamel rod is built up of equal segments separated by dark lines that gives it (striated appearance), this can be seen by partial decalcification (etching) of enamel using mild acids. This is due to the rhythmic apposition of the enamel matrix by ameloblasts, [ Daily rate of secretory activity of the ameloblasts]. By SEM the segments of 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 brown bands form semicircles do not reach the surface, they 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). **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 rythmic formation of enamel ( where periods of activity of the ameloblasts alternate with periods of rest). ** When the incremental lines are broad , it means that the rest period of ameloblast is prolonged. ** When the incremental lines are close to each other , means that there is metabolic upsets. 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 foetus. Neonatal line NEONATAL LINE Postnatal Prenatal Enamel Enamel 3- Primary enamel cuticle (Nasmyth’s membrane). _ Delicate organic membrane that cover the crown surface of newly erupted teeth. _ It is the last secretory product of the ameloblasts, _ Its thickness (0.2 micron), it has structure similar to the basal lamina of the epithelium.