Transcript for:
الخصائص الفيزيائية والكيميائية للمينا

hello and welcome to lecture number two today we are going to begin discussing the first heart issue of the tooth which is enamel i am dr hibaldo and i hope you will enjoy this lecture we will discuss the physical properties of enamel the chemical composition and finally we will discuss the histology of enamel and see some histological slides enamel which is the hardest tissue in the whole body is the heart tissue that covers the tooth and can be seen in the oral cavity this tissue withstands shearing forces shading forces are forces that work two forces that works in different directions or opposite directions also it withstands impact forces like the force would have that happens during occlusion it has high resistance to abrasion abrasion means loss of tooth structure because of friction with a hard object other than other teeth the thickness of enamel varies from up to 2.5 millimeters and a cusp area to be a further edge at the cervical margins in primary teeth where the thickness of enamel is almost uniform it is about 1.3 millimeters enamel cannot be repaired or replaced because the cells that secretes enamel which are called amyloblasts they perform apoptosis after they finish secreting enamel the enamel has low tensile strength or in other words high modulus of elasticity both means that the tooth cannot resist the forces that tends to pull it apart which means you can easily break enamel that's why it requires the support of the resilient dentine without the support enamel is easy to break or in other words it's brittle the surface enamel is harder denser and less worse than subsurface enamel why we will come to that in the histology section of this lecture the hardness and density decreases from the cusp tips to the cervical margins young enamel appears white turning to a more yellow appearance as truth translucency increases with age the chemical composition of enamel enamel is composed of 96 percent in organic material 2 percent organic material and 2 percent water by weight the inorganic composition is the same in all the heart tissues of the tooth it's calcium hydroxyapatite the difference is in the size of the crystal and how does it form or bind to each other and also to the calcium and other minerals uh composition and inside the crystal the organic composition we have three amino acids small molecules peptides large proteins complexes like amelogenins and non-amilogenins the enamel hydroxyapatite crystal are about 17 nanometers in width and 25 nanometers thickness almost the full thickness of enamel actually most crystals are hexagonal in cross-section as we can see here the core of the crystals are rich in magnesium and carbonate in comparison to the peripheries and the length of it sometimes the whole length of the enamel each crystal unit has a hydroxyl group surrounded by three calcium ions which are surrounded by three phosphate ions six calcium ions in a hexagon enclose the phosphate ions the crystal is made of a repetition of those planes of ions side by side in a stacked layers some substitutions of ions and minerals can happen in the human appetite the main substitutions that can be seen are the hydrogen phosphate and the carbonate ion can substitute the phosphate ion also strontium barium lead sodium potassium and magnesium can substitute calcium ions furthermore fluor chlor bromine iodine can substitute the hydroxide iron the ions com present in the enamel the kind of ions can influence dental caries by affecting the dissolution of the appetite crystal and the remineralization for example fluoride incorporation in the crystals inhibits caries the carbonates incorporation in the crystals promote the carious attack because it makes enamel more soluble water is present in enamel in two percent by weight five to ten percent by volume what represents is related to the porosity of the tissue because it can be found between the crystals surrounding the organic component it can be trapped within the crystalline defects forming a hydration layer ions such fluoride travel through the water component the organic matrix so mature enamel has one to two percent organic matrix that varies from 0.05 to 3 depending on the regularity of the crystals of course a mature enamel has higher and higher percentage of organic matrix this organic matrix consists of proteins that are exclusively found in enamel ninety percent of them are called amelogenins ten percent of them are non-milogenics they do not contribute to the enamel structure the highest concentration of proteins in enamel is found in a structure called tufts that is found at the gentino enamel junction we will see that in the section of histology the lipid content is one percent by weight of enamel and it may be there because it's a remnant of cell membranes that form the enamel you will be introduced into these membranes and how did they form the enamel and the truth in the embryology lectures so first we are going to talk about the amelogenins unfortunately this information are to be memorized there's nothing there is not a lot of things to understand in the components of the organic matrix of the enamel so i hope you will be patient till we um start discussing the histology of the tissue so aminogenins are hydrophobic they have low molecular weight and they aggregate into clumps they are produced by ameloblasts which are the cells that secreted in allen as i have already mentioned they spread throughout the whole developing enamel resulting in a gel matrix through which the molecules and ions spread this helps in the formation of large crystals afterward in the mineralization stage all these stages were given to you when we discuss amelogenesis and the embryology lectures the non-amerigenins such as daphthylene amyloblastin and enameline they have low molecular weight they are derived from plasma albumin they contain distinct components secreted by amyloplasts and they may have a role in mineralization along with amelogenomes do you remember this slide i'll show it to you last lecture it talks about how can we study different tissues in the oral tissue in the oral cavity so we can use the ground section where the organic substance is burned and the inorganic substance remains so due to the high mineral content of enamel 96 percent it is totally lost in the demineralized section as we can see here so enamel structure is mainly studied in the ground sections a mature enamel can be studied in demineralized sections due to its high protein content around 25 to 30 percent as you will be taught in the amelogenesis lecture the enamel formation starts with the matrix and organic matrix which happen which is mineralized in a second stage that's why the immature enamel which is not yet mineralized has a high protein content the basic structure unit of the enamel is called the rods or the prisms each prism consists of several million hydroxyapatite crystals that are packed into a long thin rod five to six micrometer in diameter and up to 2.5 millimeter in length as you can see these are the rods cut in a cross section or a limited intersection frozens this photo is taken by the electro microscope on very in a high higher magnification so the prisms are separated by what we call the inter-road substance here where crystals change direction and deviate 40 to 60 degrees leaving some space for organic material to accommodate higher magnification here prism boundaries reflect the sudden change in crystallite orientation we will discuss that again in a few minutes the prisms have have a slightly undulating course that reflects ameloblast path during seclusion in a cross-section the enamel prisms take different shapes or patterns the keyhole pattern pattern 3 predominates the prisms have head and tail region the tail of one prism lies between the heads of two adjacent prisons maybe it's hard to you to see it in this section but in a few minutes the head and the tail i will show it in another section in a few moments and per change of crystal orientation of the prism boundary is responsible for the optical appearance of the boundary which means that the boundary looks a little bit darker than the inside of the prism this section shows us how the prism stakes undulating and wavy course so the black prism comes from here but it ends coming up from here the gray one comes from here comes up from here the white one and so on because the ameloblast takes this undulating course during a seclusion of enamel as you can see here is the keyhole pattern here and it's different it's wider when we cut in different angles getting much wider when you cut in a wider angle so the keyhole pattern can be seen here very clearly there is a head and a tail the tail is between two heads the crystal's direction in the middle is different than the boundaries the crystals deviate at the boundaries as i have just explained also the crystals in the middle or in the head has a different angle than the tail the crystals in the tail also deviates this deviations at the boundaries gives us this darker color of the pattern that makes us able to recognize the pattern it's parallel to the long axis in the middle and the head and deviates as i have said at the boundaries here it's easier to understand how the shape is different according to the cutting angle in cross section when we cut a 90 degrees angle we get a perfect keyhole pattern as we go from 90 to zero the shape of the keyhole becomes more and more circular until we don't have a pattern anymore we have almost four patterns of the enamel prism's interest section the keyhole the almost circular the stacked pattern in which all the prisms are stacked above each other and the one that has no pattern every 10 to 13 layers of prism follow the same direction but the blocks of prisms above it and below it follow a different path this gives rise to banding pattern called the hunter schrager bands they are approximately 50 micrometer in width and are visible due to light reflection in different directions so it is an optical phenomena and the outer quarter of enamel all prisms run in the same direction and so there is no banding the prisms or the prism bands that are cut longitudinally are dark and they are called the perizones while the one cuts and transverse are light and they are called the diazones the angle between them is 40 degrees this pattern actually strengthens the enamel against fracture in the cusp areas as you can see here the space for the prisms is a little bit tighter than that in other areas so what happens is the prisms over these areas they become twisted as you can see here they are twisted around each other in a complex arrangement that we called ignard enamel the other surface layer of enamel about 20 to 70 micrometer in secondary teeth is a prismatic the crystallites are aligned at right angle to the surface and parallel to each other that's why the surface layer is more highly mineralized than the rest of the anova this is attributed to the absence of prison boundaries where the organic material is located as i have mentioned earlier enamel as much as any other heart tissue in the tooth is formed in increments because of periods of activity alternating with periods of inactivity this results in what we call incremental lines there is lines that represent short periods of activity we call them the cross striations and there is lines that represents long periods of activity um approximately weekly a week or 10 days of activity that's called the enamels trial so the crosstriations the clustrations appear as lines crossing the enamel prism at right angle to their long axis so what we are talking about is these lines that are crossing enamel in a right angle they reflect a diurnal rhythm or in other words a daily increments of growth they are 2.5 to 6 micrometer apart they are closer to each other near the enamel detail junction and they may represent variations in the organic matrix or the crystal orientation and composition or prism width again they are these black lines that are crossing enamel prisms in right angle the enamel straya or straya of fritzius they run obliquely across the prisms as you can see here they represent increment incremental lines uh that are approximately represents a weekly or 10-day period of activity and in this section as you can see you can see on the surface of enamel the cells that are forming the tooth which are called amyloblasts which means this tooth is still in early stages of formation and the enamel here has high organic matrix percentage in a cross section like the one we see here this is the enamel this is the dentino enamel junction and this is the dentine so the stray of ridges run circumferentially like rings of sea of three as you can see them these are straight of ridges they are seven to ten cross striations between enamel adjacent straya so each two stray africans have seven to ten crustaceans between them the enemies trail are 25 to 35 micrometer apart in the middle portion where the crustaceans are four micrometer apart and 15 to 20 micrometer apart cervically whereas the crustaceans are two micrometers per third on the surface of a newly erupted tooth the striae appears as fine grooves running circumferentially around the crown as you can see in this picture they are called pirikimata a grooves where the parikhima targets and between they are removed after eruption by attrition and abrasion attrition is the loss of total structure that is caused by uh confronting or friction with other teeth the new nato line is one of the incremental lines that that shows us difference between the enamel formation before and after birth so we call this prenatal enamel and this is postmentally added which are of course different in composition the surface enamel as we have already mentioned it's richer and trace elements especially has this carbonate it's a prismatic therefore i don't realize as i have explained in the past slides we can see some phenomenons on the surface enamel for example on protected areas of enamel small pits can could be seen on the surface they are within the purikumata and they mark the end of ameloblasts where the aminoblasts were actually sitting on the surface before they committed apoptosis they are 1 to 1.5 micrometer in depth also we can see something called enamel caps they are small elevations 10 to 15 micrometer across they result from another deposition on top of debris that happens late during tooth development when those caps are removed for one reason or another we see something called focal holes which are depressions in the surface this happens due to abrasion or attrition also in the surface enamel we can see something called enamel rocks or brush they are elevations on the enamel surface 30 to 50 micrometer in diameter they have radiating groups of crystals and they are more common and premolars so as we go deeper and deeper in enamel we reach the inaudible injunction in the areas where shearing forces are high like beneath cusps and inside the edges the enameled in team junction has a scalloped shape or pattern but in lateral surfaces it's more more smooth the enameled interior junction is less mineralized than the enamel and the detail on the argentina enamel junction we see multiple phenomena among them is the enamel spindles enamel spindles are narrow attributes that are eight micrometer in diameter and they are actually odentoplastic processes that are extended from vitantine and to the enamel to reach sometimes 25 micrometers of the enamel length as i will discuss in the density lecture chain is composed of two viewers that are lying an organic matrix inside the tb inside these two views we will have the odontoplastic processes that extends sometimes from the anterior enamel junction into the enamel dental blasts are the cells that forms and secretes dentine these enamel spindles are most commonly seen beneath cusps because of crowding that makes the space less for the london plastic processes and pushes some of the processes above the junction also at the argentino enamel junction we can see what is called enamel tufts as you can see tuft means something that looks like grass they have the same direction as enamel prisms because they are a component of enamel they are hyper mineralized areas that shows residual matrix proteins as we mentioned earlier at the beginning of the lecture they are highly they have high percentage of organic matrix tough to protein is a minor non-american protein they occur almost at a 100 micrometer interval another structure that can be seen in the inaudible it's a structural fault that run the entire enamel thickness from the surface to the interior enamel region they are hypo mineralized areas because incomplete maturation of groups of prisms had happened there as i have mentioned earlier that enamel has some kind of micro porosity the enamel pores are water filled spaces between the crystals they make three to five percent of the volume of the enamel larger ports are found at the prism boundaries because as we have seen in the slides and in the pictures at the boundaries of the prisms or the crystals deviate have deviation in the angles at the boundaries and at the tail there is another junction between the enamel and another tissue it is the cementum in what we call the cement to enamel junction the cemento enamel junction has three patterns so pattern one is where the cementum overlaps the enamel which happens in sixty percent of the cases pattern two is where the cementum and enamel meets at the bud joint which happens in thirty percent of the cases like we see here pattern three is where the enamel and the symmetry falls to meet and there is exposed the team between them another pattern the fourth button that is very rare around 1.6 of the times the enamel actually overlaps that in team it does cement i'm sorry all these patterns may be present in a single tooth but in different areas i will show you more slides about the cement normal junction in the lab section of enamel what happens to enamel with aging so enamel wears slowly with age depending on the diet and habits it darkens in color because of the reduced translucency of the tooth and secondary in teen forms and enamel things we will discuss secondary dentin in the dentine lecture because of the accumulation of surface coatings and stains you know the people who smokes a lot and drink a lot of caffeine caffeinated beverages they have darker teeth the composition of surface enamel changes as a result of exchange with oral fluids as we have discussed it depends on which uh ions are substituting the ions of the hydroxyapatite crystal so if it's fluoride it will be more resistant to carries if it is carbonated will be more prone to carries there is a decrease in care is due to enhanced mineralization i just have said it depends on the diet the ph of the mouth and the kind of substitutions that happened in the crystals a final phenomena that is maybe interested for you is what we call enamel pures they are small droplets of enamel on the root near the location because the uh sometimes uh in the stage of teeth formation some of the cells that forms enamel just slips down a little bit into the root and continue to form enamel there to form enamel pierce how does that happen what is hurtworks rootsheath you will be introduced into that in the in the embryology lectures that would be all for today next step is to see the laboratory section for enamel where we where i will show you a lot of histological slides to review and discuss the histology of enamel that we have discussed here in the theory part i wish you all the luck bye bye