Enamel PDF

Enamel Cristiane M. Franca, DDS, PhD Research Assistant Professor Division of Biomaterials and Biomedical Sciences School of Dentistry, OHSU Enamel ◉ Hardest mineralized matrix in the body, brittle, translucent ◉ Amelogenesis ◉ Ameloblasts ○ cells responsible for enamel formation ○...

Enamel Cristiane M. Franca, DDS, PhD Research Assistant Professor Division of Biomaterials and Biomedical Sciences School of Dentistry, OHSU Enamel ◉ Hardest mineralized matrix in the body, brittle, translucent ◉ Amelogenesis ◉ Ameloblasts ○ cells responsible for enamel formation ○ Life cycle – unique in the body 2 Structure of Enamel ◉ Very complex – combination of methods to study properly ◉ Ground section ○ non-decalcified tooth and viewing by polarized light Late maturation Late maturation Late maturation ground section ground section polarized light enamel dentin enamel dentin enamel 3  Structure of Enamel ◉ Demineralized histological sections ○ organic material, reveals structure. Keyhole or fish-scale Secretory – Immature enamel decalcification dentin enamel Structure of Enamel ◉ Electron microscopy ○ Rods – prisms Ribbon-like carbonatoapatite crystals ○ Rod sheath (more organic material) ○ Interrod enamel (interprismatic substance) Rod Structure Side of Tomes Process (proximal) Tip (distal) Orientation of crystals is different in rod and interrod regions Fig 7-10 Crystals grow in thickness “young” “older” Rodless enamel primary and secondary teeth! Initial Enamel – near the DEJ Final Enamel – tooth surface  Amelogenesis & life cycle of ameloblasts ◉ Amelogenesis – 3 main functional stages 1. Presecretory ○ Differentiating ameloblasts – phenotype, change their polarity, develop protein synthetic apparatus and prepare to secrete organic matrix 2. Secretory (or formative stage) ○ Ameloblast elaborate and organize the entire enamel thickness 3. Maturation ○ Ameloblasts modulate and transport ions for accretion of mineral 10 Amelogenesis & life cycle of ameloblasts ◉ Amelogenesis – presecretory, secretory and maturation ◉ Life cycle of ameloblasts – functional stages 1. Morphogenetic stage 2. Histodifferentiation stage 3. Initial secretory stage (no Tomes` process) 4. Secretory stage (Tomes` process) 5. Ruffle-ended ameloblast of the maturative stage 6. Smooth-ended ameloblast of the maturative stage 7. Protective stage 11 Morphogenetic Stage  Morphogenetic Stage Ameloblasts 1.These are cells of the inner enamel epithelium before any changes begin to take place. 2.They are cuboidal or low columnar 3.A basement membrane is present 4.The acellular zone in the papilla is apparent 5.In the papilla, the cells destined to be odontoblasts have not gotten organized or lined up next to the acellular zone Junctional complex: tight junction, zonula adherens, desmosomes) Differentiative Stage Ameloblasts B. Differentiative Stage Ameloblasts 1. These are the preameloblasts. They are more columnar than morphogenetic stage, their nuclei shift to the proximal end of the cells, and they begin to extend processes trough the basement membrane; a second junctional complex forms 2. In the papilla, preodontoblasts are “organized” – they are getting larger and lining up across from the preameloblasts. The acellular zone disappears as the odontoblasts differentiate 3. The preameloblasts induce differentiation of odontoblasts; predentin is produced by odontoblasts 4. Amelogenin is expressed by the preameloblasts and some enamel proteins secreted before basal lamina breaks down 5. Signals from odontoblasts/predentin triggers differentiation to secretory stage (mouse studies implicate BMPs) Secretory Stage Ameloblasts Early/Initial Early Secretory Stage Ameloblasts 1. Early/Initial stage ameloblasts are more columnar and the basement membrane is gone 2. Visible dentin is present across from them 3. These early ameloblasts secrete enamel matrix that will form the initial layer of “structureless” enamel (~ 5 microns at the DEJ) 4. Proximal, flat end of Tome’s process is present Secretory Stage Ameloblasts Secretory stage Ameloblasts (cont) – These ameloblasts have a distal Tome’s process, a pointed or shovel- shaped tip, that allows formation of “structured” enamel Golgi and secretory granules are prominent (cytochemical stain for golgi in low power, SEM shows golgi) The enamel matrix (immature enamel) is structured (rods) – previously called prisms The enamel matrix secreted is immediately mineralized to 30% Tomes process and rod structure Shovel-shaped tip of an ameloblast which allows formation of structured enamel Final enamel is rodless Slightly more severe acid etching is required, especially in primary teeth (more rodless enamel) Transitional Stage Ameloblasts Maturational (columnar, next to thick enamel matrix) 1.Transitional stage – decrease in cell height, volume, and organelle content; 2.Some enamel proteins are still being made and secreted during the transition 3.~ 25% ameloblasts undergo apoptosis Maturation Stage Ameloblasts Maturational stage 1. During maturation proper, protein is removed (smooth) and more mineral is added (ruffled) so that mature enamel is 96% mineralized. Ameloblasts “modulate” between two phases. Another ~25% of the ameloblasts undergo apoptosis. a. Ruffle-ended cells: i. Proximal junctions are leaky, distal junctions are tight. ii. High level of endocytic activity iii. Contain machinery to pump Ca++ into the enamel matrix iv. Produce bicarbonate ions that may prevent demineralization of the enamel crystals b. Smooth-ended cells i. Proximal junctions are tight, distal junctions are leaky ii. Little endocytotic activity iii. No Ca++ pumping activity iv. Organic breakdown products pass between leaky junctions 2. Basal lamina secreted, hemidesmosomes attach cell to basal lamina a. Form a basal lamina rich in glycoconjugates. b. Typical basement membrane components are not present (collagen IV) c. Laminin-5 is present and essential for formation of hemidesmosomes. d. Amelotin, ODAM, and SCCPPQ1, novel secretory products, are present in basal lamina e. So it is a unique structure that may allow adhesion to enamel and confer properties that could help regulate movement of material into and out of the enamel layer. Protective Stage Ameloblasts Protective stage ameloblasts 1.Form part of the Reduced Enamel Epithelium 2.Cells transition from low columnar to low cuboidal or flattened 3.Remain associated with the newly formed enamel protecting it until the time of eruption 4.Permits some modification of enamel composition, such as fluoride incorporation 5.Most of them will undergo programmed cell death during eruption into the oral cavity – thus limiting the capacity of enamel to repair/regenerate itself 6.Remainder of the protective ameloblasts & reduced enamel epithelium (REE) form the initial junctional epithelium  Enamel Formation & Composition A. Matrix formation (Organic = 1-2%) 1. Enamel proteins (Exact composition is still unclear) 90% amelogenins 10% non-amelogenins: ameloblastin (amelin, sheathlin), enamelin 2. Water (2%) between the crystals allows for incorporation of fluoride Matrix formation & Composition A.Organic: 1-2%: a fine lacy network of organic material appears between crystals 1. Enamel proteins are non-collagenous a.90% amelogenins, 10% non-amelogenins i. Amelogenins (hydrophobic proteins rich in proline, histodine, glutamine; are carefully processed - provide a framework for and participate in crystal growth. ii. LRAP (leucine rich) comes from alternative splicing of RNA transcript. iii. TRAP (tyrosine rich) is breakdown product of main amelogenin – bulk of organic in mature enamel along with nonamelogenins iv. Ameloblastin - might help ameloblast adhere to immature enamel v. Enamelin - Role in enamel biomineralization proposed. vi. can be seen in EM of mature enamel that has been demineralized 2. Water: 2% by weight - related to porosity of tissue. a.Water lies between crystals and surrounds organic material; some forms hydration shells around crystals. b.Water is of clinical importance because ions such as fluoride travel through the water component  B. Processing of Amelogenins Secretion Serine proteases TRAP Kallikrein4 (KLK4) Aggregation Processing Enamelysin Fincham et al., 1999 Fig 7-47 MMP20 B. Processing of Amelogenins & hypothetical role in crystal growth 1. Amelogenins molecules are secreted by ameloblasts during secretory stage 2. They self assemble into nanospheres a. Same diameter as the space between newly forming crystals b. May serve to space crystals into parallel arrays 3. They undergo short term processing during secretory stage a. Enamelysin (MMP20) cleaves hydrophilic C-terminal ends – so they become progressively less anionic b. Hydrophobic nanospheres further assemble, stabilizing the matrix 4. They undergo long term processing during maturation stage a. Kallikrein4 slowly degrades residual hydrophobic amelogenins and fragments of nonamelogenins b. TRAP is breakdown product of amelogenins c. Crystallites thicken and fuse 5. Studies in mice show that crystals still grow when MMP20 or KLK4 are missing so not necessary for crystal growth, but enamel is hypomineralized, rod-interrod relationship is disorganized and enamel proteins persist 4. MMP20 knock-outs have thinner enamel 5. KLK4 knock-outs have mostly normal thickness but enamel does not adhere to dentin at DEJ, not clear why… Amelogenins form nanospheres and microribbons that separate crystals and expand the matrix to accommodate subsequent elongation during the secretory stage of ameloblast life Du et al., 2005 Clinical Implications Amelogenesis imperfecta – hypoplastic or hypomaturation ○ Defect in amelogenin gene (AMELX: hypoplastic; X-linked), Thin hypoplastic enamel, lacks rods ○ Defect in enamelin gene (ENAM: hypoplastic; autosomal dominant) Hypoplastic, no defined enamel layer ○ Defect in enamelysin or kallikrein 4 genes (MMP20 or KLK4: hypomaturation; autosomal recessive) Incremental nature of enamel formation A. Incremental lines B. Striae of Retzius (weekly lines ~ 20 microns apart) C. Cross Striations (artifact or daily lines ~ 4 microns) D. Perikymata Striae of Retzius Cross Striations Enamel Spindles spindel Odontoblast processes, cross the DEJ, insinuated between ameloblasts - up to 8µm in diameter. Enamel tufts & lamellae Tufts project from DEJ toward surface, best seen in transverse sections. Lamellae extend from surface toward DEJ. Both contain greater concentrations of enamel protein than rest of enamel. No known clinical significance - not prone to caries.  Gnarled Enamel produced by “undulation” of rods at cusp tip Hunter-Schreger Bands Occur in inner two thirds of enamel layer. Optical phenomenon produced by reflected light. Correspond to differences in directions of rods. Final clinical remarks Thanks! Any questions ? You can find me at ◉ [email protected] ◉ KCRB 5001.46 40