Enamel 2023.pdf

Development, Structure and Function of Enamel Chider Chen, Ph.D. Assistant Professor Department of Oral & Maxillofacial Surgery [email protected] Lectures 1. Development, structure and function of enamel 2. Development, structure and function of dentin and pulp 3. Root development 4. Stem cells T...

Development, Structure and Function of Enamel Chider Chen, Ph.D. Assistant Professor Department of Oral & Maxillofacial Surgery [email protected] Lectures 1. Development, structure and function of enamel 2. Development, structure and function of dentin and pulp 3. Root development 4. Stem cells Total 60 test questions (15 questions per lecture) will be in a quiz given the end of August covering these lectures. Agenda 1. Tooth structure and developmental stages 2. Ameloblasts and amelogenesis 3. Enamel structure 4. Disturbances in enamel formation Textbook recommendation: • Oral development and histology. Avery JK., 2002 • Ten Cate’s oral histology. Development, structure, and function. Nanci A., 2017 Mature Tooth Tooth Development Cell proliferation 2nd month of embryonic development Epithelium-mesenchyme interaction Cell differentiation Morphogenesis Mineralization Tooth eruption Root development 3 years old Tooth development Lamina Initiation Bud Stage Cap Stage Morphogenesis Bell Stage Cytodifferentiation Eruption Matrix Apposition Ameloblasts and Odontoblasts Definitions • Odontogenesis: the process whereby a tooth develops from a few cells in the primitive oral cavity to an erupted tooth with crown and root. • Amelogenesis: enamel formation. • Enamel: a unique epithelial tissue that covers the crown of the tooth. Timing of developmental stages Primary epithelial band Bud stage Cap stage Bell stage Crown 5 weeks 6-8 weeks 8-12 weeks 12-16 weeks 18-20 weeks initiation in-growth morphogenesis differentiation mineral forms The terms (bud, cap, bell, and crown) refer to the morphological changes in the dental organ. Reconstructed early oral cavity • At about 5 weeks, the primary epithelial band forms in presumptive upper and lower jaws. • The band thickens by epithelial proliferation perpendicular to the surface. Dental and vestibular laminae Dental lamina: tooth development Vestibular lamina: forms the oral vestibule Tooth Development stages Cap stage of tooth development • The bud splits into a cap-like structure due to proliferation. • The epithelium forms the enamel organ. • Ectomesenchymal cells aggregate beneath the enamel organ to form the dental papilla. • The dental follicle (sac) forms. Bell stage • During bell stage, the undersurface of the enamel organ deepens, and cellular histodifferentiation begins. • Epithelial cells assume different appearances in preparation for formation of the hard tissue (enamel). Bell stage: epithelial cell differentiation 4 epithelial cell types during bell stage: • Outer/external dental epithelium (EDE)cuboidal cells. • Stellate reticulum (SR)- star-shaped cells • Stratum intermedium (SI)- several flattened layers. • Inner dental epithelium (IDE)- short columnar Bell stage Ameloblasts and odontoblasts • Precursors (inner dental epithelial and ectomesenchyme cells) stop proliferation. • Epithelial and ectomesenchyme cells signal to each other using secreted proteins as instructions for differentiation. • Cells elongate and change organelle components for secretory function. • Begin to secrete extracellular matrix (ECM). Late bell and early crown stage • During late bell stage, inner dental epithelium folds according to tooth type. • The stellate reticulum reduces in thickness at locations of mineral formation. • Differentiation continues down the slopes of developing crown. Ameloblasts Stratum intermedium • After first dentin is secreted and mineralized, ameloblasts secrete first enamel toward the basement membrane. • Ameloblasts then retreat peripherally forming Tomes’ process, which creates structure in enamel. • Ameloblasts are in contact with stratum intermedium (SI), which contains alkaline phosphatase (essential for mineralization). • Ameloblasts and SI are considered a functional unit. Ameloblasts morphology • As ameloblasts change in morphology, their functions changes as well. Each ameloblast goes through all stages, but at different times. • Secretory: ameloblasts produce most of the enamel matrix. • Transition: a short stage where secretion slows, and some ameloblasts die by apoptosis. • Maturation: matured ameloblasts secrete a basal lamina on the enamel surface and attach to it. Matured ameloblasts remove proteins from developing enamel and replace organic material with calcium and phosphate. • Protective: ameloblasts continue to modify enamel composition. Maturation stage ameloblasts • Exist in two morphologies. • Smooth-ended ameloblasts permit removal of peptides and water (20%). • Ruffled-ended ameloblasts transport calcium and phosphate to the enamel (80%). • Cells cycling between stages are undergoing modulation. Reduced dental epithelium • As mineralized dentin and enamel form, the dental organ compresses, and stellate reticulum is no longer visible. • The remaining layers of the enamel organ form the reduced dental epithelium. Stages of amelogenesis: proteins Secretory stage: Non-amelogenins (10% of ECM) • Enamelin: acidic; found at dentin-enamel junction (DEJ); involved in crystal nucleation. • Ameloblastin: found in rod sheath; interaction between cell and ECM. • MMP20: a protease that processes enamel proteins during secretory stage. Transition stage: • Ameloblasts shorten and organelles rearrange. • Some ameloblasts die through apoptosis. Maturation stage: • KLK4 protease is secreted and degrades most remaining enamel proteins. • Protein is removed from developing enamel, and calcium and phosphate are added. Enamel: a unique mineralized tissue • Hardest tissue in the body. • Product of epithelial cells. • Extracellular matrix (secreted material) that contains no collagen. Enamel composition Stages Secretion Maturation Protein Mineral Water 66% 29% 5% 4% 95% 1% Enamel structure: hydroxyapatite • Formula is Ca10(PO4)6(OH)2. • A unit cell is the smallest repeating unit. • Unit cells stack to form the repeating unit of the lattice. • Fluoride can substitute for OH-. Tome’s process • Before Tomes' process, enamel rods are not found. • Secretory granules containing enamel proteins are released from the ameloblast. • Tomes' process is the term given to the end of the cell which lays down the crystals of the enamel matrix. • The Tomes' processes are angled, which introduces differences in crystallite orientation, and hence structure. • The enamel matrix will eventually become an enamel rod. Rod sheath • The rod sheath is a space created by groups of crystals that orient at angels. • Small amounts of protein remain in the rod sheath. • The rod sheath separates enamel rods. Inter-rod enamel • Crystals in rod and inter-rod enamel are similar in structure and composition, but diverge in orientation. Rod sheath Rod sheath Enamel lamella and stripes of Retzius • Cross-striations are daily increments of enamel secretion. • Stripes of Retzius are prominent striations or developmental lines in enamel. • Lamellae are faults in the enamel mineral, probably due to incomplete mineralization. • Stripe end at the enamel surface in shallow furrows called perikymata. Enamel tufts • Enamel tufts extend from the DEJ into enamel. • They are feather-like structures that contain the protein tuftelin. • Enamel tufts act to prevent enamel fractures. Enamel spindles • Enamel spindles are found at the DEJ and extend into the enamel. • Enamel spindles are formed when ends of odontoblast processes are embedded in enamel during development. Enamel demineralization Type I Type II Three acid etching patterns: • Type I: most common. • Preferential removal of rods. • Type II: inter-rod enamel removed. • Type III: least frequent. • Irregular and indiscriminate pattern. Type III Junction between type I and II Fluoride-induced defects • During aging, enamel can erode as it is not regenerated. • When fluoride is adsorbed onto the crystals, enamel becomes more resistant to acid dissolution, which helps resist caries. 1-5 ppm fluoride • Excess fluoride interferes with enamel development (amelogenesis), leading to fluorosis. • Chalk white or brown in color. 8-10 ppm fluoride Age changes in enamel • During aging, enamel can erode as it is not regenerated. • Attrition of enamel may expose dentin. • Color darkness with age (extrinsic staining). • Permeability decreases with age. Pore size between crystals are reduced. • Water content decreases with age. Reduced pore size forces water out. • Nature of enamel surface layer changes due to ionic exchange with oral environment. • Brittleness increases with age. • Decreased incidence of caries: • Loss of surface areas susceptible to caries. • Change in diet- less refined carbohydrates Amelogenesis imperfecta (AI) • • • • • Disturbances in enamel formation. A genetic dysplasia. Ameloblasts are very sensitive to changes in the microenvironment. Minor physiologic changes: structural changes visible only histologically. Severe insults: damage or death of ameloblasts easily with visible defects Enamel defects with fever • Fever causes disturbance in enamel formation. • Enamel formed during this time period is malformed. • Enamel formation may return to normal with remission of fever. Tetracycline-induced defects • Tetracycline antibiotics are incorporated into all mineralizing tissues (enamel, dentin, cementum and bone). • Brown, yellow or gray band of pigmentation. • Severity proportional to dose and duration. Take home message 1. Tooth development contains five stages: dental lamina, bud stage, cap stage, bell stage and eruption. 2. Ameloblasts develop from inner dental epithelium and require communication with ectomesenchyme cells (future odontoblasts) in order to differentiate. 3. Ameloblasts changes in shape and function during enamel development. 4. The principal stages of enamel development are secretion, transition and maturation. 5. Ameloblasts secrete a unique extracellular matrix containing enamel proteins (amelogenin, enamelin, ameloblastin, and proteases MMP20 and KLK4). 6. Ameloblasts secrete first and subsequently process and remove enamel proteins as the enamel hardens. 7. Ameloblasts also direct movement of calcium and phosphate into developing enamel. Take home message 8. Tomes’ process creates structure in enamel. Before Tomes' process, enamel rods are not found. 9. Ameloblasts are lost before the tooth erupts into the oral cavity. 10. Enamel is also influenced by environmental fluoride and fever during tooth development. 11. Enamel is the hardest tissue in the body because of the high mineral amount and the orderly orientation of mineral crystals. 12. Genetic mutations in genes encoding enamel proteins or proteases lead to the inherited enamel defect amelogenesis imperfecta. Chider Chen, Ph.D. Assistant Professor Department of Oral & Maxillofacial Surgery [email protected]