Enamel composition and structure
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Enamel is the hardest and most highly mineralized tissue in the body, consisting of 96% inorganic material (hydroxyapatite) and 4% organic material. It is formed through the process of amelogenesis, which involves three stages - the presecretory, secretory, and maturation stages. Ameloblasts are the cells responsible for enamel formation and organization into rods and interrod enamel. Enamel acquires its structural properties through mineral deposition and maturation over several years. Its unique composition and structure provide protection and function for teeth.
Enamel composition and structure
- 1. Enamel: Composition, Formation and Structure Dr. Yumna Lecturer Dental Hygiene and Technology
- 2. Introduction • Enamel is a protective covering for teeth • Cells responsible for formation of enamel are ameloblasts • Enamel acquires a complex structural organization • It possess high degree of mineralization
- 3. Composition
- 4. Physical Characteristics • Enamel is the most highly mineralized extracellular matrix • It consists of approximately 96% mineral and 4% organic material • Inorganic content of enamel is a crystalline calcium phosphate (hydroxyapatite)
- 5. Physical Characteristics • Various ions strontium, magnessium, lead and fluoride may also be incorporated into the crystals of enamel • High mineral content renders enamel extremely hard • When crystals start dissolution, dental caries starts
- 6. Physical Characteristics • Hardness of enamel is comparable to that of steel • Enamel is brittle • Dentine is resilient and it maintains integrity of enamel • Enamel is translucent
- 7. Physical Characteristics • Enamel varies in color from light yellow to gray white
- 9. Structure • Conventional demineralized sections of enamel only have an empty space which was previously occupied by mature enamel because the mineral has been dissolved and traced organic material has been washed away
- 10. Structure • Fundamental organizational units of enamel are the – Rods (prisms) – Interrod enamel (interprismatic substance)
- 12. Structure • Enamel rod was first described as hexagonal and prism like in cross section • Enamel is built from closely packed and long, ribbon like carbonatoapatite crystals • Fully mature enamel crystals are no longer hexagonal
- 14. Amelogenesis
- 15. Amelogenesis • It is a two step process • Enamel first mineralizes to only 30%, later on crystals grow wider • Organic matter and water are lost and mineral is added to reach full thickness of enamel • At this stage greater than 96% mineral content is found
- 16. Amelogenesis • Ameloblasts secrete matrix proteins and are responsible for creating and maintaining an extracellular environment favorable to mineral deposition • Amelogenesis has been described in as many as six phases
- 17. Amelogenesis • Generally it is subdivided into three main functional stages referred to as the – Presecretory stage – Secretory stage – Maturation stage
- 18. Presecretory Stage • Differentiating ameloblasts acquire their phenotype • Change their polarity • Develop extensive protein apparatus • Secrete organic matrix of enamel
- 19. Secretory Stage • Also called formative stage • Ameloblasts elaborate and organize the entire enamel thickness • Results in the formation of highly ordered tissue
- 20. Maturation Stage • Ameloblasts modulate • Ameloblasts transport specific ions required for concurrent accretion of enamel • Ameloblasts are considered cells that carry out multiple activities throughout their life cycle
- 21. Maturation Stage • When differentiation of the ameloblasts occurs and dentin starts forming, these cells are distanced from the blood vessels • Vascular supply is achieved by blood vessels invaginating outer enamel epithelium • There is loss of stellate reticulum
- 22. Light Microscopy of Amelogenesis • At the late bell stage, most of light microscopic features of amelogenesis can be seen • In the region of cervical loop, low columnar cells in inner enamel epithelium are identifiable
- 24. Light Microscopy of Amelogenesis • An important event for the production of enamel is the development of cytoplasmic extensions on ameloblasts called Tome’s process • Tome’s process give the junction between the enamel and the ameloblast a saw-toothed appearance
- 25. Light Microscopy of Amelogenesis • As the inner enamel epithelium is traced coronally in a crown stage tooth germ, it cells become tall and columnar
- 27. Light Microscopy of Amelogenesis • Blood vessels invaginate deeply into the cell, without disrupting the basal lamina associated with the outer aspect of enamel organ to form a convoluted structure called papillary layer
- 28. Light Microscopy of Amelogenesis • Finally when enamel is fully mature, the ameloblast layer and the adjacent papillary layer regress and together constitute the reduced enamel epithelium • At this stage ameloblasts stop modulating, reduce their size and assume a cuboidal appearance
- 30. Light Microscopy of Amelogenesis • As the tooth passes through oral epithelium, incisally reduced enamel epithelium is destroyed but remain cervically to form the junctional epithelium
- 33. Presecretory Stage Morphogenetic Phase: • During bell stage shape of the crown is determined • Cells of inner enamel epithelium still can undergo mitotic division • They are cuboidal or low columnar cells
- 34. Presecretory Stage Differentiation Phase: • As the cells of inner enamel epithelium differentiate into ameloblasts they elongate • Ameloblasts become polarized • These cells can no longer divide
- 35. Presecretory Stage • Junctional complexes are present between ameloblasts • They play an important role in amelogenesis by tightly holding together ameloblasts and pass on required nutrients to them
- 36. Secretory Stage
- 37. Secretory Stage • Ameloblasts reflects their intense synthetic and secretory activity • The cell granules migrate to the distal extremity of the cell that is into Tome’s process
- 38. Secretory Stage • When enamel formation begins, Tomes’ process comprises only a proximal portion • The content of secretory granules is • Secretion from the proximal part of the process and from adjoining ameloblasts, results in the formation of enamel pits
- 39. Maturation Stage
- 40. Maturation Stage • Amelogenesis is a slow developmental process • It can take as long as 5 years to complete • Maturation stage ameloblasts are referred to as post secretory cells
- 41. Maturation Stage • In this stage messenger RNA and protein signal for – Amelogenin and – Ameloblastin • The two enamel proteins found in maturation ameloblasts
- 42. Maturation Stage Transitional Phase • Ameloblasts undergo programmed cell death (apoptosis) • Approximately 25% cells die during transitional phase • Other 25 % cells die as enamel maturation proceeds
- 43. Maturation Stage • In embryogenesis, cells die at specific times during development to permit orderly morphogenesis • Two major ways by which cell death can occur are – Necrosis (pathological) – Apoptosis (physiological)
- 44. Maturation Stage • Specialized proteinases (caspases) also inactivate cellular survival pathways and activate factors that promote death
- 45. Maturation Stage Maturation Proper: • Next principal activity of ameloblasts is the bulk removal of water and organic material from the enamel to allow introduction of additional inorganic material • The most dramatic activity is the modulation of cells
- 46. Maturation Stage • They maintain the environment that allows accretion of mineral content • Maintain pH for functioning of the matrix degrading enzymes
- 47. Enamel Proteins
- 48. Enamel Proteins • The organic matrix of enamel is made of non- collagenous proteins only and made up of enamel proteins and enzymes • 90% of proteins are heterogenous group of low molecular weight proteins known as amelogenins • Remaining 10% are enamelins and ameloblastins
- 49. Enamel Proteins • Amelogenins are hydrophobic proteins • They have molecular weight between 5 and 45kDa • Amelogenins regulate growth in thickness and width of crystals • Ameloblastins are concentrated near the cell surface at sites where they are secreted
- 52. Rod Interrelationships • Rods tend to maintain in group • Rods run in a perpendicular direction to the surface of the dentin with a slight inclination towards the cusp
- 54. Striae of Retzius • They are identified using ground sections of calcified teeth • They are seen as a series of dark lines extending from dentino-enamel junction towards tooth surface • In cross sections they appear as concentric rings
- 55. Striae of Retzius • The basis for their production is still not clear • A proposal suggests that they reflect appositional or incremental growth of the enamel layer
- 57. Neonatal Line • The neonatal line when present, is an enlarged stria of Retzius that reflects changes occurring at birth • Accentuated incremental lines also are produced by systemic disturbances (e.g. fever) that affect amelogenesis
- 59. Cross Striations • Human enamel is known to form at a rate of 4µm per day • Ground sections of enamel reveal periodic bands or cross striations make up rods of enamel • Alternating expansion and contraction of bands is sometimes visible • This pattern reflect diurnal rhythmicity in rod formation
- 61. Hunter Schreger Bands • The bands of Hunter and Schreger are an optical phenomenon produced by changes in direction between adjacent groups of rods • They are seen more clearly in longitudinal ground sections • Found in inner two thirds of enamel • Appear as dark and light alternating zones
- 63. Gnarled Enamel • Over the cusps of teeth the rods appear twisted around each other in a seemingly complex arrangement known as gnarled enamel
- 65. Enamel Tufts and Lamellae • They are like geological faults • No known clinical significance • Seen in transverse sections of enamel •
- 66. Enamel Tufts • Enamel tufts project from dentino-enamel junction into the enamel • Contain greater concentration of enamel proteins then rest of enamel • They occur developmentally
- 68. Enamel Lamellae • They extend for varying depths from the surface of enamel • They contain linear, longitudinally oriented defects filled with organic material • Cracks in enamel can be sometimes mistaken for lamellae but generally donot contain organic matter
- 69. Enamel Spindles • Before enamel forms, some developing odontoblasts processes extend into the ameloblast layer • When enamel formation begins, become trapped to form enamel spindles • The shape and nature of DEJ prevent shearing of enamel during function
- 71. Enamel Surface • The striae of Retzius often extend from the dentinoenamel junction to the outer surface of enamel where they end in shallow furrows known as perikymata • Perikymeta runs in circumferentially horizontal lines across the face of the crown
- 73. Enamel Surface • Structure of enamel varies with age. • In unerupted teeth enamel surface consists of a structureless surface layer (finalenamel) that is lost abruptly by abrasion, attrition and erosion in erupted teeth
- 74. Enamel Surface • As the tooth erupts it is covered with pellicle, consisting of debris from enamel organ • Salivary pellicle always reapperas on surface of teeth shortly after mechanical polishing of teeth • Dental plaque forms readily on teeth
- 76. Age Changes of Enamel • With age enamel becomes progressively worn in regions of masticatory attrition • Wear facets increase in old ages
- 77. Age Changes of Enamel • Characteristics of aging include – Discoloration • Teeth darkens with age – Reduced permeability – Modifications in the surface layer
- 78. Age Changes of Enamel • Darkening of teeth can be caused by addition of organic matter • It may also be caused by darkening of dentin layer • Enamel becomes less permeable with age • Young enamel acts as semipermeable membrane
- 80. Defects of Amelogenesis • Many conditions produce defects in enamel structure • Defects occur because amelobalsts are cells sensitive to changes in environment
- 81. Defects of Amelogenesis • Defects in enamel can be caused by – Febrile disease – Nutrient deficiency – Protein malformation in enamel – Disarrangement of ameloblasts – Maternal issues – Fluoridation
- 83. Fluoridation • If fluoride is incorporated into the hydroxyapatite crystals, the crystal becomes more resistant to acid dissolution • fluoride play role in caries prevention • Presence of fluoride enhances chemical reactions that lead to the precipitation of calcium phosphate
- 86. Acid Etching • Use of fissure sealants, bonding of restorative materials to enamel and cementing of orthodontic brackets involve acid etching • In first step etching removes plaque and other debris along with a thin layer of enamel
- 87. Acid Etching • Second step: it increases porosity of the exposed surface of enamel which helps in proper bonding for restoratives and adhesive materials