Enamel
Upcoming SlideShare
Loading in...5
×
 

Enamel

on

  • 3,335 views

 

Statistics

Views

Total Views
3,335
Views on SlideShare
3,335
Embed Views
0

Actions

Likes
3
Downloads
93
Comments
0

0 Embeds 0

No embeds

Accessibility

Upload Details

Uploaded via as Microsoft PowerPoint

Usage Rights

© All Rights Reserved

Report content

Flagged as inappropriate Flag as inappropriate
Flag as inappropriate

Select your reason for flagging this presentation as inappropriate.

Cancel
  • Full Name Full Name Comment goes here.
    Are you sure you want to
    Your message goes here
    Processing…
Post Comment
Edit your comment

    Enamel Enamel Presentation Transcript

    • Enamel 04/09/2013
    • Physical characteristics • Hard, brittle, totally acellular , highly mineralized • Secretory product of stratified squamous epithelium • Calcified tissue • Hydroxyapatite crystal arrange in prism or rods Density:-  decreases from the surface of enamel to the dentino- enamel junction. Thickness:-  thickness over the cusps of the molars where it measures 2.5 mm & incisal edges of incisors where it is 2.0 mm.
    • 1. Forms a protective covering (2 mm – knife edge). 2. Forms a resistant covering (suitable for mastication). 3. The hardest calcified tissue in human body. 4. enamel is very brittle but the underlying dentin provides some resilience 5. The specific gravity is 2.8. 6. Khn-343 7. Acts as semipermeable membrane (selectively permeable). 8. Color: yellowish white to grayish white depends on translucency.
    • • Enamel gains mechanical strength by interweaving HAP crystals • Enamel rod – 5-12 million/tooth • Appatite crystal is hexagonal • Enamel initially starts with a high protein content, but these are removed and the voids backfilled with HAP as the tooth matures
    • Hydroxyappatite
    • Chemical properties • 96% inorganic - by weight • 4% organic - by weight • inorganic crystalline calcium phosphate – hydroxyapatite • various ions like strontium, magnesium, lead and fluoride are present at some point during enamel formation • In volume the organic matter and water are nearly equal to the inorganic contents.
    • 1) Enamel rods(prisms) 2) Rod shealths 3) Cementing inter-rod substance. STRUCTURE OF ENAMEL
    • Enamel rods • Basic unit of enamel
    • Cross section • Cross section of enamel rod shows the key hole pattern • Head represents the rod and key shows the inter rod region • Head is directed towards the occlusal aspect and tail towards the cervical region of the tooth
    • Cross section of enamel
    • Enamel • built from closely packed ribbon- like hydroxyapatite crystals – 60 to 70 nm in width and 25 to 30 nm in thickness • the rod is shaped somewhat like a cylinder and is made up of crystals that organize with their long axes parallel to the longitudinal axis of the entire rod – this organization is tighter around the center of each rod • the interrod region surrounds each rod – its crystals are oriented along different axes from the rod • Rods are oriented at right angles to the dentin surface • In cervical and central part of a permanent teeth they are horizontal
    • Characteristics - Enamel rod/prism  Number: 5 – 12 millions.  Direction: Run in oblique direction and wavy course.  Length: greater than the thickness.  Diameter average: 4 µm.  Appearance: Have a clear crystalline appearance.  Cross-section: hexagonal, round, oval, or fish scales.
    • • Enamel Rod: Basic Structural Unit Cross section
    • • Head of enamel rod is formed by one ameloblast and tail is formed by three ameloblast • Thus each rod is formed by four ameloblast
    • Enamel Rods
    • Submicroscopic Structure Of Enamel Rods  Keyhole or paddle-shaped.  Separated by interrod substance.  About 5 µm in breadth and 9 µm in length.  The bodies are near the occlusal or incisal surface.  The tails point cervically.  The crystals; parallel to the long axis of the prism heads.  Deviate about 65° from the tails.
    • Keyhole shaped E. rods Hexagonal ameloblasts Note crystal orientation Enamel Rod’s Shape
    • Enamel Rod • Longitudinal section
    • Crystals in rod and inter-rod enamel are similar in structure but diverge in orientation
    • Rod sheath • the boundary between rod and interrod is delimited by a narrow space containing organic material – rod sheath  A thin peripheral layer.  Darker than the rod.  Relatively acid-resistant.  Less calcified and contains more organic matter than the rod itself.  Electron Microscope : often incomplete.
    • Enamel rods sectioned in cross- section • In this electron micrograph enamel rods are cut perpendicular to their long axis. The ligherareas are the rod coresin which hydroxyapatitecrystals are tightly packed in alignment with each other. The darker areas surrounding the rod cores are the rod sheathsin which the crystals are loosely packed at various angles. There are two main parts to a rod: the rod headand rod tail. The head has the central core (light area), and is sometimes referred to as the "rod". The tail is made of the rod sheath (less mineralized enamel). During development, one ameloblast(in position 1 in the inset diagram) makes the rod core for the rod at position 1, while three other ameloblasts(in positions 2, 3 and 4) produce the rod tail of rod 1. The tail is located between 2 and 3 and above 4. • Legend:A, Rod core; B, Rod sheath; C, Rod tail; D, Rod head
    • Box diagram of human enamel • This diagram represents a 25 X 25 X 25 μm of enamel. • •It demonstrates the arrangements of hydroxyapatitecrystals in the enamel rods in three planes of section. • •One rod is highlighted in blue to demonstrate the typical human rod shape. • •In the rod core, hydroxyapatitecrystals are aligned with the long axis of the rod. • •In the tail the crystals are aligned diagonally or perpendicularly to the long axis of the rod.
    • Alternating rod directionality • Hunter Schregerbands are alternating light and dark bands seen in a section of enamel when cut longitudinally and illuminated in a special way. • •The bands are produced by the orientation of groups of rods. • •If the light passes through rods cut in cross-section, the band appears light. • •If the light passes through rods cut in longitudinally, the band appears dark. • Legend: A, Rods cut longitudinally; B, Rods cut cross- sectionally
    • Enamel Crystal • Crystals length: 0.05 – 1 µm. • Thickness: about 300 A°. • Average width: about 900 A°. • Cross sections: somewhat irregular.
    • Enamel Crystal Longitudinal Section Transverse Section
    • Interprismatic substance • Cementing E. rods together. • More calcified than the rod sheath. • Less calcified than the rod itself. • Appears to be minimum in human teeth.
    • Striations • E. rod is built-up of segments (dark lines). • Best seen in insufficient calcified E. • In a longitudinal section dark lines are seen that shows the daily deposition of enamel (rhythmic manner of E. matrix formation). These lines are known as cross striation • Segment length: about 4 µm.
    • Cross-striations Cross striations
    • Direction of rods • Near the edge or cusp tip they are oblique • At the cusp tip they are almost vertical • Run from DEJ to surface of enamel • Usually at right angles to the Dentin surface. • Follow a wavy course in clockwise and anticlockwise deviation full thickness of enamel • At the cusps or incisal edges: gnarled enamel. • At pits and fissures: rods converge in their outward course.
    • Straight enamel rods -longitudinal labiolingualsection • The enamel rods project in the direction of the arrow. • Can you see the striaof Retzius?
    • Enamel cut • In enamel cut in perfect cross- section the shape of the enamel rod exhibits a "keyhole"-shaped pattern. However, in a normal cross- section of enamel, as seen here, most rods are cut obliquely. • This is because they do not travel in a straight line. • The micrograph on the left is produced by differential interference microscopy while the micrograph on the right is from transmitted light microscopy.
    • Wavy course of enamel rod • A more spiral course is noted at cusps & incisal areas Gnarled enamel
    • Gnarled enamel • Optical appearance of enamel cut in oblique plane • Bundles of rods appear interwine more irregularly • Makes enamel stronger • Seen in the incisal or cuspal region • The enamel at the cusp of the tooth generally exhibits a wavy pattern. This enamel is called gnarled enamel. This is NOT hypo-mineralized.
    • Gnarled enamel • Enamel rods are general not straight throughout their length. • In the cuspal region, the rods are very wavy. • This is referred to as gnarled enamel. • In this section, you can see the end of an odontoblasticprocess penetrating the enamel just past the DEJ. • This structure is called an enamel spindle. • Legend • Legend: A, Gnarled enamel; B, Enamel spindle
    • Direction of Enamel Rods
    • Hunterschrager bands • Optical phenomenon seen in reflected light • Alternate light and dark bands • Seen in ground longitudinal section • Due to abrupt change in the direction of enamel rod • Originate from the DEJ.
    • Hunter-Schreger Bands
    • Hunter-Schreger Bands
    • Hunter-Schreger Bands This is Due to: 1. Change in the direction of E. rods. 2. Variation in calcification of the E. 3. Alternate zones having different permeability and organic material.
    • Enamel -transverse ground section • In a transverse section of tooth, the striaof Retziusappear as concentric bands parallel to the dentino-enamel junction (DEJ). In addition to the "hypo-mineralized" dark striaof Retzius, there also exist hypo-mineralized areas perpendicular to the DEJ. These are enamel lamellae(that traverse the entire thickness of enamel) and enamel tufts(that traverse the inner third of enamel adjacent to the DEJ
    • • sectionLegend: A, Striaof Retzius; B, Enamel tuft; C, Enamel lamella; D, DEJ
    • Strae of retzius • Incremental lines of growth • Eccentric growth rings • DEJ to outer surface of enamel • Where they end as shallow furrows known as perikymata
    • Incremental Lines of Retzius: • Brownish bands in ground sections. • Reflect variation in structure and mineralization. • Broadening of these lines occur in metabolic disturbances. • Etiology 1. Periodic bending of E. rods. 2. Variation in organic structure. 3. Physiologic calcification rhythm.
    • Microscopic picture of Enamel Brown striae ofRetzius
    • Incremental Lines of Retzius:
    • Neonatal Line • The E. of the deciduous teeth and the 1st permanent molar (It is incremental line that is the boundary between the enamel forms before and after the birth) • The neonatal line is usually the darkest and thickest striaof Retzius. • Etiology – Due to sudden change in the environment and nutrition. – The antenatal E. is better calcified than the postnatal E.
    • Neonatal Line
    • Enamel Lamellae • Are thin, leaf like structures, • Develop in planes of tension. • Extends from E. surface towards the DEJ. • Confused with cracks caused by grinding (decalcification). • Extend in longitudinal and radial direction. • Represent site of weakness in the tooth and three types; A, B, and C.
    • Enamel lamellae • Filled with organic material and water • Type A B C can be seen • A- poorly calcified rod segment • B- Degenerated epithelia cells • C- organic matter
    • Enamel Lamellae Type A Type B Type C Consistency Poorly calcified rod seg. Degenerated cells Organic matter from saliva Tooth Unerupted Unerupted Erupted Location Restricted to the E. Reach into the D. Reach into the D. Occurrence Less common Less common More common
    • Enamel lamellae • In this ground cross- section of tooth, you can see enamel lamellae and enamel tufts You can also see the neonatal line. • •What do all three of these structures have in common? • Answer: They are all hypocalcified. • Legend: A, Enamel lamella; B, Enamel tuft; C, Neonatal line
    • Enamel Lamellae
    • Enamel Lamellae
    • Enamel Tufts • Arise from DEJ. • Reach to 1/5 – 1/3 the thickness of E. • In ground section: resemble tufts of grass. • Do not spring from a single small area. • The inner end arises at the dentin. • Consist of hypocalcified E. rods and interprismatic substance. • The extend in the direction of the long axis of the crown (best seen in horizontal sections).
    • Enamel tufts • Runs short distance in the enamel • Forms in the formative stage in the enamel
    • • Enamel tufts are less mineralized areas of enamel in the inner third of enamel adjacent to the DEJ. They resemble tufts of grass. • •They are wavy due to the waviness of the adjacent rods. • •Structures rich in organic matter (i.e. less mineralized) that project to the surface of the enamel are enamel lamellae. • Legend: A, Enamel tufts; B, Enamel lamella
    • Enamel tufts -two planes of focus • Enamel tufts consist of several unconnected "leaves" of hypo-calcified enamel. • •They display a wavy twisted appearance. • •Enamel spindles are the processes of odontoblastsprojecting into the enamel. • Legend: A, Enamel spindle; B, Enamel tuft
    • Enamel Tufts
    • Enamel Tufts
    • Decalcified tooth • In a decalcified section of tooth, only the organic material is left behind. • •In this micrograph you can see an enamel lamella and enamel tufts. • Legend: A, Enamel lamella; B, Enamel tuft
    • Dentino-Enamel Junction • Scalloped junction – the convexities towards D. • At this junction, the pitted D. surface fit rounded projections of the enamel. • The outline of the junction is performed by the arrangement of the ameloblasts and the B. M.
    • DEJ • Convexity of the enamel fits into the concavity of the dentin • Spindle tufts and lamellae are present at DEJ
    • Dentino-Enamel Junction
    • Odontoblastic Processes and Enamel Spindles • The odontoblasts processes may cross DEJ (before the hard substance is formed) to the E. and ends as E. spindles. • Odontoblasts process trapped in the enamel • More in the cuspal region • They are filled with organic matter. • The processes and spindles are at right angle to the surface of the dentin. • The direction of spindles and rods is divergent. • Spindles appear dark in ground sections under transmitted light.
    • Enamel Spindles • Odontoblastprocesses usually end at the DEJ. However, sometimes the ends of the process become embedded in the enamel as it forms. • •These very small, usually straight structures that you can see adjacent to the DEJ are enamel spindles. • •They are only about one tenth the length of an enamel tuft. Legend: A, Enamel spindle; B, Odontoblastprocesses in dentin
    • • In this high magnification of the DEJ you can clearly see the bifurcation of the ends of some of the odontoblastprocesses as well as enamel spindles. • Legend: A, Enamel spindle; B, Odontoblastprocess; C, Enamel rod
    • Odontoblastic Processes and Enamel Spindles
    • Surface Structures a. Structureless layer (E. skin) b. Perikymata c. Rod ends d. Cracks e. Enamel cuticle
    • a. Structureless layer • About 30 µm thick. • In 70% permanent teeth and all deciduous teeth. • Found least often over the cusp tips. • Found commonly in the cervical areas. • No E. prisms. • All the apatite crystals area parallel to one another and perpendicular to the striae of Retzius. • More mineralized than the bulk of E. beneath it.
    • b. Perikymata • Transverse wave like grooves. • Thought to be the external manifestation of the striae of Retzius. • Lie parallel to each other and to CEJ. • Number: – About 30 perik./mm at the CEJ. – About 10 perik./mm near the incisal edge. • Their course is regular, but in the cervical region, it may be quite irregular. • Powdered graphite demonstrates them. • It is absent in the occlusal part of deciduous teeth but present in postnatal cervical part (due to undisturbed and even development of E. before birth)
    • The relationship between the striae of Retziuz and surface perikymata Striae of Retziuz Perikymata
    • Perikymata (imbrication lines) Are external manifestations of Retzius striae
    • c. Rod ends • Are concave and vary in depth and shape. • Are shallow in the cervical regions. • Deep near the incisal or occlusal edges.
    • Rod ends
    • d. Cracks • Narrow fissure like structure. • Seen on almost all surfaces. • They are the outer edges of lamellae. • Extend for varying distance along the surface. • At right angles to CEJ. • Long cracks are thicker than the short one. • May reach the occlusal or incisal edge.
    • Cracks
    • e. Enamel cuticle 1. Primary E. cuticle (Nasmyth’s membrane). 2. Secondary E. cutile (afibrilar cementum). 3. Pellicle (a precipitate of salivary proteins.
    • Primary enamel cuticle • Covers the entire crown of newly erupted tooth. • Thickness: 0.2 µm. • Removed by mastication (remains intact in protective areas). • Secreted by postamloblasts. • EM: similar to basal lamina.
    • Secondary enamel cuticle • Covered the cervical area of the enamel. • Thickness: up to 10 µm. • Continuous with the cementum. • Probably of mesodermal origin or may be elaborated by the attachment epithelium. • Secreted after E.O. retracted from the cervical region during tooth development.
    • Pellicle • Precipitate of salivary protein • Covers the crown • Re-form within hours after mechanical cleaning . • May be colonized by microorganisms to form a bacterial plaque. • Plaque may be calcified forming calculus.
    • Life Cycles of the Ameloblasts • According to their function, can be divided into six stages: 1. Morphogenic stage. 2. Organizing stage. 3. Formative stage. 4. Maturative stage. 5. Protective stage. 6. Desmolytic stage.
    • Morphogenic stage. • React by differential growth • Produce shape of the crown • Terminal bar appears • Basal lamina separates the inner enamel epithelium and cells of the dental papilla • Pulpal layer adjacent to the basal lamina is a cell free zone • At cervical region – cell is relatively undifferentiated
    • Organizing stage. • Inner enamel epithelium interact with the cells of dental papilla which differentiate into odontoblast • Cells become elongated • Proximal part contain nuclei • Distal end is nucleus free zone • Dentin formation begins • Cell free zone disappear
    • • As dentine is formed nutrition supply of the inner enamel epithelium changes from dental papilla to the capillaries that surround the outer enamel epithelium • Reduction and gradual disappearance of the stellate reticulum
    • Formative stage. • Formatve stage starts After the dentine formation • Enamel matrix formation starts • Development of blunt cell process on the ameloblast surface which penetrate the basal lamina and enter the predentin
    • Maturative stage. • Maturation starts after most thickness of enamel matrix formation in occlusal and incisal area. In cervical area matrix formation is still in progress • Ameloblast reduce in length • Cells of stratum intermedium takes spindle shape
    • Protective stage. • After enamel calcification cells on ameloblast can no longer be differentiated from stratum intermedium and outer enamel epithelium • These layer forms reduced enamel epithelium • Protect the enamel from connective tissue until the tooth erupts, if it contacts then anomalies develop enamel may be resorbed or cementum cover may form (afibrillar cementum)
    • Desmolytic stage. • Reduced enamel epithelium induces atrophy of connective tissue separating it with oral epithelium thus fusion of the two epithelia can occur • Premature degeneration of the reduced enamel epithelium may prevent the eruption of he tooth
    • Amelogenesis 1. Organic matrix formation (follows incremental pattern – brown striae of Retzius). 2. Mineralization.
    • Organic Matrix Formation a. Amelodentinal membrane. b. Development of Tome’s processes. c. Distal terminal bars. d. Ameloblasts covering maturing enamel.
    • Enamel matrix • Secretory activity of ameloblast starts after the small dentin layer formation • Ameloblast lose their projections separating them from predentin • Islands of enamel matrix deposit along the dentin • This layer is known as dentino enamel membrane
    • Tomes process • Surface of ameloblast facing the enamel is not smooth • The projection of ameloblast into the enamel matrix is tomes process. • There is an interdigitation of cells of ameloblast and enamel rod because long axis of rods and ameloblast are not parallel
    • • Picket fence arrangements • Atleast two ameloblasts are involved in the synthesis of each enamel rod
    • Distal terminal bars • Appear at the distal end of the ameloblast • Separate the tomes process from the cell proper • They are localized condensation of cytoplasmic substance
    • Ameloblasts covering maturing enamel • These are shorter than the ameloblast covering incompletely formed enamel • During enamel maturation 90% initially secreted protein is lost • Organic content and water is lost in mineralization
    • dpTP=distal portion of Tome’s process ppTP=proximal portion of Tome’s process Sg=secretory granules(E. protein) Organic Matrix Formation
    • Ameloblasts are perpendicular to the rods (arrow=cell membrane, p=Tome’s process, s=incomplete septum)
    • Depression in enamel surface which were occupied by Tome’s processes
    • Mineralization a. Partial mineralization (25-30%). b. Maturation (gradual completion of mineralization).
    • Maturation • Maturation seems to begin at the dentinal end of the rod • Each rod mature from the depth to the surface • The sequence of maturing rod is from cusps or incisal edge toward the cervical line • Incisal and occlusal region reach maturity ahead of the cervical region
    • • Original ribbon shaped crystal increase in thickness more rapidly than in width
    • Crystal Mineralization Recently formed crystals Mature crystals
    • Abnormalities • Interference during E. matrix formation may cause Enamel hypoplasia. • Interference during Enamel maturation may cause Enamel hypocalcification. • Each condition may be caused by systemic, local, or hereditary factors.
    • Abnormalities Enamel Hypocalcification Enamel Hypoplasia
    • Amelogenesis • two-step process • first step produces a partially mineralized enamel – approximately 30% mineralized • second step involves an influx of additional mineral content – coincident with the removal of organic material and water – results in 96% mineralization • the influx of mineral results in growth of the crystals in width and thickness
    • Amelogenesis- life cycle of ameloblast • ameloblasts – derived from the inner dental epithelium • secrete matrix proteins that are responsible for creating and maintaining and extracellular environment favorable to mineral deposition • possess a unique life cycle – each stage reflects its primary activity during enamel stages • can be divided into three main stages – presecretory, secretory, maturation stages
    • Amelogenesis Figure 7-14 Representative micrographs of amelogenesis in the cat. A, Tooth formation shows an occlusal-to-cervical developmental gradient so that on some crowns finding most of the stages of the ameloblast life cycle is possible. The panels on the right (B corresponds with B1 and C with B2) are enlargements of the boxed areas: B, Secretory stage, initial enamel formation; C, secretory stage, inner enamel formation. D and E are from the incisal tip of the tooth (see Fig. 7-15). D, Midmaturation stage, smooth-ended ameloblasts; and E, late maturation stage, ruffle-ended ameloblasts. Am, Ameloblasts; D, dentin; E, enamel; N, nucleus; Od, odontoblasts; PL, papillary layer; RB, ruffled border; SB, smooth border; SI, stratum intermedium.
    • Presecretory stage • morphogenetic phase – during the cell stage – DEJ and shape of the crown is determined – cells of the inner dental epithelium are cuboidal or low columnar with large centrally located nuclei and poorly developed Golgi – separated from the dental papilla by a basement membrane • Organizing and differentiation phase – as the cells of the IDE differentiate into ameloblasts they elongate and their nuclei shift toward the stratum intermedium – the basement membrane fragments by the cytoplasmic projections of the ameloblasts – during the formation of predentin • this allows contact between the pre- ameloblasts and pre-odontoblasts – the Golgi complex increases in volume and migrates distally to occupy a major portion of the cytoplasm – the amount of RER increases significantly and most of the mitochondria clusters in the proximal region of the cell • -therefore the cell becomes polarized with most of the organelles distal to the nucleus
    • – at the distal end of the cell – extensions form called Tome’s processes - against which enamel forms – research now shows the these differentiating ABs secrete enamel proteins at the early stage – even before the basement membrane disintegrates • pre-ameloblasts also express dentin sialoprotein transiently – which is also an odontoblast product – adjacent ABs align closely with each other – form junctional complexes between them keeps them aligned • these complexes encircle the cells at the distal end of the cell (adjacent to the stratum intermedium) • formed of fine actin filaments radiating from these complexes into the cytoplasm
    • Secretory Stage • cells acquire intense synthetic and secretory activity • enamel proteins are translated by the RER, modified by the Golgi and packaged into secretory granules – migrate to the distal Tome’s processes • secretion is constitutive – the secretory granules are not stored for long within the cells • the contents of the secretory granules are released against the newly formed dentin along the surface of the Tome’s process • little time elapses between the secretion of enamel and its mineralization
    • • as the initial enamel layer forms – the ABs migrate away from the dentin surface and develop a distal portion of Tome’s process – extension from the existing proximal portion of Tome’s process – the pTP extends from the distal junctional complex to the surface of the enamel layer – the dTP interdigitates into the enamel beyond the initial layer – the cytoplasm of both processes is continuous with that of the body of the AB – so once the initial enamel layer forms the AB only has a pTP – the dTP forms once the enamel forms into rods – when the dTP forms – the enamel proteins are secreted at two sites located at defined ports along the dTP – the dTP lengthens as the enamel layer thickens – it also becomes thinner as the rod grows in diameter – eventuallyu squeezed out of existence
    • Figure 7-31 In three dimensions, interrod (IR) enamel surrounds the forming rod (R) and the distal portion of Tomes’ process (dpTP); this portion is the continuation of the proximal portion (ppTP) into the enamel layer. The interrod (IGS) and rod (RGS) growth sites are associated with membrane infoldings (im) on the proximal and distal portions of Tomes’ process, respectively. These infoldings represent the sites where secretory granules (sg) release enamel proteins extracellularly for growth in length of enamel crystals and, consequently, the thickening of interrod and rod enamel.
    • Maturation Stage • both tooth eruption – the enamel hardens • change in the physiochemical properties of the enamel • actually because the pre-existing HA crystals of the enamel grow in width and thickness and NOT because new crystals are made • Tome’s processes are not apparent at this stage • the ABs are generally referred to as post-secretory cells at this stage – although they still secrete proteins • made up of a transitional phase and the maturation proper phase – transitional phase – after the full thickness of the enamel has formed • the ABs undergo significant morphological changes that prepares them for the maturation of the enamel • reduction of AB height and a decrease in their volume and organelle content
    • – Maturation Proper Phase – ABs become involved in the removal of water and organic material • also undergo apoptosis so that approximately 25% of the ABs die during the transitional phase and an additional 25% die during the MP phase • characterized by the modulation of the cells – cyclic creation, loss and recreation of a highly ruffled apical surface and a smooth surface • occurs in waves traveling across the crown of the developing tooth – from least mature to most mature enamel regions • can happen every 8 hours in some species • significance is unknown – could be related to calcium transport and completion of enamel mineralization • the calcium ions are required for active crystal growth because the cellular junctions at the ruffled end are more leaky • these ABs show enhanced endocytic activity and numerous lysosomes – for the withdrawl of enamel proteins from the maturing enamel matrix • BUT the main mechanism for organic matrix removal is the production of bulk-degrading enzymes into fragments small enough to be able to leave the enamel layer and be taken up by the AB
    • Ruffled and Smooth ameloblasts
    • Enamel proteins • amelogenin – accumulate during the secretory stage – undergo minor short-term and major long-term processing to form smaller fragments – these fragments form the bulk of the final organic matrix of maturing enamel – prevents crystals from fusing during their formation and must be removed to permit crystal growth – least concentrated at positions where the interrod and rod crystals grow in length = enamel growth sites – form aggregates called nanospheres that surround the enamel crystals along their long axis
    • Non amelogenins – enamalin – small degredation during the secretory stage which decreases in the deeper zones of the enamel • crystal nucleation and growth – ameloblastin – undergoes rapid degredation – the intact protein is found near the enamel-forming surface while the fragmented forms are found in the deeper zones of the enamel • promotes mineral formation and crystal elongation • also known as amelin and sheathlin • highest concentration can be found at the enamel growth sites • secreted together with amelogenins
    • • sulfated glycoproteins – short half life in the enamel • tuftelin – localizes specifically at the DEJ and participates in its establishment – not specific to enamel • enzymes: metalloproteinases – e.g. MMP20 or enamelysin (short-term breakdown), serine proteinases (bulk degredation), phosphatases • dentin sialoprotein – transiently expressed
    • Mineral pathway • introduction of minerals span the secretory and maturation phases • calcium moves from the blood supply through the enamel organ to reach the enamel • a smooth tubular network has been described that is found in ABs and opens onto the enamel • this network is similar to the ER/sarcoplasmic reticulum • calcium is likely to be routed from high-capacity stores associated with the ER • no matrix vesicle are associated with mineralization – as is the basis for collagen-based mineralized tissues like bone • almost an immediate formation of crystallites within the enamel secreted against the dentin – so there is no pre-enamel
    • Enamel structural organization • Striae of Retzius – in longitudinal sections they are a series of dark lines extending from the DEJ toward the tooth surface – forms becuase of a weekly rhythm of enamel production resulting in structural alterations of the rods – OR could be due to appositional deposition of successive layers of enamel • Cross striations – forms at 4um intervals across the rods – rhythmicity?? • Bands of Hunter and Schreger – optical phenomenon produced by the changing orientations of adjacent groups of rod • Gnarled enamel • Enamel tufts and lamellae – no clinical significance – like geological faults – project from the DEJ for a short distance into the enamel – branched and contain greater concentrations of enamel that the rest of the enamel – abrupt changes in the directions of the rods arising from the DEJ