 Objectives: 1-Understand the Physical and Chemical characteristic features of enamel

2-Recognize the surface and histological structure of enamel.

 (1) Definition; Enamel is the hardest mineralized tissue in the human body that covers the anatomical crown of teeth. It forms a protective covering of teeth to resist the stresses and forces of mastication.  (2) Origin ectodermal in origin from the IEE.  Formative cells: which are lost as the erupts and hence enamel can not renew itself. Oral ectoderm

*It is a mineralized epithelial tissue that is totally acellular. Dental *Inert. lamina *Non-vital.

*Insensitive. Inner *If wear or Caries, it can’t replaced enamel Or regenerated (can’t renew itself). epithelium *Permeable. *Unique crystalline structure. *Unique matrix protein.

 (3) Physical properties of enamel: A)Color: Yellowish white to grayish white (according to thickness, degree of translucency, degree of calcification & homogeneity of enamel).

B)Thickness: Variable (max. on cusp tips {2-2.5mm.}& kinfe edge at cervical margin).

C)Hardness: Hardest calcified tissue in the body due to: [↑↑content of mineral salts & its crystalline arrangement]. Hardness of permanent teeth ˃ deciduous. Hardness at the surface ˃ DEJ. Hardness at cusp & incisal edge ˃ hardness at cervical line.

D)Brittleness: Enamel is brittle, especially when looses its elastic foundation of healthy (i.e. undermind enamel).

E)Permeability: Enamel can act as semipermeable membrane, permitting complete or partial slow passage of certain ions & dyes. Mainly from the saliva to the outer layer of enamel, but to a lesser degree from the pulp to the inner enamel layer across the dentin. GROUND SECTION DECALCIFIED SECTION

Methods of Studying ” hard tissues” HISTOLOGICAL PREPARATION OF TEETH GROUND SECTIONS ENAMEL Show enamel, dentin and bone. Do not show cells and soft tissues. PULP CAVITY

CEMENTUM DENTIN DEMINERALIZED PULP SECTIONS CAVITY Show cells and organic material. Do not show mineralized components. (4) Chemical composition:

By weight;

-96% inorganic substances in - 4% organic substance and water. the form of hydroxy appetite (1-2%organic material& 2-3 % water). crystals (0.05-1 mic length& The organic substance present between 300 angstrom thick and 900 the crystals as a fine network or angstrom width) forming a cortex(envelop) or a cover 3Ca (Po ) .Ca(OH) surrounding each crystal. 3 4 2 2. - - Some molecules may replace - Organic material is proteinaceous and the hydroxyl radical as fluoride, contain some mucopolysaccarides. - The protein is non-collagenous protein Mg, Strontium, carbonate and called enamel protein enamelin(with sulfate. This is the basis for the high molecular weight) or amelogenin , national project of fluorination amino acids( proline, glutamine, Lucien, of drinking water. glycine and histidine.)..

**By volume; 50% inorganic substances and 50% organic substances.(1:1)  (5) Histological structure; Enamel is difficult to study in decalcified section, due to its high crystalline nature, so it is studied by ground section.

** Enamel rod or prism: is the structural unit of enamel a) Number; 5 million in lower central & 12 million in upper molars. b) Diameter; small at inner side (DEJ) and wide at outer surface. The ratio between diameter of the rods at inner & outer enamel surface is (1:2) , this due to difference in the surface area between inner & outer surface of enamel. Wavy Course of Enamel Rods

c) Course: Enamel rods follow a wavy course from DEJ outwards, just before outer surface of enamel, they become straight. • A more spiral course(complicated or twisted or braided) is noted at cusps & incisal areas Gnarled enamel. to give max. strength against masticatory forces. d) Length; longer than enamel thickness due to its wavy course. e)Direction: perpendicular to the dentin surface In primary teeth, rods are horizontal, at the cervical and central part of the crown, and then change generally to become in oblique direction until they become vertical in the region of incisal ridge or cusp tip. In permanent teeth, the directions of enamel rods are similar to the primary teeth in the occlusal and middle thirds of the crown. In cervical region, the enamel rods deviate from the horizontal to apical direction.

 By light microscope **T.S. Enamel rods appeared in cross section by light microscope as hexagonal, oval, round and as fish scales. Enamel rods don’t not have a regular geometry. By light microscope, the rod appeared to be surrounded by rod sheath. Rods are separated from each others by interrod region. Enamel rods have clear crystalline appearance, permitting light to pass through them. ** prism sheath : is the peripheral part of enamel rod that forms incomplete envelop around the prism. Less calcified than rod itself. **interprismatic substance: separate the enamel rods from each other. Highly calcified as enamel rods, it has different refractive index.

**L.S - The cylindrical prism appear to be divided horizontally into equal segments by less calcified dark lines giving:, striated appearance , which are called (short increments or Cross-striations within longitudinal ground section of enamel (Lines at right angles with long axes of prisms)

 Electron microscope; - The rod is shaped somewhat like a cylinder and is made up of crystals with long axis that run, for the most part, parallel to the long axis of the rod particularly for crystals along the central axis of the rod.

- The interrod region surround each rod, and its crystals are oriented in a direction different from those making up the rod. - The boundary between rod and interred enamel in this region is delimited by a narrow space containing organic materials known as the rod sheath. - This difference in the crystal angulation results in difference in the refractive index of the prism & interprismatic substance ( is a function of Tomes’ processes).

** In the enamel prism & interprismatic region, the crystals are so tightly packed, so no or minute spaces were found between them for the organic material, so organic material only forms an envelope around them.

** While the abrupt change in crystal orientation in the rod & interprismatic region, leads to crystals not tightly packed, allowing more space between them for more organic material to be present , this account for the prism sheath to be visible even in LM.

Rod sheath

Rod

Interrod substance  Electron microscope of enamel ** Cross section: (Keyhole appearance) _The head of keyhole represent part of enamel rod with apatite crystals ⁄⁄ to long axis of the rod ( 0-40̊ ), Directed occlusally. _ The tail represents the interprismatic region, directed Cervically (where crystals fan out until become ┴ to the Long axis. **N.B : T.S. of enamel prisms revealed different patterns:- Such as: Hexagonal, round, oval, fish scales and keyhole patterns.

Enamel rods

Enamel rods Rod Sheath ) َو َسا ِر ُعىاْ إِلَى َم ْغ ِف َر ٍة ِّمن َّربِّ ُك ْم َو َجنَّ ٍة َع ْر ُض َها ال َّس َما َوا ُت َواألَ ْر ُض أُ ِع َّد ْت ِل ْل ُمتَّ ِقي َن ( 1- Hunter Schreger bands (optical phenomenon) dark and light bands ** Alternating dark & light bands , start at DEJ & end shortly before outer surface of enamel. They present at middle & cervical thirds of the crown and absent in the region of gnarled enamel. This optical phenomenon is seen when examining LS ground section of enamel under oblique reflected light, this is due to the wavy course of enamel rods.

B A

Hunter Schreger bands Hunter Schreger bands 2- Incremental lines of Enamel  1- Cross striation  2- Brown Striae of Retzius  3- Neonatal line 1) Cross striations or short increments,: _ The enamel rod is built up of equal segments separated by dark lines that gives it (striated appearance), this can be seen by partial decalcification (etching) of enamel using mild acids. This is due to the rhythmic apposition of the enamel matrix by ameloblasts, [ Daily rate of secretory activity of the ameloblasts]. By SEM the segments of uniform length (4 microns).

Cross striation

Enamel rods and cross striations 2) Brown striae of Retzius(Incremental lines of Retzius:-  They are incremental growth lines indicating the successive rhythmic apposition of enamel layers during formation of enamel.  They represent weekly rhythm in enamel production.

**In L.S. ground section of enamel : At the incisal ridge & cusp tip, they appear as brown bands form semicircles do not reach the surface, they arise from DEJ upwards and outwards surrounding the tip of dentin. At the middle & cervical thirds they reach the outer surface of enamel and become represented as series of transverse depressions called (perikymata). **In T.S. : the incremental lines of Retzius appear as concentric rings similar to the growth rings in a cross section of a tree. **Cause: May be due to the rythmic formation of enamel ( where periods of activity of the ameloblasts alternate with periods of rest). ** When the incremental lines are broad , it means that the rest period of is prolonged. ** When the incremental lines are close to each other , means that there is metabolic upsets. Brown Striae of Retzius

L.S. T.S. Longitudinal ground section showing deposition of the striae of Retzius ( Arrows in the enamel layer). 3) Neonatal line -Neonatal line is an accentuated incremental line that apparently reflects the great physiologic changes occurring at birth in enamel.

-It separates the prenatal formed enamel from the postnatal one, (before and after birth).

-It is therefore present only in the deciduous teeth as well as the first permanent molars. It is due to abrupt change in nutrition and environmental condition that occur at birth.

-The prenatal enamel is thought to be more homogenous than the postnatal enamel , due to more protected conditions & constant nutrition of the foetus.

Neonatal line NEONATAL LINE

Postnatal Prenatal Enamel Enamel 3- Primary enamel cuticle (Nasmyth’s membrane).

_ Delicate organic membrane that cover the crown surface of newly erupted teeth. _ It is the last secretory product of the ameloblasts, _ Its thickness (0.2 micron), it has structure similar to the basal lamina of the epithelium. _ It is organic material, worn off gradually from the surfaces exposed to mastication or tooth brushing, but may remain for life in sheltered areas, as gingival sulcus & depth of pits and fissure on the occlusal surfaces. 4- Amelodentinal junction (ADJ) _Appear s as scalloped line with the convexities towards the dentin. _ It is thought that these scallops provide undercuts , which causes firm attachment between E & D, but it is found that firm attachment still present even in smooth DEJ. _**The cause of firm attachment between E & D appear to be due to interdigitation at DEJ between the fibrils of the first formed layer of dentin (mantle dentin) & the fibrils of the organic matrix of the first formed layer of enamel and consequently the interdigitation between the hydroxy apatite crystals that are deposited during mineralization of both structures (E & D). 

DENTINO-ENAMEL JUNCTION (DEJ)

Dentin Striae of Retzius DEJ

DEJ

Striae of Retzius Dentin 5- Enamel lamellae ** Thin leaf like structure, extend from the enamel surface to a considerable distance in the enamel till DEJ , sometimes extend into the dentin. ** Appear dark in transmitted light, and best seen in TS ground section.

** There are three types of enamel lamellae: 1)Enamel lamellae type A( true enamel lamellae):- Limited to the enamel. It is formed of organic substance with slight calcification (i.e hypocalcified enamel rod segment). _ Caused by mild stimulus (as abnormal tension), leading to some disturbance in enamel calcification. 2) Enamel lamellae type B: _ If more severe stimulus occur, a crack may be formed & may reach the ADJ. _ The crack occurs before eruption of teeth, may be filled with **cells of enamel organ, so the cells in the depth of the crack will degenerate, while those on the surface remain vital & produce hornified cuticle, or filled with **cells of the adjacent CT , that differentiate into cementoblasts, may laid down cementum. 3) Enamel lamellae type C:- _ Severe stimulus, the crack occurs after eruption of teeth, so will be filled with **organic material from the saliva & may extend to the dentin.

Clinical cosideration of enamel lamellae:- Act as means of spread of dental caries.

Enamel lamellae

In enamel Extending in enamel and only dentin  Enamel lamellae;

ENAMEL

DENTIN 6- Enamel tufts _ It is a ribbon like structure, starts from ADJ to about 1/5 – 1/3 of the thickness of enamel. _ It is hypocalcified enamel rod & interrod substance. _They resemble tufts of grass when examined in ground thick section with low magnification. Best examined in TS. _Enamel tufts are more numerous than enamel lamellae. - Tufts contain greater concentration of enamel proteins than the rest of enamel

Enaml tufts A: enamel tuft B: enamel Lamella

7- Enamel spindles _ They are odontoblastic processes that pass across the ADJ into the enamel, they are thickened at their end, so termed enamel spindles. _The odontoblastic processes extended between the inner enamel epithelium before hard substances are formed. _ They are best demonstrated in LS ground sections, where the organic content of the spindles disintegrates and replaced by air, so the spaces appear dark in transmitted light.

Enamel Spindles  (8) Surface structure of enamel;  Structureless enamel,  Perikymata,  Rod end,  Cracks,  Afibrillar Cementum

1-OUTER STRUCTURELESS ENAMEL _ Prismless enamel occurs in the outermost 30 microns of enamel.

_ In all deciduous teeth & 70% of the permanent teeth.

_ Most common toward the cervical enamel, less common on cusp tip & incisal ridge.

_ No prism outline & the hydroxy apatite crystals are // to each other & ┴ to the surface. _ Tome’s processes are absent .

_ Hypercalcified than the bulk of enamel beneath it. ***__Inner structureless enamel layer : It is the first formed layer of enamel against the dentin surface, it is formed before the ameloblasts develop their Tome’s processes therefore lacks prismatic structure ,as the hydroxy apatite crystals are // to each other & ┴ to the dentin surface.

1 – OUTER STRUCTURELESS ENAMEL

REMEMBER: THAT THERE IS AN INNER STRUCTURELESS ENAMEL

30 um thick 2-Perikymata(imbrication lines) _ Transverse wave like grooves believed to be the external manifestations of the brown striae of Retzius. _ They are continuous around the tooth & // to each other and to CEJ. _ 30 Perikymata/mm in the region of CEJ. _10 Perikymata/mm near the occlusal surface or incisal edge.

The relationship between the striae of Retzius and surface perikymata (arrows). A B

Ground section of enamel showing the relationship between the striae of Retzius and surface perikymata. B, S.E.M. of the labial surface of a tooth, showing the perikymata. 3-Enamel Rod Ends

•The enamel rod ends are concave , vary in depth& shape .

•SHALLOWER CERVICALLY

*DEEPER OCCLUSALLY& Incisally 4-CRACKs

They extend for varying distances along the surface and appear as jagged lines in various regions of the tooth surface.

5-AFIBRILLAR CEMENTUM _ In 60% of cases , the cervical area of enamel is covered with cementum [afibrillar type]. _ Cause, when the reduced enamel epithelium retracts from the cervical region during tooth development.

Enamel

Cementum Dentinenamel junction

Age changes of Enamel  Attrition  Decreased Permeability  Increased Hardness (ionic exchange)  Color changes Age changes of enamel: 1- Attrition: - It’s a physiological wear of tooth hard structure in occlusal or incisal surfaces as a result of tooth to tooth contact, or wear of proximal surfaces of tooth due to slight mobility of teeth in their sockets. - Attrition occurs in permanent & deciduous teeth. Attrition in permanent is more sever & may reach to dentin surface because permanent tooth has longer life span. Attrition in deciduous may be sever because they are less hard. - Attrition appears as small polished shiny areas on cusp tip or incisal ridge followed by gradual reduction in height of cusp and flattening of Dentin occlusal surface resulting in decrease in vertical height.

2- Permeability: - permeability decreases by time esp. from outer surface while from dentin surface permeability remains constant.

Recently Main path Old enamel Erupted teeth 3- Atleration in chemical composition of enamel:

- Ionic exchange between E & saliva last through the life so, the E become more mature. It’s inorganic content increase up to 98% - Localized increase in fluoride and nitrogen make E. more resistance to decay.

4- Discoloration: Teeth darken with age. Although it could be due to the addition of organic material to enamel from the environment, it also may be due to deepening of dentine color seen through the progressively thinning layer of translucent enamel.

Amelogenesis LIFE CYCLE OF AMELOBLAST The life cycle of the inner dental epithelial cells are divided according to their function into:

1. Morphogenic stage. 2. Differentiation ( Organizing) stage. Deal with IEE 3. Secretory ( Formative) stage. 4. Maturative stage. Deal with Ameloblasts 5. Protective stage. 6. Desmolytic stage. Deal with REE 1. MORPHOGENIC STAGE:

 Inner dental epithelium interact with the mesenchymal cells of the dental papilla determining the shape of the dentin-enamel junction.

 The cells of the inner dental epithelium become cuboidal or low columnar with large central nuclei. Golgi bodies and centeriols are located in the proximal end of the cell (facing stratum intermedium). Mitochondria and free ribosomes are scattered through the cytoplasm.

 Inner dental epithelium separated from dental papilla by delicate basement membrane.

 Cell free zone of the dental papilla adjacent to the basement membrane.

2.DIFFERENTIATING (Organizing) STAGE:  As the ameloblasts are differentiated, they elongate ( tall columnar=40μ) and their nuclei shift proximally while golgi and centrioles shift to distal end (reversal of function polarity).

 The epithelial cells come into close contact with the peripheral connective tissue cells of the dental papilla inducing them to be differentiated into (Induction).

 Adjacent ameloblasts are closely aligned to each others and develop junctional complexes (Distal and proximal terminal webs).

 As the first layer of dentine forms it cuts off the source of nourishment from dental papilla. Ameloblasts are supplied by blood vessels from dental follicle.

3)Formative (Secretory) stage:

 Begin after first layer of dentin is formed which induce IEE to differentiate to ameloblast (reciprocal induction).

 There are increase in number of cell organelles.

 Synthesis of Enamel proteins occur in RER, then pass to Golgi complex where condensation & package into membrane bound secretary granule.

 The granule migrate to distal end & their content are released against first formed layer of dentin.

 After formation of first layer of E, ameloblast migrate away from dentin & develop conical process (Tome’s process). Tome’s process contain secretory granule. DP OB

Am

Tomes processes 4)Maturative stage:  After full thickness of E has formed. The ameloblast undergo significant ultrastructure changes: a. Reduction in height of cell, (decrease in volume & content) b. Withdrawal and disappearance of Tome’s process. c. Shift of remaining cell organelle to distal part of cell. d. The excess cell organelles are enclosed in autophagic vacuoles are digested by lysosomal enzymes e. Folding of distal plasma membrane to form striated border to increase surface area of distal end of cell so called (Ruffled border ameloblast) f. Another form of maturative ameloblast has smooth distal end (smooth ended ameloblast). Both forms are involved in process of E maturation.

Ruffled border and Smooth border Ameloblasts Leaky proximal junctions Tight proximal junctions

Introduction of minerals Removal of water and proteins 5.Protective stage:

 Near completion of enamel maturation the ameloblasts lose their ruffled border and secrete a thin continuous layer that covers the enamel this is the primary enamel cuticle.

 The ameloblasts lose their regular arrangement and join the other layers of the dental organ to form the reduced enamel epithelium. (3-4 stratified flattened epithelial layer).

 Reduced enamel epithelium protects enamel from the connective tissue of the dental follicle until tooth erupt. If cells of dental sac come in contact with enamel, tooth resorption or cementum deposition (afibrillar cementum) occur.

6. Desmolytic stage:  Reduced enamel epithelium proliferates and elaborates enzyme that induce atrophy and destruction of the connective tissue separating the erupting tooth from the oral epithelium.

 Premature degeneration of the reduced enamel epithelium may retard or prevent the eruption of the tooth. life cycle of ameloblasts as would occur in human tooth. 1, Morphogenetic stage; 2, Differentiating stage; 3, Initial secretory stage (no Tomes` process); 4, Secretory stage (Tomes`process); 5, Ruffle-ended ameloblasts of the maturative stage; 6, Smooth-ended ameloblasts of the maturative stage; 7, Protective stage.

AMELOGENESIS (ENAMEL FORMATION) It consists of two steps:  1- Secretion of organic matrix which is almost immediately partially mineralized.

 2- Full mineralization of the matrix as a result of Maturation activity of Ameloblasts.

Amelogenesis 1)Formation of enamel matrix: - Synthesis of E soccur in RER, then pass to Golgi complex where condensation and polysaccharide fraction is added & packed into membrane bound secretary granule. - The granules migrate to distal end & their contents are released against the first formed layer of dentin. - After formation of first layer of E, ameloblast migrate away from dentin & develop conical process (Tome’s process). Tome’s process contain secretory granule. - Mineralization of E protein occurs immediately and the crystals of first layer of dentin interdigidate with crystals of E. - Tome’s process has two region: a. Blunt distal end B. proximal end The secretion of E. proteins occurs through these two regions and this is responsible for formation E. rod and interrod structure of enamel

II. Mineralization and maturation: a. **First stage (partial mineralization) - Deposition of 25-30% of total mineral content occurs immediately after E. protein synthesis. b. **Second stage (maturation): - Mineralization begins before enamel matrix reach its full thickness. - Begins under cusp tip and proceeds cervically and from dentin surface to outer surface. - Maturation of E. gradually increase by deposition of minerals and growth of preexisting crystals then fusion of growing crystal with one another. - Deposition of more minerals occur by rapid influx of calcium and phosphates, so inorganic content increase up to 96%. And the growing crystals gain space by reabsorption of E. protein by ameloblast. - Secondary maturation: done by deposition of mineral from saliva after tooth eruption, so mineral content reach up to 98%

DEFECTS OF AMELOGENESIS

 Hypoplasia (affect enamel matrix) and Hypocalcification (affect enamel maturation).

 Hypoplasia appeared as pining, furrowing or total absence of enamel.

 Hypocalcification is manifested by opaque or chalky areas.

Causes: 1- Systemic factors Nutritional deficiencies, febrile diseases, endocrine disorders, Chemical intoxication (excess Floride ingestion, Tetracycline antibiotics) 2- Local Factors: Pulpitis, periapical infection of deciduous teeth. 3- Genetic Factors: Generalized affecting both .

Linear enamel hypoplasias manifested as horizontal grooves in several teeth of this Inuit specimen.

Linear enamel hypoplasias manifested as horizontal grooves in several teeth of this Inuit specimen Linear enamel hypoplasia enamel hypoplasia

fluorosis intoxication (mottled enamel hypocalcification enamel)