Category Archives: Enamel

Age and functional changes In Enamel

Posted by on February 4, 2012 No comments

Age and functional changes In Enamel

Enamel is a non-vital tissue and is not capable of regeneration.

1-    Attrition

It is the physiological wearing away of the tooth hard substance. It is mainly encountered in the occlusal and incisal surfaces. Proximal attrition may occur due to the slight mobility of the teeth during mastication.

Attrition occurs in deciduous and permanent dentition, however sever attrition is not found on deciduous teeth due to their short life span. The nature of the diet and certain habits may influence the degree of attrition.

Men exhibit more sever attrition than women due to their greater masticatory force.

Clinically: it appears first as a small polished facet on a cusp tip or ridge or slight flattening of an incisal edge.

There is gradual reduction in cusp height, depth of fissures and consequent flattening of the occlusal inclined planes resulting in a decrease in the vertical dimension.

Due to the proximal attrition, the contact points flattened and shortening of the dental arch length occur.

In case of severe attrition, dentin may be exposed. Loss of surface structures such as perikymata and rod ends may occur.

2-    Color changes of the tooth

The teeth may become darker in color due to;

  • Deepening of dentin color seen through the progressively thinning layer of the translucent enamel.
  • Addition of organic material from the environment on the enamel surface.

3-    Alteration in the chemical composition of enamel

After tooth eruption, ionic exchange between enamel and saliva occurs which may lead to localized increase of certain elements (e.g. fluorine) in the superficial enamel layers by age. This exchange increases the inorganic contents to 98% and known as secondary maturation.

A decrease of the water content of enamel by age was also suggested.

The alteration in the organic portion of enamel is questionable. These alternations may lead to reduction in the caries incidence.

4-    Permeability

Enamel is semi-permeable allowing ionic exchange which leads to its surface modification. Enamel crystals may acquire more ions and increase in size by age. This leads to decreased permeability. The permeability decreases or even disappears from the outer surface, while that from the dentinal surface may remains.

Amelogenesis 2

Posted by on February 4, 2012 1 comment

Amelogenesis 1

Amelogenesis 2

Two processes are involved in the development of enamel.

  1. Organic matrix secretion.
  2. Mineralization and Maturation.

1- Organic matrix secretion

The ameloblasts begin their secretory activity shortly after dentin formation is initiated.

Synthesis of enamel protein occurs in the rough endoplasmic reticulum, from where it is passed to Golgi complex in which it is condensed and packaged into membrane-bound secretory granules.

These granules migrate to the distal extremity of the cell and their contents are released against the newly formed mantle dentin. This discharged material is the organic matrix of the first formed enamel.

Little interval exists between the secretion of enamel protein and the appearance of inorganic crystals within it. These crystals are packed in this first-formed enamel and interdigitate with the crystals of dentin. Some consider that dentin crystals form the nucleation sites for enamel crystals.

The first enamel layer formed along the dentin surface before the development of Tomes ‘ processes of the ameloblasts accounts

later for the rodless (Prismless) enamel seen in ground sections of the tooth crown.

As this first enamel is formed, the ameloblasts migrate away from the dentin surface and each ameloblast develops a short conical projection known as Tomes’ process a distinction between the cell body and the Tomes process is evident by the terminal bar apparatus which appear at the light microscope as :

*    Localized condensation of the cytoplasm.

•    Thickening of the cell membrane.

The terminal bars separate Tomes’ process from the cell body.

They are observed during the enamel secretory stage of the ameloblast, and they play an important role in mineralization by separating the mineralization front from the intercellular compartment. This helps the cell in controlling the transport of calcium to the mineralization front.

Secretion of further enamel protein now occurs at Tomes ‘ process. Coincident with this, the mineral crystals forming at the cell membrane of the process assume a preferential orientation and this orientation is greatly responsible for much of enamel structure.

It has been postulated that the cell membrane of Tomes’ process plays a major role and that crystals are aligned along lines of flow developed within enamel proteins as it is secreted and oriented along the axis of enamel protein fibers.

The enamel is formed of rods. Each ameloblast forms one enamel rod and a portion of the surrounding interred region. It is said that four ameloblasts contribute to form the shape of one keyhole and that one cell form the head of the keyholed rod, while the adjacent three cells form the neck and tail.

2- Mineralization and maturation of enamel matrix

Mineralization of enamel matrix takes place in two stages.

  • In the first stage an immediate partial mineralization of about 25-30% (varies depending on the species) of the total mineral content occurs in the matrix as it is laid down.
  • The second stage of maturation occurs very rapidly after the first stage. It is characterized by the gradual completion of mineralization, till the inorganic content reaches 96%.

Maturation begins before the matrix has reached its full thickness. The first matrix deposited and the first enamel mineralized is along the dentino-enamel junction. The pattern of matrix formation and mineralization continues peripherally to the tips of the cusps and then laterally on the crowns. Finally the matrix of the cervical region mineralizes.

Morphological studies have revealed material resembling enamel matrix between the process of the striated border and within vacuoles inside the ameloblasts. Enzymes such as acid phosphatase and amino peptidase which are catabolic enzymes become demonstrated within the ameloblasts.

Maturation involves the rapid influx of calcium and phosphate ions. This action permits the rapid crystal growth which occupies spaces formed, as organic material and water are removed.

Removal of proteinous material during maturation is selective in that, most of the amelogenins are removed leaving behind the high-molecular weight protein. Enamelins become tightly bound to the hydroxy apatite crystal surfaces.

So maturation involves the resorption of enamel matrix by the ameloblasts throw:

  • Removal of large amounts of water.
  • Removal of certain parts of enamel proteins.
  • Growth of the pre existing crystals.

Table shows the changes in the chemical make up of the enamel matrix upon maturation.

It is found that 90% of the initially secreted protein is lost during enamel maturation. There is also evidence that maturation of enamel continues after eruption by the exchange of ions from the saliva lead to 98%) inorganic contents (Secondary maturation).

Amelogenesis 1

Posted by on February 4, 2012 1 comment

Amelogenesis 2

Amelogenesis 1

The cells forming enamel, the ameloblasts differentiate from the inner enamel epithelium.

Ameloblasts begin enamel deposition after a few micrometers of dentin have been deposited.

Dentinogenesis begins at the late bell stage of tooth development. Shortly thereafter, amelogenesis begins.

The life cycle of the ameloblasts In the early development, the inner dental epithelium is formed of single layer of cuboidal cells supported by a basement membrane which separates them from the dental papilla.

These cells undergo differentiation to become active secretory
ameloblasts. They, thus exhibit a number of morphologic changes as
they differentiate and pass through the following functional stages.
1-Morphogenic    

  1. Organizing
  2. Secretory

    4- Maturative

    5- Protective

    6- Desmolyfic

    1- Morphogenic stage :

    The cells of the inner dental epithelium interact with the adjacent mesenchymal cells determining the shape of the dentino-enamel junction of the crown.

    The adjacent pulpal layer is a cell free zone contains fine argyrophilic fibers and cytoplasmic processes of the superficial cells of the dental papilla.

    During this stage, the cells assume columnar shape with large oval nuclei that almost fill the cell body. Golgi apparatus and the centerioles are located in the proximal end of the cell (facing the stratum intermedium) while the mitochondria are dispersed throughout the cytoplasm.

    ‘ Life cycle of amelobiasts.

    The cells become tall columnar and the cell free zone between it and the dental papilla disappears.

    Thus the epithelial cells become in close contact with the connective tissue cells of the dental papilla which differentiate into odontoblasts. The odontoblasts begin to secrete dentin during the terminal phase of the organizing stage.

    A reversal of the functional polarity of these cells take place by the migration of the centriols and Golgi regions from the proximal ends of the cells into their distal ends. The nucleus free zone at the distal end of the cell become almost as long as the proximal parts containing the nucleus and mitochondria.

    There are desmosomal attachment at the proximal and distal part of the cells.

    3-    The Secretory stage

    The ameloblasts enter their secretory stage after the formation of a layer of dentin.

    The ameloblast becomes a highly polarized cell with the majority of its organelles situated in the cell body distal to the nucleus. The Golgi complex increases in volume and the amount of rough endoplasmic reticulum and cytoplasmic inclusions are significantly increased.

    The newly formed ameloblasts begin to secrete enamel matrix which constitute the non prismatic layer near the DEJ.

    As the first increment of enamel is formed, the ameloblasts migrate away from the dentin surface and each ameloblast develops a short conical projection known as Tomes ‘ process. A distinction between the process and cell body is marked by the distal terminal bar. The process only contains secretory granules, while the ameloblast cytoplasm contains abundant synthetic organelles.

    Tomes ‘processes extend into the newly forming enamel giving the junction between ameloblasts and enamel a Saw tooth like appearance.

    At the end of the enamel matrix formation there is a brief transitional phase involving a reduction in height of the amelobiasts and a decrease in their organelle’s content.

    Tomes ‘processes at the distal ends of the amelobiasts disappear.

    During this period most probably the outer structureless or prismless enamel is formed.

    secretory amelobiasts showing five substages.

    AG, absorption granules; AP, apical contact specialization (hemidesosomes); Av, autophgagic-vacuales Oysosomss); BTJ, butb type of contacts; CV, coaled (absorplive?) vesicles; D, desmosomes; DG, dense (secretory) granules; G, Golgi apparatus; GER, granular (rough) endoplasmic reticulum; Gr, pale (secretory?) granules; L1.L2, L3, lysosomes; LG, lipid granules; M, mitochondria; MG, mitochondrial granules; SB, striated border; TB, terminal bars; TJ, tight junctions; TW. terminal web.

    4-    Maturative stage

    As the amelobiasts complete the matrix deposition phase, they undergo significant ultrastructural changes linked to the assumption of their new function of enamel maturation.

    Excess organelles associated with synthesis are enclosed in autophagic vacuoles and digested by lysosomal enzymes.

    This development is followed by a shift in many of the remaining organelles to the distal part of the cell and a complex folding of the distal plasma membrane to form a ruffle border.

    This ruffle border greatly increases the surface area of the extremity of the ameloblast and indicates that rapid transport of material is taking place across the plasma membrane.

    Cytoplasmic vacuoles containing material resembling enamel matrix are present in the cell body.

    As the enamel maturation nears completion, the amelobiasts lose their ruffle borders and secrete a material between the now flattened distal end of the cell and the enamel surface.

    This material appears morphologically identical to a basal lamina.

    Hemidesmosomes also form along the distal cell membrane at this time.

     

    5-    Trotective stage

    Then the amelobiasts loose their regular appearance and become cuboidal and often can no longer be differentiated from the cells of the other layers of the epithelial dental organ.

    These layers together form a stratified epithelial covering of the enamel, the so called Reduced Denial Epithelium (Reduced Enamel Epithelium).

    The reduced dental epithelium protects the enamel from the surrounding connective tissue until tooth eruption.

    If the connective tissue comes in contact with the enamel, enamel can be either resorbed or covered by a layer of cementum.

    6- Desmolytic Stage:

    At the time of eruption, the reduced dental epithelium proliferates and seem to elaborate enzymes (desmolytic enzymes) that induce atrophy and destruction of the connective tissue separating it from the oral epithelium, so fusion of the two epithelia can occur to form the attachment epithelium.

    Premature degeneration of the reduced dental epithelium may also prevent tooth eruption.

Structure of Enamel

Posted by on February 3, 2012 No comments

Structure of Enamel.

 

Enamel is essentially a tightly packed mass of apatite crystals and most of its structural features are the result of a highly organized pattern of crystal orientation.

Enamel is retained in ground sections and can be studied with the light microscope by means of transmitted light.

 

Structure of enamel is: a- Rod or Prism, b- Interprismatic regions. C- Prism sheath.

 

  1. The Enamel Rod or Prism :

 


« The enamel rods normally have a clear crystalline appearance permitting light to pass through them. In cross section, under light microscopy, they appear hexagonal, round or oval. In human enamel, rods may resemble fish scales.

  • The number of rods varies in different teeth. It reaches about 5 millions in the lower incisors and up to 12 millions in the upper first molar.
  • The enamel rods or prisms extend the full thickness of enamel. However the enamel adjacent to the dentin surface (30 um thick band) lacks a prismatic structure as all the crystals are uniformly aligned roughly perpendicular to the dentin. Prismless enamel also occurs in the outer most 30 um or so of enamel of all primary teeth and in (70%) of the enamel of permanent teeth. Again the crystals in these region are aligned perpendicularly to the surface.
  • The length of most prisms is greater than the thickness of the enamel because of the oblique direction and wavy course of the prisms.
  • The average diameter of the prism is 4 micron near the dentinal surface and increases to about 8 micron at the surface to compensate for the greater outer surface.
  • The prisms tend to be maintained in rows arranged circumferentially around the long axis of the tooth. The prism in each row runs in a direction generally at right angle to the dentin surface. They run almost horizontally at the cervical and the middle thirds of the crown of the deciduous teeth. Then they change gradually to increasingly oblique direction until they become almost vertical in the region of the incisal ridge or cusp tip.

 

The arrangement of the prisms in the permanent teeth is
similar in the occlusal two thirds of the crown to that of the
deciduous. However, in the cervical region, the rods are tilted
apically.    


Each prism, as it runs to the surface has an undulating course bending to the right and left in the transverse plane of the tooth (except in the cervical enamel where prisms have a straight course), and up and down in a vertical plane.

 

Over the cusps or incisal edges of the teeth near the dentin surface, the prisms appear twisted around each other in a complex arrangement known as gnarled enamel.

 

Cross striations: Human enamel is known to form at a rate of approximately 4 um per day. Ground sections of enamel reveal what appears to be periodic bands or cross striations (short increment) occurring at 4 um intervals across the prisms.

 

Scanning electron microscopy reveals alternating constrictions and expansions of the prisms in some regions of enamel which may account for this banded appearance in ground sections.

The submicroscopic structure of the enamel prism:

 

The enamel prism is shaped some what like a cylinder and is made up of crystal whose long axis run for the most part parallel to the longitudinal axis of the rod. This is particularly true for crystals along the central axis of the prism. However, crystals more distant from the central axis flare laterally to an increasing degree as they approach the prism boundary.

 


 

 

 

 

 

 

 

 

 

 

 

 

 

Key hole Model of Enamel Structure:

The cross-sectional appearance of the human enamel prism and interprismatic substance similar to the outline of a keyhole. Some authors have suggested that the “keyhole” structure is the basic unit of enamel and refer to its components as the head and tail. The head is directed occlusally and the tail is pointed cervically. When enamel cut longitudinally, sections pass through the heads or bodies of one row of rods and the “tails” of an adjacent row. This produces an appearance of rods separated by inter-rod regions. The apatite crystals appear needle-like or some what ribbon like depending on the plane of sectioning. They are oriented parallel to the long axis in their bodies and deviate about 65 ° from this axis as they fan out into the tails of prism The apatite crystals of the mature enamel are the largest of all the other mesodermal calcified


structures in the body. They have an average thickness of 300A, average width of 900A and their length range from 0.05-1 micron. They are about 10 times larger than the crystals of mineralized connective tissue organic matrix probably forms an envelope surrounding each apatite crystal.

Drawing of keyhole pattern of human enamel indicating orientation of apatite crystals

 

 

b- Interprismatic Substance (Regions):

 

Crystals follow a confluent pattern from the central axis of the prism, continuing their lateral tilting until they lie nearly perpendicular to the prism in the associated inter-prismatic region.

 

 

C- Prism sheath:

 

Prism sheaths are formed along the interface between groups of crystals having markedly different angulations. They do not completely encircle a prism because of the confluent orientation of prismatic crystals with those of the cervically associated interprismatic region.

 

 

These boundaries, or sheaths contain more enamel protein than other region since the crystals are not tightly packed.

The prism sheath is seen well only at about the three fourths (3/4) of each prism where the prismatic crystal meet those of the adjacent inter-prismatic region at sharp angles.

This irregular junction accounts for the fish-scale appearance of enamel seen in cross-sections of de-mineralized developing enamel or in, etched ground sections.

 

Incremental Lines and Patterns in Enamel : * Incremental lines of Retzius or Brown Striae of Retzius :

 

They are incremental growth lines indicating the successive, rhythmic or daily apposition of enamel layers during formation of the crown.

In longitudinally cut ground section, they are seen as series of dark brownish bands surrounding the tip of dentin.

In transverse ground section, they appear as concentric rings.

 

They are prominent in most permanent teeth, less prominent in postnatal deciduous enamel, and rare in prenatal enamel.

The incremental lines are accentuated by systemic disturbances and changes in nutrition.

The structural bases for the production of Retzius lines is still uncertain.

 

They may be attributed to:

  • Periodic bending of enamel rods.
  • Variation in structure and mineralization probably related to daily increments.
  • Physiologic calcification rhythm probably related to daily increments.
  •  

     

 


 

 


 

 

 

Neonatal line:    

This is accentuated incremental line that 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 environment at birth.

The prenatal enamel is thought to be more homogenous than the postnatal enamel.

 

• Hunter-Schreger bands



They are an optical phenomenon caused by changes in prism direction. They are seen most clearly in longitudinal ground sections viewed by oblique reflected light.

 

They are found in the inner four fifths (4/5) of the enamel originating near the DEJ and do not reach the surface of enamel

They appear as alternating dark and light bands of varying

width.

 

Some investigators claim that they may be due to:

  • Change of prism direction (bnding)
  • Variations in enamel calcification.
  • Zones of different permeability.

 

• Enamel Lamellae :

They are thin organic bands leaf like structures that extend from the enamel surface toward the DEJ. They may end in enamel, or sometimes penetrate into the dentin.

They consist of organic material with very little mineral content. They may be confused in ground section with artificial cracks caused by grinding. By careful decalcification, the cracks disappear, while the lamellae persist.

 

Three types of lamellae are known;

1-    Type A Lamellae :

Lamellae may develop in planes of tension where rods cross such a plane, a short segment of the rods may not fully calcify. So Type A lamellae are composed of poorly calcified rod segment and are restricted to the enamel. They occur before eruption during enamel mineralization.

 

2-    Type B Lamellae :

With severe tension, a crack may develop which becomes filled with the surrounding cells of the non-erupted tooth. Lamellae of this type may reach into dentin according to the severity of the crack.

If cells from the dental organ fill a crack in the enamel, those in the depth degenerate, whereas those close to the surface may remain vital and produce a hornified cuticle in the cleft. If connective tissue invades a crack in the enamel, cementum may be formed.

 

3- Type C Lamellae :

If the tension occurs after tooth eruption the crack will be filled with organic matter originating from saliva.

This type is most common and may extend also into dentin.

 

•    Enamel tufts :

Tufts consist of hypomineralized enamel rods and inter-prismatic substance in adjacent layers of enamel and radiating from the dentino-enamel junction in different directions into the enamel for about (1/5) to (1/3) of its thickness so give the impression of a tuft of grass. Enamel tufts extend in the direction of long axis of the crown, so better seen in thick transverse section and rarely in longitudinal sections. They contain greater concentration of enamel protein than rest of enamel.

 

•    Enamel Spindles :

Before enamel formation occurs, some newly forming odontoblastic processes push between adjoining ameloblasts which when enamel formation begins, become trapped to form enamel spindles with rounded ends. These structures do not follow the direction of enamel prisms. They are observed equally well in transverse and longitudinal ground sections at the area of DEJ.

 

•    Dentino Enamel Junction ( DEJ )

The junction between enamel and dentin is established as these two hard tissues form and is seen as scalloped profile in sections with the convexities directed toward dentin.

Scanning EM of the junction shows it to be a series of ridges which increase surface area and may enhance adhesion between enamel and dentin.

TS of a tooth crown showing (A) enamel tufts, (B) enamel lamella (C) enamel, (D) dentino-enamel junction, (E) dentin AND (f) spindle.

 

Surface Structures of Enamel:

The surface of enamel is characterized by several formations. • Perikymata

 


These are external manifestations of the brown striae of Retzius. They run in a linear horizontal plane across the face of the crown as wavy grooves. They are more concentrated near the cemento-enamel junction, and parallel to the cervical line.

•    Enamel rod ends

These are concave and vary in depth and shape. They are shallower in the cervical region and deeper near the incisal or occlusal edges.

 

•    Cracks

They are actually the outer edges of lamellae. They extend for varying distances along the surface and appear as jagged lines in various regions of the tooth surface.

 

•    Salivary Pellicle and Dental Plaque

Salivary pellicle
is an organic deposit on the tooth surface, always reappears very shortly after teeth have been mechanically polished. Within a day or two after the pellicle has formed, it becomes colonized by micro-organisms to form a bactereial plaque, especially in the more protected areas of the teeth.

The important role of the dental plaque in fostering caries as well as other clinical implications of these surface accumulations is well known.

 

•    Primary enamel cuticle ((Nasmyth’s membrane):

The delicate organic membrane that covers the crown of newly erupted teeth. It is the last product of the ameloblasts and organically attached to the prisms and inter-prismatic substance. Its thickness is about 0.2 micron.

The primary enamel cuticle is gradually worn off from the surfaces exposed to mastication or tooth brushing. It may remains in the gingival sulcus and depth of pits and fissures as well as around the proximal contact areas.

Enamel

Posted by on February 3, 2012 No comments

Enamel

 

Enamel is the hardest calcified tissue in human body. It forms a protective covering of the entire crown surface.

Enamel is unique because it is a mineralized epithelial tissue, while bone, dentin and cementum are mineralized connective tissue. Mature enamel is the only tissue that is totally acellular. It is also avascular and non sensitive.

 

Physical properties :

« Enamel is extremely hard due to its high mineral content and its crystalline arrangement. Enamel of the pennanent is harder than of deciduous teeth. The hardness is greater at the surface and decreases toward the dentino- enamel junction (DEJ).

  • Enamel is brittle, and an underlying layer of more resilient dentin is necessary to maintain its integrity.
  • It is semipermeable permitting complete or partial passage of certain molecules.
  • Enamel has variable thickness, attaining its maximum thickness on the cusps of molars and premolars of about 2-2.5 mm. thinning to almost knife edge at the cervical margin.
  • Enamel is translucent and varies in color from light yellow to grayish white. This variation in color depends on its thickness as the underlying yellow dentin is seen through the thinner regions.

 

Chemical properties :

Enamel consists mainly of inorganic phase which constitute 96% of its weight. The inorganic substances are mainly calcium and phosphates in the form of hydroxy apatite crystals.

The basic crystallographic formula for the unit cell of hydroxyapatite is Cal0 ( P04 )6 ( OH )2.

There are minor substitution in the crystal lattice (e.g. carbonate, magnesium, lead and fluoride).

Most ion substitution result in a small increase in the solubility of the crystal except fluoride, lead and Zinc which cause decrease in the solubility of enamel crystals.

The organic substances and water constitute the remaining 4% of enamel by weight (1-2% organic material, 2-3% water).

The nature of the organic element of enamel is incompletely understood.

The proteins of the enamel matrix are unique among mineralized tissue because they are not a fibrous collagen protein like dentin, cementum and bone.

The following investigations give an idea about the nature of the proteins;

  • Histologic staining reactions of enamel matrix during development resemble keratinizing epidermis.
  • Chemical analysis of mature enamel matrix indicates that the amino acid composition is not closely related to keratin.
  • X-ray diffraction studies reveal that the molecular structure is typical of group proteins called cross-B-proteins and suggests the presence of acid mucopolysaccharide and glyoproteins.

Two classes of enamel proteins are described,

  • Amelogenin: which is known to constitute approximately 90% of enamel protein in the secretory stage and 50% after maturation. They have a molecular weight of 22-30 KDA (Kilodaltons) and are rich in proline, glutamic acid, histidine and leucine.
  • Enamelin: It constitutes approximately 10% of enamel protein in the secretory stage and 50% after maturation. It has higher molecular weight (48-70 KDA). It is acidic and rich in proline, glutamic acid, alanine, serine and asparatic acid.

It has been proposed that with progressive mineralization, amelogenins are removed, selectively, while enamel ins remain tightly bound to the mineral.

Enamel protein is generally considered a rather amorphous gel with an ability to flow under pressure and occupies all existing space between enamel crystals.