Oral Histology Lecture 7

Issue 7- 25/03/11 ~ Issue title : “Amelogenesis ” ~ written by : Osama Yousef

Internal Enamel Ep-ithelium

A dental importance

Deposits and later modifies enamel and it is responsible for the formation of enamel only when the tooth differentiate and be-come Ameloblasts (enamel forming cells), read this issue to find out more about the other layers.

in this issue we will stumble upon a very important example which is tooth etching read this issue to find more about it.

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Amelogenesis

Life cycle of Enamel

The process by which the enamel is formed you will read about the diffrent stag-es this process has you will read about toms’ process and many other sturctures and more in this issue.

it consists of 5 stages in this is-sue we disscuess the first one which is presecrotery stage read this issue to know more about it.

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PageOneLecture title :

Basic Info Index Writing staff

Teacher Name : Lecture Date :Publish Date :

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Number of Pages :Duration :

“a page that sums it all “

Amelogenesis

Ashraf ShaweshWe/16/03/11Th/25/03/11

8 Pages45 minMohamed Haroon

page one...................................................................1Introduction (slide 2).............................................1Life cycle of Enamel: (slide 3)...............................2Presecretory stage (slide 5 + 6).............................4Secretory stage (slide 8+9+10)..............................5Mineralization process (slide 11+12 ..................7Termination of secretory stage : (slide 14 + 15 + 16...............................................................................8Transition stage .....................................................8

Thank you for making this issue possible : Osama YousefMohamed Haroon

Last time we finished the related topics about the tooth development and now we will talk about a new subject which is “Amelogenesis” .

Introduction (slide 2) Amelogensis is: the process which by it the Enamel is formed. Amelo means Enamel , Genesis means formation .

First of all we know that we have different layers in Enamel organ like (Internal Enamel Epithe-lium IEE , External Enamel Epithelium EEE , Stratum intermedium SI and Stellate Recticu-lum and all of these four layers are derived from the ectoderm )

Internal Enamel Epithelium:what it dose is that it deposit and later modify enamel and it is responsible for the formation of enamel only when the tooth differentiate and become Ameloblasts (enamel forming cells). So Internal Enamel Epithelium differentiates and gives Ameoblasts and then the Enamel is formed.

But what about the other layers? Actually they also play an important role in forming the Enam-el (please refer to table 1 next page) note that these functions are not completely understood.

You should note that the Enamel dose not form all at once, the process starts at the cusp tips then >> Incisal edges then finally >> around the cervical extension of the crown, this is why at 30 days you will see active Internal enamel Epithelium cells that have been converted into Ameoblasts started to form Enamel but at the same time other areas will still have Internal Enamel Epithelium that have not yet been converted) “By the time enamel is completed each ameloblast will have completed the same life cycle “, we have a life cycle for each enamel cell or ameloblast when the enamel is formed their life cycle ends.

“In the teeth of continuous growth (e.g. rodents) , amelogensis is present through-out the animal’s life “ this means that the process of Amelogenes is an engaged and continuous process in the rodents Amelogenesis will produce enamel for the entire life of that rodent ,why is that ? Because they have long incisors and these are subject to tooth ware. But in the case of humans and all Mammals the process actually becomes muted and stops at some point of their life.

table 1 showing the other layers of enamle

an example about rodents ani-mals a rat

Life cycle of Enamel: (slide 3)1-presecrotery stage: before the secretion of enamel 2-secrortery stage: where enamel is secreted and deposit to hard tissues3-Transition stage: temporary stage between secretory stage and maturation stage 4-Maturation stage: the developed enamel matures here 5-post-maturation stage , please read table 2 and see figure 1 for the important info

figure 1 showing the diffrent stages of Amelogensis

table 2 read this while vieiwing figure 1

figure 2 showing EDJPresecretory stage

Just one side note is that the nucleus location in presecrotery stage (cell 3) and the secro-tery stage (4a and 4b) notice how different the location is. The nonfunctional Ameloblasts will keep protecting the tooth while it forms they just cover the tooth , but once the tooth is formed this layer is lost ( the nonfunctional ameloblsts layer ) this is why we cannot replace enamel if it is lost.

1-Presecretory stage (slide 5 + 6)“All ameloblastic activities before secretion of enamel matrix “, we can divide this stage into two sub-stages:• 1- the differentiation of Internal Enamel epithelium cells IEE (becom-ing ameloblast) The differentiation process starts at cusp tips and incisal edges and ends cervically also the shape of the cell will change from cuboidal to polarized columnar (note that polarized refer to a nucleus located on a one pole ; polarized ) , the nucleus become near the Stratum intermedium SI , and the cells are linked by gap junctions and they are organized and finally the ba-sal lamina marks the future enamel-dentine junction (EDJ) (please see figure 2) you can see the junction between the two structures notice how on one side the nucleus of the cells are polarized and they are long coloumnar cells these are the >> ameloblasts .Notice on the other end we have Dentine and basal Lamina which together forms the enamel-dentine junction

• 2-thesubsequentresorptionofbasallamina:The basal lamina starts to break down once the odontoblasts have differentiated; now the odontoblasts and ameloblasts become intimate in contact to allow “signal exchange” which in turn allows the production of enamel.

In presecretory stage the changes do not only occur in amenoblast or the basal lamina they also occur in the outer mesenchymal cells of Dentine (dental papilla). Note that “Differ-entiation into dontoblasts precedes that of IEE cells which follows deposition of first pre-dentine “this means that the layer of Dentine is formed before the Enamel layer and once the first pre-dentin layer is deposited, IEE will differentiate into ameloblasts.

“Terminally differentiated pre-ameloblasts are joined at SR end by desmosomes forming proximal terminal web “ meaning that IEE cells and SI cells are joined by Desmosomes to form proximal terminal web appearance .Note that other cells are joined by gap junctions as we said earlier desmosomes are only for the terminally differentiated pre-ameloblasts . In histology and embryology science, Terminal means >> “end “, proximal =” toward the enamel organs “ note that distal = “away from enamel organs “ so each cell has two endings we are interested in the proximal ending .

Dentino-enamel Junction

“fully differentiated ameloblasts have proximally-place nucleus “ meaning that the nucleus are near the enamel organs , “ Basic enamel matrix is assembled in the endoplasmic re-ticulum ER and carried to Golgi apparatus before packaging into secretory granules “ the reason why they move proximally and assemble at the ER is because they need to synthe-size protein . After the protein is synthesized in ER they are carried via golgi apparatus , here you can see the small dots which represents the granules and also notice the location of the nucleus . “secretory phase begins with the formation of a thin layer of enamel matrix as ameloblasts retreat from EDJ “ meaning that the enamel starts as a thin layer which will retreat gradually from the Enamel-Dentine junction

Secretory stage :

random example about a polarized nucleus

Subsequently ameloblasts develop Tomes’ processes at the secretory end” , so after the formation of thin layers the cells start to develop what we call “toms’ process “ these processes play an important function which is keeping the ameloblasts in contact with the DEJ in order for the enamel to stay strong so the toms’ process crystals provide anchor and support .Here is an example presented by the doctor “ imagine we have a wall , if we want this wall to be strong we need to place the blocks in specific orientation that provides the maxi-mum supporting ability” .

“Ameloblasts are joined by a proximal terminal bar apparatus (web) consisting of junctional complexes & desmosomes” , here the proximal ends are joined by desmosomes , why do we want them to be joined ? Because we want to separates the area of enamel structure from the surrounding atmosphere and why do we want to separate it from the atmosphere? Because in this space there is fluid contains fat or minerals and we don’t want this fluid to get in contact with the enamel. This is why these cells are joined in the proximal terminal bar apparatus in a special way to prevent any 1-fluid from entering + 2-minting the integrity of the enamel also note that this web or apparatus “3-separates developing enamel from enamel organ”

“Shape of Tomes’ processes TP determines the prismatic structure of enamel “this is because we have crystals (toms’ processes) that are found in different orientation this will be reflected on the struc-ture of enamel as we said above, now considering the enamel , it is not a random hard tissue in fact it is very organized hard tissue and it is formed by Prisms that are interconnected in very organized and fashioned way., who makes these Prisms? Toms’ process them-selves , so we have enamel >> Prisms >> toms’ process (responsible for arranging the crystals in different location ) .Without Toms’ processes we will have very weak enamel

“Interpit ‘prongs’ develop between TP to deposit first the enamel matrix of the prism’s pe-ripheries” , now “ interpit prongs “ are areas in-between toms’ processes where enamel is first formed , this is what we need to know about them only

the tip of tomes’ processes will deposit the core of the prism of enamel and the sides of tomes processes will deposit the periphery in other words the middle of prisms will form the tip of toms’ and the periphery of these prisms will form the prongs , “TP infills the residual pits as they retreat from the prism’s core” here the idea is given in two examples the first one in the aspect of the Tomes’ processes and the second one in the aspect of Primes note that the two are the same.

“Then, TP infills the residual pits as they retreat from the prism’s core” the prism’s core is the middle of the prism so Toms’ process will move from the core until reaching some point in the enamel the doctor said he will explain this in more details later on .

Mineralization process : We said that the enamel production is two stages first the production of proteins and the second stage is the production minerals, Minerals actually are not disorganized they are stored in form of crystals that have different orientations.” Organic matrix layers get mineralized immediately by hydroxyapatite crystals” hydroxyapatite crystals = calcium , phosphate and hydroxyl groups , it is important to note that the cell pattern in forming enamel is building a layer of protein then immediately calcification (mineralization )and so on … but the cell pattern for Dentine formation is different we have protein synthesis then 4-5 hours break and then we have calci-fication that is why in dentine if you see a layer that is being formed you will only see protein at first. So enamel mineralization comes immediately after the protein synthesis unlike the Dentine

“First crystals are thin and smaller than mature crystal “ note that the first crystals are thin and small and are immature but this changes when they mature

Crystallites align perpendicular to the surface of TP , here you can see (the figure below) how the crystals have different orientation , as what we have said earlier the tips of the tomes pro-cesses secret minerals forming the core of the prism , the side of the tomes processes or the area between the tomes processes deposits minerals in different orientation which represents the periphery of the prism , that is why the direction of mineralization in the core is different from the direction of the periphery , all this arrangement leads to a very strong structure , so the difference in the crystal orientation in the core and periphery gives a stronger and more stable design or structure for the prism

As each TP has two fronts This produces the abrupt (sudden) change in crystallite orientation this explains the difference between the prism’s core (rod) and boundaries (interrod) and evi-dent after etching enamel surface , in etching enamel surface we will have a core that is less resistant than the periphery >>the core becomes deep etching and the periphery is raised which will give a rough surface or (honeycomb appearance ) this appearance is of dental importance for us for example if I want to apply a sticky martial to the etched tooth because we want to pre-vent saliva from entering for example , it is wise to put it in the deepest part of this etched tooth which is here the : core so this is shortly the dental importance of it .

Secretory stage

crystals arrangement in mineralization processhoneycomb appearance

“Each rod (prism’s head) is formed by a single ameloblast and Four ameloblasts contribute to each inter-rod region (tail)” , now each ameloblast will form the core(core) but the periphery is formed by four ameloblasts (tail) now look to the side of the structure not the honeycomb appearance but the side at the right you will find the different orientation of the crystals it is important to note that as the crystals are closer to the periphery as in (*) they have different orientation. Also notice that the change is graduate in one rejoin but it is very abrupt in two different rejoins as in 1, 2 and 3 (the above figure)

Termination of secretory stage : (slide 14 + 15 + 16) :TP retracts and a thin layer of aprismatic enamel is formed at the surface , this means that TP will be gone expect for a small layer , this small layer will not be prismatic it will be aprismatic

Once the full thickness of enamel matrix has been laid down (this is brought by as we said protein synthesis and mineralization )

A final important note about secrotry stage, by the end of secretory stage the final result between the two processes of Protein synthesis and Mineralization are not equal, in fact protein is more than minerals content or the calcified enamel in the enamel matrix, later on in “maturation stage” the protein to mineralization content will be more equal (more minerals content).

Transition stage : “Initially deposited enamel is high in protein & water & low in minerals “ as we just said above , the doctor gave some numbers (protein = 70% and minerals are = 30% ) to quailze these numbers (the protein and minerals ) “Maturation involves converting the initially deposited enamel into the fully mineralized enamel” so here we remove the non calcified and we put a calcified enamel

Transition stage is the stage where the ameloblasts change from a secretory to maturation forms

Enamel secretion stops & amelogenin(the protein) is removed

Ameloblasts become reduced in height & number and Protein synthesis organelles are reduced this is because there is no more production of protein so there is no need for these organelles to be present

Blood vessels invaginate enamel organ and reach the proximal end of ameloblasts

Ameloblasts form a basal lamina over the immature enamel

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the end