Oral Histology Lecture 6

8/7/2019 Oral Histology Lecture 6

1/16


2/16

Last time we started a new topic which is Development of the tooth & its

supporting structures;we will continue this lecture and start a new topic.

The hall was going to be occupied by the faculty of science so the Dr. told us that

he was going to release us early yotleq sara7na

We said that we have different stages of tooth development, the first stage is

the Bud stagewhen we see the tooth as a spherical invagination at this stage we

cannot identify anything related to the enamel organ or any structure.

After the bell stage we have the Cap stagelike in slide #10 and invagination in

the oral Epithelium into the mesenchymal tissue,

It has actually developed a concavity there that is why it is called Cap stage, and

this tissue that invagenates and goes down from the dental lamina into the

mesenchymal tissue we call it Enamel organ, and it will lead to the formation of

Enamel.

As they find it is very important in the formation of dentine but it will not form

dentine! It is there to send the signal that is responsible for the formation of

dentine .

Enamel Organ: is an epithelial structure that arises from the dental lamina, finally

it will give the enamel of the tooth.


3/16

Slide (10) so the whole structure is called enamel organ it has a concavity that

looks like a cap so it is called Cap stage of enamel organ, at this stage we canidentify different groups of cells, we can identify the border line/ or the

Peripheral cellsand the core, -like the outer cells and the inside cells-.

So there are:

Central portion & Peripheral cells.

The peripheral cells are divided into: External enamel epithelium EEE; outside &

Internal enamel epithelium IEE; and the IEE are very important, because they

will finally differentiate to become the ameloblasts and secrete enamel! (Ya3ne el

cells el mawjoden fe manteqet el concavity).


4/16

Later on what happens? This concavity deepens; starts to become deep, until

the tooth looks like a bell thats why it is called the Bell stage, as u can c slide

#17 the enamel organ has now the shape of a Bell! And this is actually what

converts the shape is actually the division and proliferation of the cells, these

cells that are called IEE they start to divide, and to orientate themselves

(bewajho anfes-hom) to give the 3D shape of the crown, (lazm yetnazamo b

tareqah monasebeh jedan 7ata yekon shakel tholathi el ab3ad lal crown), by this

way they establish the three dimensional shape of the crown!

So this is called morphodifferentiation or morphogenesis; because it involves

processes that will give out the shape /morpho of the tooth.

And we called the IEE that will make those cells capable of producing enamel is

called histogenesis; because it will lead to the formation of hard tissue -- all the

time when we have a question in the exam about those two points students dont

understand them! -- So we have to understand what is said not just memorize the

written words --

The process of differential growth /differential cell division that leads to the

establishment of the three dimensional shape of the crown is called

morphogenesis, but the internal process that takes place inside the cells to makethose cells capable of producing enamel is called histogenesis /or

histodifferentiation.

Notice in the picture that we have the two cusps!


5/16

Lets give an example lets say that this tooth that is going to form is a premolar

because it has two cusps, in case of an incisor we will have an incisor edge, in case

of the canine we will have one cusp and so on.

So the shape of the enamel organ in the bell stage is related to the shape of the

crown, so when we talk about morals it will be different from canine different

from premolars different from incisors and so on!

What happens in the early bell stage? We have Differential cell division along

IEE, those cells IEE in the cap stage they start to divide but not in a haphazard

way (3ashwa2eyah), they divide in a very organized way to give the structure/shape of the crown.

It is called differential because division is not the same at all of the places in the

IEE cells! (B ma3na a5ar sor3et el inqisam ma betkoon nafs-ha) the rate of

division is not the same, so it is differential at certain area we will have a very

fast division at another area we have slow division; because we want to build the


6/16


7/16

start dividing and they will give cysts (akyas fe al fak), next year we will learn this

maybe in-sha2-Allah we will take it in oral pathology! Lets hope that someone of

us will remember this info.

We talked about morphology now at the same time the cells undergo

Histodifferentiation, which leads to the appearance of 4 distinct layers (el 5layahele canat very simple epithelial cells balashat tetmayaz w balashat te5talef 3n

ba3ad be7ayth a3tatne b el nehayeh arba3 anwa3 mo5talefeh mn el 5alayah).

Now the EEE cells these are Cuboidal cells the Stellate reticulum cells these

are located inside enamel organ star shaped cells, we have Stratum intermedium

SI cells these cells support, the IEE cells, finally we have the IEE cells, in

addition to the cells we have the surroundings of the crown, so we start seeing:

Dental papilla and Dental pulp.

As u can c in slide 17:

We have 4 types of cells which are the EEE cells the cells that are found in the

concavity are the IEE cells the cells in the middle these are the SR cells and

we have a group of cells that are directly under lining IEE cells which we call the

SI cells

Lets now see the function of each of these cells:

What are the functions of EEE cells?! Maintaining the shape of the enamel organ,

this is very important in the soft tissue stage because any trauma would

destroy the crown 3D shape if the EEE cells wont maintain it and also it has the

function to Exchange of substances between enamel organ and dental follicle

enamel organ is Avascular (has no vessels), so we have to exchange nutrients to

feed these cells.

*So EEE cells function is to: Maintain the shape of the enamel organ

& Exchange substances between enamel organ and dental follicle.


8/16

The EEE cells and the IEE cells when they meet at the margins below at the

cervix of the tooth we call them Cervical Loop, at this cervical loop we have

increased cell division .

The other group of cells SR these are fully developed at this stage, they are star

shaped cells (zai shakel el najmat), they have extensions which are connected(mutada5leh b ba3dha) and the function of these cells is: protection of underling

tissue against physical disturbance, (hai el 5layah fe benha epitheliam madeh

btemtas el sadamat) it has some elasticity, if the enamel organ has a trauma or

any kind of shock this will absorb the shock, because we dont want the 3D shape

of the tooth to be affected, so they protect it by having a cushion like material

between the cells it absorbs traumas and shock all the time, as a result of

protection they maintain the tooths shape, the hydrostatic pressure ((helps topass the nutrients to the IEE cells)) its hydrostatic pressure is in equilibrium

with that of the dental papilla allowing the proliferation of IEE to determine

crown morphogenesis inside this area of the SR we have hydrostatic pressure

it has 100% has to be the same as the hydrostatic pressure of the dental papilla

otherwise the tooth will have different shape (ra7 ye5talef shakel el tooth), lets

assume that the HP in the dental papilla is bigger than the HP in the SR space the

tooth will become more spherical, because all of the cells are going to be pushedoutside, and trans versa there will be shrinkage, and the tooth will become smaller

all the time remember that we have a state of equilibrium between the dental

papilla and the SR cells space.

The 3rd group of cells is called Stratum intermedium SI cells these cells are

located underling IEE cells, and they are 2-3 layers of flattened cells over IEE,

and they may be concerned with protein synthesis, transport of materials to andfrom IEE cells and concentration of cells, so it has a direct relation in supporting

the active IEE cells it pass the nutrients and it has something to do with the

protein synthesis it helps, so these cells are very important.

Finally the most important cells are the IEE cells these are columnar cells

(toleyeh) that become elongated starting from the cusp tips and incisal edges.


9/16

Those IEE cells are connected to each other very tightly through attachment

that is called Desmosome, which is one of the surface cell attachments, and they

connect them with stratum intermedium cells.

We have basement membrane all the time between the IEE cells and the dental

papilla cells, so this basement membrane is very important in separating theenamel organ from the surrounding ectomesenchyme!

Lets ask a very important Q why are those cells (IEE) elongated at the cusp

tips but they are cuboidal here? Its because the process of differentiation

starts at the cusps tips.

Come back after one month and we will find that all of these cells are columnar,

but till now this means that the cells that are ready to form enamel are only atthe cusps tips, so this means that enamel is first formed at cusp tips, in other

words the last enamel that has formed is at the cervical part and the first enamel

that has formed is at the junction between enamel and dentine.

Dental papilla is still less differentiated than enamel organ.

A question about vascular component I guess (not heard)! The answer: is that we

are interested in providing nutrients to the cells in differentiation and they dontget fluids (sara7ah el Dr. En3ajag w batalt a3rf afham sho be7ke bs its a good

question(!

Dental papilla all the time is behind, and its differentiation is less, notice that we

have blood vessels in Dental papilla and blood vessels in the area of tooth germ,

but we dont have blood vessels in the enamel organ and the nutrition of the IEE

(the answer of the Q) can be provided in 2 ways either from the SR cells or


10/16

through blood vessels in the Dental papilla

Slide #15 is a very good view of the cells(=

The late bell stageis an appositional stage it is a stage when we can see hard

tissue formation, at the last step of tooth formation hard tissue starts to form.

What hard tissue do we have here in the crown? We have the Enamel & Dentine.

Dentine formation all the time precedes enamel formation, although the

differentiation of enamel organ is all the time ahead of dental papilla!

Appearance of a lingual down growth of EEE this structure the down growth

(emtedad) of EEE is very important in deciduous teeth this structure is very

important because it will give the successor lamina, we have dental lamina that will

give the primary tooth, but the lingual extension of the EEE will give the

successor tooth if any tooth doesnt have this lingual extension we are sure then


11/16

that this tooth is not going to have a permanent tooth there will be only a

deciduous tooth.

In case that one of the teeth is missing the permanent tooth for it is going to

be missing too so some Moms come to the clinic and their child didnt have a

deciduous tooth and they ask if he will have permanent one the answer is thathe wont)=

So loosing the deciduous tooth will lead to the loss of this lingual extension and so

no permanent tooth later on.

Appearance of a lingual down growth of EEE

In deciduous tooth germs successor laminagives rise to tooth germs of

permanent successor teeth.

In permanent tooth germs, we will also have this but we will have it as transient

structure that disappears eventually.

Non successor teeth will also have such as extension but it has no function, it is

transient (ya3ne 3ebarah 3n structure mo2aqat)

>> All teeth have lingual extensions but they are only active for deciduous teeth >>

Lets think about 1st, 2nd, and 3rd morals their lamina? Those teeth take their

lamina from the tooth that is in front of it immediately what is the tooth that

is in front of the 1st permanent molar? Its 2nd deciduous molar so 2nd deciduous

molar in addition to having lingual extension it also has a distal extension.

Lingual extension gives 2nd premolar and the distal extension gives the 1st molar.

Behind second deciduous molars dental lamina grows backwards to budoff permanent molars successively.

Successively means at the same time, in other words we have a distal extension

behind the deciduous 2nd molar for the 1st permanent molar we also have a distal

extension behind the 1st molar for the 2nd molar, and the 2nd molar will have distal

for the 3rd molar, But the 3rd molar doesnt have a distal extension.


12/16

Also in the late bell stage we have an interaction between structures raised from

ectoderm and structures raised from mesoderm, we said that for the tooth to

form there should be some interaction between enamel organ and surrounding

ectomesenchymal tissue, long time ago they made experiences they had an enamel

organ and took it away from the mesenchymal tissue they put it in a tissue culture

no tooth formation, and then they had an ectomesenchymal tissue and put it

alone in a tissue culture and no tooth formation.

That means that there is an interactive interaction both of them should be

together so the tooth would be formed, this interaction is important, this

interaction is said to be Reciprocal (tbadoli)

(zai lama te7ke l sa7bak 3asheshne b 3asheshak bte3tamdo 3ala ba3ad)

*Dentine & enamel formation commences at:

-Cusp tips

-Incisal edges

All the time Dentine & enamel forms at the Cusp tips and incisal edges firstly and

then they continue all over the tissue.

*Developing ameloblasts (Preameloblasts), we said that the IEE cells they will

become columnar and they will be called ameloblasts, but before that it is Pre

Ameloblasts it is a very mature IEE cells, and they induce adjacent mesenchyma

cells to become columnar and differentiate into odontoblast and they lay down

predentine & dentine, this is the 1st dentine to be produced before calcification,

and this dentine induces ameloblasts to secrete enamel.

*mesenchymal cells columnar differentiate into odontoblast secrete

predentine & dentine mature ameloblasts secrete enamel.

So cells that form dentine go (back) inside and cells that form enamel go outside.


13/16

A pic that we dont have! >> 1st part of dentine the first layer is the one at the

cusp tip this is followed by enamel then I have dentine then enamel. And so on

So the cells that form enamel they go outward, but cells that form dentine they

go inward, they are close but going apart (bekono 7ad ba3ad bs bebta3do 3n

ba3ad).

The full formation of the cusp doesnt mean that the enamel has stopped; there

will be enamel formation at the sides!

All the time the last enamel to be formed is the enamel beside the cervical line.

Q: What is the last dentine that is formed?A: the one nearest to the pulp.

Q: What is the last hard tissue to be formed?!

A: It will be the dentine.

V.I.I: enamel formation is an ending process it has a start and an end, and once

the last layer is formed the cells that are responsible for the formation of enamewill disappear but dentine formation is not an ending process, it is continuous

process, this process of dentine formation only stops when the pulp is removed (if

u remove the tooth or remove the pulp).

So the last layer of hard tissue to be formed is dentine and the first one is

dentine as well =)

A question not heard as well as the answer!

Another Q: why when we remove the pulp dentine formation stops?!

Because when we remove the pulp we also remove the cells that form dentine

Now we have some Transient structures that may develop, like: Enamel knot,

Enamel cord, and Enamel niche.


14/16

Enamel knot: sometimes we see a localized mass of cells in the center of IEE

cells; they bulge into dental papilla, they are Non-proliferative cells, they produce

signaling molecules, disappear & contribute cells to enamel cord like in slide 20

(3oqdeh bekoon fe)

Sometime these masses of cells extend from SI into SR forming the Enamel

cord, it has no specific function, and they may be involved in mechanica

transformation of cap stage into bell stage.

And they are termed enamel septum if it reached the EEE cells, when an enamel

septum proliferation meets the EEE it might make what is called Enamel navel A small invagination at junction with EEE

*Enamel niche

Area enclosed by the double attachment of enamel organ to dental lamina

those structures will finally disappear.


15/16

* Nerves and Blood Vessels:

As we know nerve supply starts as a plexus under dental papilla during cap stage

nerves spread & penetrate dental papilla with onset of dentinogenesis when

dentine formation process start dentinogenesis nerves start to penetrate the

dental papilla.

Blood vessels invade dental papilla at early bell stage we need blood supply to

have nutrients for the active cells.

Blood vessels are evident in dental follicle in close association with EEE which

they never penetrate important thing that no penetration

Because it is very important to preserve the 3D structure of the tooth before

hard tissue formation after that it is more stabile and there will np zai el

sabbeh b el saqif w el 5shabat ta3oon el sqaleh! B3d ma tenshaf awal tabaqet

isment fena ensheel el 5ashab w ne3tamad 3ala el esment el hard so bld vessels

after that wont affect the integrity of the cells.

We are late in this course and the Dr. hopes that we catch up or

mayoneez :P :P"

4give me 4 any mistake

I am very very sorry for being late but its not my mistake

Good luck to all w in-sha2-Allah good marks lal kol bel exam

W

Ur sister: Nada Nammas


16/16

Oral Histology Lecture 6

Documents

Transcript of Oral Histology Lecture 6