Dental Anatomy Lecture 4

باسم الله الرحمن الرحيم

Oral histology lecture #4 of the final material …

Development of root part1

Root development:

Sometime after enamel completions … remember that there is no root formation before the tooth has completed its crown …

Interaction between … dental follicle and Epithelial root sheath

This means that root formation is similar to crown formation … crown formation depends on interaction between enamel organ and dental papilla … here (in root) we have interaction but between a structure called Epithelial root sheath {of Hertwing} and dental follicle which are the cells that surrounds the developing tooth

Epithelial root sheath is a structure derived from cervical loop region of enamel organ

The onset coincides (yatazamn) with axial phase of tooth eruption

As long as the tooth has not undergone any root formation it is held in place (la yata7arak) … once the 1st part of the root forms the tooth starts to move axially (ezan 7araket el senn b al bozoo3’ a7ad asbabha ennoh el root betkawan) for sure when the tooth is inside the bone it moves but not axially … so we have interaction between: dental papilla, dental follicle, and Epithelial root sheath these three structures are going to be discussed next

Slide 3 … just to remind you of what happens we have bud stage, cap stage, bell stage, before bell stage we don’t have any root formation even during bell stage we don’t have any root formation … as you remember bell stage is divided into two phases early and late, in early phase we have movement of cells to establish the 3-D shape of the crown, and in the late stage we have the beginning of hard tissue formation … once all enamel is formed now it is time for root to start to develop

Epithelial root sheath (ERS)

Forms at the late bell stage … so it forms after hard tissue formation

Amelogenesis and dentinogenesis are well advanced … (lazm yekon 3ende amelogenesis w dentinogenesis otaqademeh w 3ala washk enha tentahi lama el ERS etbalesh tetkawan)

What is the epithelial root sheath? It is a double layered structure composed of *Internal & *External enamel epithelium

If you remember from the enamel organ we have: internal enamel epithelium and external enamel epithelium, we have between these two cells also SR and SI, what happens now (in root formation) the two layers of cells that are the internal and external enamel epithelium they join together or fuse together to form bi-layered structure which is called ERS

It is located at the cervical margin of the developing crown

Just to have a clearer view slide 5 … in red here this is external enamel epithelium, and we have the ameloblasts or the internal enamel epithelium in the crown we have some cells between them which are the ST and SI, now what happens in root formation the ST and SI disappear which leads to some sort of joining between the internal enamel epithelium and external enamel epithelium at the cervical margin of the tooth forming the ERS of Hertwing (after the scientist who discovered it)

Notice that we don’t have ST or SI in the root this means that there is no enamel formation because for it to form it needs ST and SI … and here something else would form not enamel

Apical proliferation … we also have division of cells toward the apex of the tooth… and this apical proliferation leads to extension of the ERS … mapping out the shape of the root (correct it in the slides it is root not crown) now this structure goes apically and it gives the 3-D shape of the root {as we said that the IEE gives the 3-D shape of the crown}

Also the ERS migrates apically and as it goes it makes the 3-D shape of the root

A very important info: this sheath is never a continuous sheath, in other wards if you came to the tooth in any stage of root development you will not see this sheath as a continuous sheath why? Because when

this sheath proliferate from the edge of the crown it will be responsible for the formation of the 1st part of the root, and then it will go down and form another part of the root and so on … once the 1st part of the root has formed this sheath disintegrates or precedes down

Kol ma hada el sheath ada 2ela takowon juzu2 mn el root yanqasim hada el sheath… faqat bekon active b al makan ele bekawen el root feeh

It will not appear continuous … only appear as a sheath in the apical parts of the root … the areas that had already formed from the root adjacent to it the sheath discontinue

Sometimes some mistakes may take place … sometimes the ST and SI are retained are kept instead of being disappeared at that location we will see some enamel formation at the root … and this is called Enamel pearl as you see in slide 6 the molar notice we have enamel structure on the root …

We know that root should be covered with cementum not enamel, why in this case we see enamel instead of cementum here? Because at that spot which is called enamel pearl SI and ST failed to disappear, normally they should disappear just leaving IEE and EEE, when they failed to disappear this led to formation of enamel at the root… enamel pearl is usually present at the root trunk of maxillary molars … it is not very common less than 1% of the

cases … so it is a localized areas of enamel on the root surface … usually it is located in inter-radicular regions of molars … and this is because of retention (7abs) instead of disappearance … retention of ST and SI inside the double layered ERS

Let’s hope the dude playing with his laptop is viewing something related to the material :P

ERS in multi-rooted teeth:

When we have more than one root what happens? In single root we have the primary apical foramen (like in pic A) we have one hole for the root … and this leads to the formation of only one root

But when we have two roots forming notice that we have extension of this root sheath, and then these extensions fuse together dividing the primary foramen (apical foramen of the root) into two apical foramina … and in cases when we have three roots we will have three extensions fusing together and dividing the structure into three parts

Notice that the ERS is not only apically or vertically directed it is also angled internally … as you can see it goes apically but the margin of this apical root sheath is tilted inside … so this part

which horizontally located {the horizontal portion of the root sheath} is called Epithelial diaphragm the whole thing is ERS and the horizontal part of it is called the Epithelial diaphragm

We are going to imagine that we have one tooth and this tooth has this inflection {the horizontal inflection of the ERS} and if you want to look at this tooth from the apical view the outer line represents the edges of the tooth and the internal represents the edges of the diaphragm

That is why it looks like a circle or a circular band

Primary apical foramen subdivide into a number of secondary apical foramina

In-growth of epithelial shelves from the margins of ERS … shelves fuse together dividing the single foramen into foramina … the fuse near the center of the root … the # and location of the shelves corresponds to the # of roots … we have two shelves two roots in cases we have three shelves we will have three roots

And this is under the inductive role of dental papilla … so dental papilla is actually involved in their stability

And as we said in dental anatomy the root starts as a single root we call it root trunk and then into two or three {and usually between the cervical third and the middle third of the root}

This is genetics so the cells are coded genetically and they know when to induce the proliferation of the shelves that leads to division of the root

Ingrowth is believed to occur along paths of low vascularity … some scantiest say this ingrowth of these shelves is related to areas of low vascularity, areas of low bl pressure … the next pic is just a summary of what happens:

Primary apical foramen:

The primary apical foramen is a description of the hole that is found inside the root while the root is developing …

When I see a tooth that is still developing I have a hole this is called or due to the primary apical foramina … but if I have a completed tooth {in X-sec} it won’t appear this way

There will be very think dentine layer, and we just have a small root canal in the middle of the root … and this means that a developing root should all the time have knife edge development of dentine … and because of this knife edge development we have big holes or big cavities … big foramen at the end of each root

While the root is being formed the apex is wide … as you see in the slide and it is open … it is surrounded by thin regular knife edge of dentine … while the root is forming …

A permanent tooth erupts with about 2/3s of its formed root … this need 3 or more years before root completion … but in deciduous teeth we need 1 to 1.5 years until root completion

By root completion, the wide apical foramen becomes very narrow … as the root starts to be completed or as the root completes {reach its completion} the wide apical foramen starts to get narrowed down … until it become a small narrow hole

In dental anatomy exam the Dr. brings Qs about the root and if the apical foramen is empty from inside this means this tooth is still not completed its root {still developing its root} and it is not fractured … this means that we have some sort of internal build up of dentine … so this dentine would transform this big foramen into small apical foramen … just enough to allow the neurovascular bundle to pass.

Growth of ERS:

Occurs apically … and it encloses the dental papilla … as we can see and the area in the middle it would be the dental papilla ^^;

This except in apical foramen {the fact that the ERS encloses dental papilla}

So the growth of the ERS encloses the dental papilla but not totally except at the apical foramen

Margin of ERS is angled internally to form Root diaphragm … the angles are internal as we said

Dental follicle lies external to ERS … so the cells from outside are the dental follicle cells!

Dental follicle is responsible for the formation of cementum, periodontal ligament PDL, and the alveolar bone

Root diaphragm

Diaphragm is the angled edge of the ERS … it is a circular band as we discussed

Root diaphragm is sandwiched between the undifferentiated mesenchyme of:

Dental papilla &

Dental follicle

Inside the root area it is the dental papilla … but at this stage the dental papilla and dental follicle are still undifferentiated

Commencement of root dentinogenesis

We said ERs is composed of two layers IEE and EEE … the active cells are the internal cells of the ERS so they induce the peripheral cells of the dental papilla {the cells located just next to them}, and upon this induction these dental papilla cells differentiate into odontoblasts … and then root dentine is deposited

As you can see in the picture the epithelial root sheath which is composed of two layers internal and external … now just next to

the internal layer there is undifferentiated dental papilla cells … so they induce those cells to become odontoblasts and they lay down root dentine

Root dentine is deposited … which leads to disintegration (tajazu2) of the ERS … ERS cells lose continuity and form epithelial rests (of Malassez) in PDL …

And those rest cells of Malassez stay in the PDL after the root has completed

The ruminants of the ERS cells are important in the development of dental cysts {if you ever heard of someone having a cyst in his jaw?} we call these cysts odontogenic cysts … for example one tooth needs root canal treatment and you left it and didn’t treat it this will lead to papiloma … sometimes these rest cells of Malassez if induces somehow it may proliferate and make a cyst in the jaw and we call this “radicular cyst” it doesn’t take place in everybody but in people with chronic infection

Just remember that those rest cells of malassez are present in everyone but do not get activated only in people with special situation

Root dentine is exposed to the undifferentiated cells of the dental follicle (dentine ada ela tajazu2 el root seath f sar juzu2 mn el dentine exposed)

Exposed root dentine is to the undifferentiated cells of the dental follicle because of that these cells differentiate into cementoblasts and lay down cementum

This picture is very nice and summarizes the whole process …

Dental follicle near the diaphragm … is divided into 3 layers … dental follicle which surrounds the tooth from outside … 1. The first layer is the cells just next to the formed dentine 2. We have intermediate cells 3. And we have the cells outside … cells just next to dentine they differentiate into cementoblasts and they lay

down cementum … the cells located in the middle they differentiate into fibroblasts and they will form the periodontal ligament … the cells located outside they differentiate into osteoblasts and lay down the alveolar bone

Again we have three layers:

Inner investing layer … cells located just next to the newly formed dentine … they differentiate into cementoblasts and those are Cuboidal cells on root dentine surface … and they lay to the formation of cementum … they are Ectomesenchymal in origin so this means they are from neural crest

Intermediate layer: these are mesodermal in origin and they differentiate into fibroblasts forming the PDL

Outer layer: the cells located outside they differentiate into osteoblasts to form alveolar bone

Follicular cells are obliquely oriented along the root surface … as we can see the cells are becoming oblique they become the fibroblasts of PDL … fibroblasts secrete collagen which become embedded in developing cementum and alveolar bone (Sharpy’s fibers) we will talk about this …

How can the senn :P :P

How can the tooth be attached to the alveolar bone? As you can see in the pictures … when dentine is formed if we have ropes and attach them to dentine … part of these ropes will have cementum forming over them! … Ya3ne bekon fe menha juz2een juzu2 embedded in cementum w juzu2 free

And this free end will attach in a similar way to the bone

So PDL has fibers called PDL fibers the edges of these fibers are attached to cementum and in the other side are attached to bone and by this way we attach the tooth to the bone

“Zai el sabeh Matalan lama et7ut 7bal b el esment kaif bekoon shakelha tal3ah mn el esment nafs el fekrah hoon…

Root completion

When the final root length is achieved … when the ERS descends down to the length wanted for the tooth until it reaches it ya3ne what happens? Proliferation of epithelium in the diaphragm lags behind that of the pulp or connective tissue … {btenbanah tabaqat saree3ah mn el dentine asra3 mn el proliferation}

Usually dentine’s building speed is less than the ERS growth downward now when the root formations comes to an end this has to stop and the dentine building up becomes faster and faster and the ERS proliferation lags behind!

The wide apical foramen is reduced first - to the width of the diaphragmatic opening itself

After wards it further narrows by apposition of dentine and cementum at the apex of the root

Accessory root canals

So far you know that each tooth has to have one root canal but sometimes we have accessory canals

If continuity of root sheath is broken or not established prior to dentine formation a defect in the dentinal wall of the pulp occurs

If we suppose that the root dentine is going in its own way and suddenly finds a structure Bl vessel for Ex so at this area there will not be ERS … no induction for odontoblasts and no dentine building up … and we will end up with a space (fara3’) this empty space is the accessory root canals

So accessory root canals is developed when the continuity of the RES is broken by any structure that passes in its direction!

Root elongation & tooth eruption

It is well known that when the root develops the tooth starts to move anteriorly {lanafred fe ensan ma 2eloh ejren w sar 2eloh ejren hal ejreeh ra7 te5tareq el ard?} and we will talk about this later on …

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