3- Development of the Root (1)

7/28/2019 3- Development of the Root (1)

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Dr. Tahani Abualteen

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Development of the root & the periodontium

Root development: Root development starts sometime after enamel completion

Enamel formation has to be completed and all the thickness of enamel must be deposited andalso fully mature before any part of the root starts to develop

For the root to develop there must be an interaction between three components:Dental follicle

It is the ectomesenchymal tissueoutside the enamel organ and responsible for theformation of the tooth supporting tissues (cementum, PDL and alveolar bone)

Epithelial root sheath of Hertwig

Epithelial root sheath is an ectodermal structure derived from the cervical loop region ofenamel organ (which is a double layered circular structure made of IEE & EEE found at

the cervix of the crown after its complete formation) This sheath separates the dental papilla (on the inside) from the dental follicle (on the

outside)

Dental papilla

It is the ectomesenchymal tissueinside the enamel organ and responsible for the formationof dental pulp and dentine

** Both of the crown and root portions of the tooth require an interaction between ectodermal

and ectomesenchymal components to form

Onset of root development coincides with axial phase of tooth eruption Once the root starts to form and elongate, the tooth starts to erupt (moves vertically up) When the crown starts to have its root, the root elongates (action), and because it can't

penetrate the bone underneath it, the tooth erupts (reaction)

This picture is a summary of what we have covered previously: Tooth germ = enamel organ + dental papilla + dental follicle Enamel organ is responsible for the formation of enamel and it is composed of (IEE + EEE + SR + SI)


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Tooth germ passes into three stages that are named according to the shape of enamel organ (bud,cap and bell)

In the early bell stage morphogenesis 3-dimensional shape of the crown is established In the late bell stageHistogenesis hard tissue formation (enamel and dentin) begins

Dentinogenesis starts before amelogenesis When enamel formation is complete, root development begins When root development begins, tooth eruption begins too When tooth completes its eruption and emerges in the mouth, 2/3s of its root is formed, and it

needs 2-3 years (if permanent) or 1-1.5 years (if deciduous) to have its root completely formed

after emergence into mouth

Tooth is only functional, when it makes contact with its opposing tooth (when reaching theocclusal plane)

** Eruption = the full dynamic process from beginning of tooth movements until occlusal

contact with opposing tooth is achieved

Epithelial root sheath (ERS): ERS forms at the late bell stage from the cervical loop area of enamel organ ERS appears while amelogenesis and dentinogenesis are well-advanced but starts to function in

root formation after amelogenesis terminates and completes

ERS is a double-layered circular sheath that iscomposed of:

o Internal enamel epithelium (columnar cells fromthe inside)

o External enamel epithelium (cuboidal cells fromthe outside)

Unlike enamel organ, the ERS doesn't havestellate reticulum (SR) and stratum

intermedium (SI) in between EEE and IEE

So in ERS we only have EEE and IEE but wedon't have SR and SI and because SR and SI are

so important in enamel formation, then NO

enamel formation takes place in the root

portion of the tooth (enamel is only found in

the crown portion of the tooth)

So all of the components of enamel organ are important and we should have them all togetheras a group to be able to form enamel and what confirms this fact is that in the root portion of the

tooth we only have EEE and IEE and we don't have SR and SI so that no enamel forms

ERS undergoes apical proliferation (ERS proliferates toward the apex of the root and away fromthe crown) and this is important for mapping out the 3-dimensional shape of the root

Coronal proliferation, cell division and movement of IEE cells of enamel organ in the crownis very important for the 3-dimensional structure of the crown

Apical proliferation, cell division and movement ofIEE cells of ERS in the root is also veryimportant for the 3-dimensional structure of the root


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ERS is never a continuous sheath and it undergoes disintegration The ERS cells in the upper part of the root proliferate and form

the first part of the root, and as long as this part is being

formed, ERS is intact, but once this part is fully formed, the

ERS cells disintegrate and disappear and then another ERScells in a lower part of the root will proliferate and form the

second part of the root, and as long as this part is being formed,

ERS is intact, but once this part of the root is fully formed, the

ERS cells disintegrate and disappear .

ERS cells are responsible for the formation of root dentine and once dentine is formed at certain region along the root,

these ERS cells disintegrate and the process of root dentine

formation continues apically so that each time dentine is

formed the ERS cells adjacent to that dentine disintegrate

and that's why when we view the root we will NOT see the

ERS as a continuous sheath

When ERS cells disintegrate, they are removed from the root portion of the tooth & placed intothe PDL where they remain as islands of non-functioning cells called (epithelial rests of Malassez)

which if proliferate later in response to inflammation, this will give rise to cysts

Enamel pearl: It is a localized area of enamel on root surface Normally we have no enamel formation in the root portion of the tooth

because the ERS only has EEE and IEE but NO SR and SI in between


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Sometimes the ERS retains the SR and SI in between the EEE and IEE and so a small piece ofenamel forms in the root and this piece is called the enamel pearl

o Enamel pearls are usually located in the furcation regions of maxillary molars ERS in multi-rooted teeth:

Once enamel formation is completed, ERS starts root development and something called "Primaryapical foramen" appears if we view the tooth and the ERS from below

If the tooth is single rooted This Primary apical foramen remains as one foramen If the tooth is multi-rooted This primary apical foramen subdivides into a number of "secondary

apical foramina"

On the right, this is a forming root and a developing ERS as viewed from below:o At first, we will be having a primary apical

foramen

o If the tooth is single rooted, then this primaryapical foramen won't divide and it will

proceed as one foramen till apically

o If the tooth has two roots, then two horizontalextensions called "epithelial shelves" grow

and extend from the margins of ERS toward the

center of the root where they fuse and divide

the original primary apical foramen into two

secondary apical foramina (each of which will

continue its root formation on its own)

o If the tooth has three roots, then threehorizontal extensions called "epithelial

shelves" grow and extend from the margins of

ERS toward the center of the root where they fuse and divide the original primary apical foramen

into three secondary apical foramina (each of which will continue its root formation on its own)

o Growth and extension of the epithelial shelves is believed to occur along paths of low vascularityo Number and location of the epithelial shelves: It corresponds to the number of the roots May be under inductive role of dental papilla

o If the tooth is multi-rooted and the primary apical foramen has to subdivide into a number ofsecondary apical foramina then this subdivision doesn't occur at the junction between the crown

and the root. We will have a distance between the end of the crown and the beginning of the root

division and this distance is called "root trunk"

The area where primary

apical foramen divides into

a number of secondary

apical foramina and where a

number of epithelial shelves

fuse together is called"Furcation area"


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Primary apical foramen: While the root is being formed the apex is wide "open"

and surrounded by thin regular knife edge of dentine

A permanent tooth:o Erupts with about 2/3s of its root formedo Its root needs 2-3 years after eruption to have its root

formation completed

A deciduous tooth:o Erupts with about 2/3s of its root formedo Its root needs 1-1.5 years after eruption to have its root formation completed

By root completion, the wide apical foramen becomesvery narrow and only wide to transmitthe neurovascular bundle

** How to differentiate between fractured tooth (after root formation completion) and a still

developing tooth by looking at its apical foramen?!- Fractured tooth narrow apical foramen- Still developing tooth wide apical foramen

Growth of the ERS: Growth and proliferation of ERS occurs apically (toward

the apex of the root)

ERS is never a continuous sheath so that ERS cells at upperparts of the root form the root and then disintegrate while ERS

cells at the lower parts of the root are still forming the root Dental papilla lies internal to ERS and forms dentine and

pulp

ERS grows apically to enclose the dental papilla except atthe apical foramen where the dental papilla is continuous

with the dental follicle

Margin of ERS is angled internally to form the "Rootdiaphragm"

Dental follicle lies external to ERS and forms cementum,PDL and alveolar bone

Root diaphragm: It is the portion of the ERS that is angled internally toward the dental

papilla

It is a circular band (with internal & external circles) It is sandwiched between two proliferation zones of undifferentiated

mesenchyme:

1. The undifferentiated cells of Dental papilla inside (that will giveodontoblasts that will lay down root dentine on the inside)

2. The undifferentiated cells of Dental follicle outside (that will givecementoblasts that will lay down cementum on the outside)


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Root diaphragm is the only portion of the ERS that doesn't undergo disintegration and appearsalways as a continuous band

Commencement of root dentinogenesis & cementogenesis: Commencement = beginning ERS is made up of two layers (internal & external layers which are continuation of IEE & EEE of

enamel organ)

Cells of internal layer of ERS induce theperipheral undifferentiated cells of the dental

papilla to differentiate into odontoblasts to start

depositing the root dentine

Once the root dentine is laid down, the ERS losesits continuity and disintegrate leading to the

formation of the epithelial rests of Malassez in

the PDL

Disintegration of ERS cells allows the depositeddentine to become exposed to the peripheral

undifferentiated cells of the dental follicle

which will then differentiate into cementoblasts

to start depositing cementum

** Exposure of dentine to dental follicle only occurs in the root but not in the crown

** The process of root dentinogenesis & cementogenesis happens in each piece of the root

individually and it isn't neccissary to see them in all areas of the root at once !

o This picture shows the crown androot portions of a developing tooth

o Enamel in blow and dentine inbrown

o Ameloblasts in white andodontoblasts in yellow

o Enamel organ = ameloblasts + SI+ SR + EEE

o Epithelia l Root Sheath = EEE andIEE ONLY at the cervical marginof the crown

o ERS separates dental papilla onthe inside from dental follicle on

the outisde cells of internal

layer of ERS induce the

undifferentiated cells of the

dental papillaodontoblasts

are formed root dentine is deposited ERS cells adjacent to the deposited dentine

disintegrateroot dentine becomes exposed to the undifferentiated cells of the dental follicle

cementoblasts are formed cementum is deposited


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Dental follicle near the diaphragm: Dental follicle near the diaphragm is made ofthree layers:

1- Inner investing layer: It is the inner layer of the dental follicle which lies in

proximity to the epithelial root sheath and the root dentine It is ectomesenchymal in origin (originate from neural crest

cells)

Cells in this layer differentiate into cementoblasts (cuboidalcells) which lay down cementum on the top surface of root

dentine

2- Intermediate layer: It is the middle layer of the dental follicle It is mesodermal in origin

Cells in this layer differentiate into fibroblats which form the periodontal ligament (PDL) PDL is the structure binding the cementum of the root to the alveolar bone

3- Outer layer It is the outer layer of the dental follicle and the farthest away from the epithelial root

sheath and the root dentine

It is mesodermal in origin Cells in this layer differentiate into osteoblasts which lay down the alveolar bone

** Neural crest cells aren't found in all regions of dental follicle , but they are only found

in the inner investing layer

** Fordeciduous teeth

bone is forming as the root is forming BUT forpermanentteethbone is lready formed before root starts to form

** Enamel and epithelial root sheath ectodermal in origin

** Dental pulp and dentine and cementum ectomesenchymal in origin

** PDL and alveolar bone mesodermal in origin

Dental Follicle after commencement of cementogenesis: Follicular cells are obliquely (not horizontally) oriented along the root surface Once the root dentine is deposited, epithelial root sheath cells disintegrate & dentine contacts the inner

undifferentiated follicular cells which then differentiate into cementoblasts & lay down cementum

Intermediate undifferentiated follicular cells differentiate into fibroblasts & start forming PDLo These fibroblasts travel obliquely along the root surface and secrete mostly oblique bundles of

collagen fibers which become embedded in the developing cementum from the inside before

mineralization and then they get mineralized and they become also embedded in the developing

alveolar bone from the outside before mineralization and then they get mineralized

o The edges of these bundles which are embedded into cementum from one side and into alveolarbone from the other side and get mineralized are called Sharpys fibers

Outer undifferentiated follicular cells differentiate into osteoblasts & start forming alveolar bone


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Root completion: In molars of herbivores (e.g. horses, cows, goats and sheep)

root formation is a continuous process and apical foramen

remains widely open

** Teeth of herbivores are always abraded due to the nature offood they have and this necessitates continuous formation

In carnivores & omnivores (e.g. humans) root formation is anending process and apical foramen needs to narrow sometime

** Teeth of carnivores and omnivores aren't abraded in the same way of

herbivores because the nature of food is different and thus they should have

some sort ofroot completion at some time not to end up having occlusal

problems (e.g. maxilla and mandible moving away from one another)

When the final root length is achieved the proliferation of epithelium inthe diaphragm lags behind that of the dental papilla or the dental follicle Final root length is genetically determined inside the ERS itself Root development depends on the interaction between three components

(epithelial root sheath that is sandwiched between the dental papilla cells from the inside and

the dental follicle cells from the outside)

Usually the rate of growth of these three components is the same When the final root length is achieved and root formation is to be terminated, the rate of

proliferation of ERS cells slows down so that it becomes slower than the rate of proliferation

of dental papilla cells from the inside and dental follicle cells from the outside

So that more dentine will be forming from the inside & more cementum will be forming from the

outside and this narrows the wide apical foramen &terminates the process of root formation

** Along the length of the root, dentine is always a thin knife edge structure because ERS is

moving quickly from one segment of the root to another and forcing odontoblasts to follow it

and this doesn't give odontoblasts enough time to build thickness of dentine

** ERS only slows down when final root length is achieved and root formation is to be completed and

this gives odontoblasts enough time to build thickness of dentine and narrow apical foramen

The wide apical foramen is reduced:o First to the width of the diaphragmatic opening itselfo Further narrowed by apposition of dentine and cementum at the apex of the rooto Apical foramen should be wide only to transmit the neurovascular bundle

Accessory root canals: They are small secondary canals extending from the main pulpal canal to the surrounding

periodontal ligament (PDL)

When do these accessory canals occur?! If continuity of epithelial root sheath is broken or not established prior to dentine formation

then a defect in the dentinal wall of the pulp occurs

Normally, ERS cells disintegrate at certain segment along the root ONLY after complete rootdentine formation at that area


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If ERS cells disintegrate prematurely or their continuity isn't established (because there was a bloodvessel in the area so that they just looped around it) before any root dentine starts to form, then we

will have an area that is devoid of dentine and then we will be having an accessory canal

extending from the dental pulp (inside) to the PDL (outside)

Root elongation & tooth eruption: Let's imagine that my upper body half is the crown

and my lower body half is the root then if I want

to grow taller, I can't penetrate the ground but

instead I can move up

The same applies for the tooth, if the root wants togrow longer, then the tooth can't penetrate the

bone underneath it but instead it can move

upward (trying to erupt) and allowing the root to

elongate farther more

Notice that root elongation occurs in an oppositedirection to tooth eruption

Root growth theory of eruption:1- Old theory is called "cushion hammock ligament" This theory isn't accepted anymore It says that we have a hammock ligament from one edge of the root

diaphragm to the other edge

This ligament is fibrous with fluid filled interstices and it isattached on either sides to alveolar bone

This ligament has a very resistant base thus it prevents boneresorption from below so that the tooth can't penetrate the bone

underneath it but instead it will move up (erupts) and allows the

root to elongate

2- Current theory is called "pulp limiting membrane" This theory is the one accepted these days This theory rejects and corrects the old theory:

They found later that this hammock ligament is not attached toalveolar bone but instead it merges with the developing PDL

So that this hammock ligament doesn't have a very resistantbase

They found that if we remove this hammock ligament surgically,then this will NOT affect the eruption process and the tooth can

still move up and erupt

So that the hammock ligament is NOT that important in theprocess of eruption!!

The current theory states that there will be changes in vascular permeability in the apicalregion (between the inside & outside of the tooth) that leads to dense accumulation of tissuefluids that pushes the root and the bone apart and contribute to tooth eruption


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Formation of principal PDL fibers: Principal PDL fibers are the first formed fibers in the PDL and they pass into 4 stages

Stage 1before eruption:o PDL fibers before eruption are loosely structured collagenous elements that aren't making

any bundle

o In permanent molars: Dentogingival and oblique fibers are organized Dentogingival fibers = fibers connecting the tooth with the gingiva Oblique fibers = fibers extending obliquely from the root surface to the surrounding bone

o In premolars: Only dentogingival fibers are well-developed

Stage 2tooth penetration:o Tooth penetration = tooth is moving up & starting to penetrate alveolar mucosa to appear in moutho PDL fibers during tooth penetration are well differentiated but not spanning the PDL space

(they are concentrated cervically but arent found in all parts of the root and aren't connecting

the root to the bone yetbecause the tooth is still erupting and it isnt functioning yet)

o In permanent molars: Oblique fibers are well-differentiated but NOT spanning the full thickness of the PDL space

o In premolars: Only alveolar crest fibers are organized

** Organization of PDL fibers in molars is more advanced than that in premolars & anteriorsStage 3reaching occlusion:

o Reaching occlusion = the erupting tooth contacts the opposing tootho In permanent molars: Fibers in the cervical region are organized

o In premolars: Fibers in the cervical region are prominent Fibers in the apical part of the root are undeveloped

Stage 4functional:

o Functional = erupting tooth reached the opposing tooth and now it is functioning in masticationo It is the only stage where there will be a classical organization of principal fibers of PDL


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Additional notes on formation of principal PDL fibers: Collagen fibers are only organized in function (they only gain their classical organization after

functioning)

Eruptive force caused by traction of collagen in PDL development is debatable One of the theories on tooth eruption and root elongation is that as the PDL tries to develop, the

traction of collagen fibers in the PDL leads to eruption This theory is debatable and not 100% clear because collagen fibers in the PDL are only organized

after the tooth starts functioning and not before eruption so they can't actually cause the tooth to erupt

Change in obliquity & thickness of fibers as the tooth reaches the functional position Obliquity (inclination) of PDL fibers decreases as the tooth reaches its functional position Thickness of PDL fibers increases as the tooth reaches its functional position

During eruption resorption of the bone in the base of the socket is predominant If we view a tooth during eruption we will find that bone in the apical region is being resorbed

mainly

So that bone deposition at the apical region of the tooth is not accepted as a cause of eruption However, species may differ so that (in dogs for example) deposition of bone in the base of

the socket is predominant

Bone activity is related to the distance the tooth has to erupt Distance greater than root length is associated with bone deposition

- Bone resorption at the base of the socket predominates BUT If the distance between the formingtooth and the surface of the oral mucosa is very long and greater than the length of the root of

that tooth then we should have deposition of bone at the base of the socket otherwise we will be

having a space underneath the root

- Like for example the upper canines which have the greatest distance away from the oralmucosa, that's why in such teeth we should have bone deposition near the apex of the tooth to

compensate for the upward movement of the tooth

- Other teeth are very close to the oral mucosa and thus they don't need bone deposition at thebase of the socket but instead they mainly have bone resorption

3- Development of the Root (1)

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