development of the root & pdl

8/2/2019 Development of the Root & Pdl

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DEVELOPMENT OF THE

ROOT & PDL

Oral Histology

Dent 206


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Root development

Some time after enamel completion

Interaction between

Dental follicle

Epithelial root sheath (of Hertwig)

A structure derived from cervical loop region of enamelorgan

Dental papilla

Onset coincides with axial phase of tootheruption


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Epithelial root sheath (ERS)

Forms at the late bell stage

Amelogenesis anddentinogenesis are well-advanced

Double-layered

Internal enamel epithelium

External enamel epithelium

No SR or SI

Apical proliferation

Mapping out the shape of theroot

Never a continuous sheath

disintegration


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Enamel Pearl

Localized areas of enamel on root surface

Usually in inter-radicular regions of molars

Retention of SR & SI inside the double-layered ERS


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ERS in mutli-rooted teeth

Primary apical foramen subdivideinto a number of secondary apicalforamina

Ingrowth of epithelial shelves from

the margins of ERS Fuse near the center of the root

Number & location of shelves Corresponds to the number of roots

May be under inductive role of dentalpapilla

Ingrowth is believed to occur alongpaths of low vascularity


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Primary apical foramen

While the root is being formed

Apex is wide open, surrounded by:

Thin regular knife edge of dentine

A permanent tooth Erupts with about 2/3s of formed root

Needs 3 or more years before root

completion

By root completion, the wide apical

foramen becomes very narrow

Only wide to transmit the

neurovascular bundle


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Growth of ERS

Occurs apically

Encloses dental papilla

Except at primary apical

foramen

Margin of ERS is angled

internally to form

Root diaphragm

Dental follicle

Lies external to ERS

Forms cementum, PDL &

alveolar bone


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Root diaphragm

The angled edge of ERS

A circular band

Sandwiched between the undifferentiatedmesenchyme of

Dental papilla

Dental follicle


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Commencement of root dentinogenesis

Cells of internal layer of

ERS

Induce peripheral cells of

dental papilla

Differentiate into

odontoblasts

Root dentine is deposited


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Commencement of cementogenesis

Root dentine is deposited

ERS cells lose continuity

Form epithelial rests (of

Malassez) in PDL

Root dentine is exposed tothe undifferentiated cells of

the dental follicle Differentiate into

cementoblasts

Cementogenesis


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Dental follicle near the diaphragm

3 layers

Inner investing layer

Ectomesenchymalneural crest

Cells differentiate into cementoblasts

Cuboidal cells on root dentine surface

Intermediate layer

Mesodermal origin

Cells differentiate into fibroblasts

Forming PDL

Outer layer

Mesodermal origin

Cells differentiate into osteoblasts

Forming alveolar bone


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DF after commencement of

cementogensis

Follicular cells areobliquely oriented along

the root surface Become the fibroblasts

of PDL

Fibroblasts secretecollagen which becomeembedded in developingcementum and alveolar

bone (Sharpys fibers)


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Root completion

When the final root length is achieved, the

proliferation of epithelium in the

diaphragm lags behind that of the pulp or

connective tissue

The wide apical foramen is reduced first -

to the width of the diaphragmatic openingitself

further narrowed by apposition of dentine and

cementum at the apex of the root


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Accessory root canals

If continuity of root sheath is broken or notestablished prior to dentine formation a defect inthe dentinal wall of the pulp occurs


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Root elongation & tooth eruption


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Root growth theory of eruption

Changes in apical connectivetissue

Old view - cushion hammock

ligament Fibrous with fluid filled

interstices

Attachment on either sides toalveolar bone

Resistant baseprevents boneresorptioneruptive force


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Root growth theory of eruption

Current viewpulp limiting

membrane

No attachment to alveolar bone

But merges with developing PDL Surgical removalno effect on

eruption

Theory

Changes in vascular permeability

Dense accumulation of tissue fluid

Fluid force bone and root apart

Contribution to eruption


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Formation of principal PDL fibers

Stage 1before eruption

PDLloosely structured collagenous elements

Permanent molars

Dentogingival & oblique fibers are organized

Premaloars

Only dentogingival fibers are well-developed


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Stage 2tooth penetration

PDLfibers developed but not spanning PDL

space

Permanent molars

PDL well-differentiated

Oblique fibers

PremolarsOnly alveolar crest fibers are organized


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Stage 3reaching occlusion

Permanent molars

Fibers in cervical region are organized

Premolars

Prominent cervical fibers

Undeveloped in apical part of the root

Stage 4functional Classical organization of principal fibers


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Additional notes on formation of

principal PDL fibers

Collagen fibers are only organized in function

Eruptive force caused by traction of collagen in

PDL development is debatable

Change in obliquity & thickness of fibers as

the tooth reaches the functional position

Inclination decreases Thickness with function


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Additional notes on formation of

principal PDL fibers

During eruptionresorption of the bone in thebase of the socket is predominant

Bone deposition is not accepted as a cause oferuption

However, species differ (deposition in dogs)

Bone activity is related to the distance the toothhas to erupt

Distance greater than root lengthdeposition


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Cementogenesis

Primary (acellular) cementum

Cervical portion of the root

Secondary (cellular) cementum

Apical region of the root

Furcation areas


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Formation of acellular cementum

Starts cervically and proceedsapically

Cells of ERS

Do not enlarge as in enamel organ

Separated by a basal lamina on bothsides

Induction of root dentinogenesis

Odontoblasts retreat inwards

Form first-formed root predentine No odontoblastic process yet

Structureless

Hyaline layer when fully mineralized


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Formation of

acellular cementum

ERS in contact with predentine

Disintegration of ERS cells

Exposure of follicular cells to

unmineralized hyaline layer Differentiation into

cementoblasts

Cementoblasts secrete collagenfibrils

Deep parts of fibrils interminglewith those of the hyaline layerstrong union

Peripheral parts extend into PDL(fibrous fringe)


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Formation of acellular cementum

Mineralization of

Hyaline layer

Fibrous fringe

Thus, first formed acellular cementum are

firmly attached to root dentine

Fibers of PDL are still parallel to root surface

& not yet attached to fibrous fringe


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Formation of primary (acellular)

cementum

Slow increase in thickness of acellular

cementum

Establishment of continuity between fibrous

fringe and PDL fibers

Continued slow mineralization of collagen


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Types of acellular cementum

Acellular intrinsic fiber cementum

Acellular cementum lining the root before itsfringe fibers are linked with those of PDL

In permanent teeth, exist until tooth erupts into themouth

Acellular extrinsic fiber cementum

Acellular cementum lining the root after its fringe

fibers are linked with those of PDL

Increases in thickness slowly & evenly throughoutlife


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Mineralization in acellular cementum

Presence of crystals is adjacent dentine is believed toinitiate mineralization in cementum

Adjacent PDL fibroblasts may play a role

Very slow process No calcospherites

No layer of unmineralized precementum

Incremental lines

High content of ground substance

Low content of collagen

Closer together than in cellular cementumslow formation


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Bisphosphonate

Bisphosphonate inhibit cementogenesis

Interferes with mineralization of initial root

dentine

No mineralization of the hyaline layer

No cementogenesis


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Acellular afibrillar cementum

A thin layer overlying cervical margin enamel

Damaged reduced enamel epithelium

Follicle cells contact enamel Differentiation into cementoblasts

Cementoblasts secrete afibrillar matrix that

calcifies


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Secondary (cellular) cementum

Appears at the time of eruption

Reflects increase in rate of cementogenesis

Early inductive changes are similar to those inacellular cementum

Cells of forming cellular cementum

Appear as a distinct layer of basophilic cells (RER

active protein synthesis)

More cytoplasmic processes


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Cellular cementum


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Effect of high rate of cementogenesis

in cellular cementum

High rate of formationappearance ofprecementum

Less mineralized than acellular cementum

Multipolar mode of mineralizationcellsincorporated within matixcellularappearance of cementum

Cementoblasts beomce cementocytes Generation of new cementoblasts at surface

Incremental lines more widely spaced


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What Can Go Wrong

Cementum will not form if epithelial cells are incontact with dentine


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Acellular cementum

Root dentine

Fibres of

Periodontal

Ligament

CementumEpithelial

Rests


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Cellular cementum

development of the root & pdl

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