(a largely ectomesenchymally derived unit )

Dr.Jignesh R. PatelDr. Shrikar Desai

Contents :

•Introduction

•Development of teeth

•Stages of tooth growth

•Hertwig’s epithelial root sheath & root formation

•Development of cementum/ cementogenesis

•Development of PDL

•Development of alveolar bone

•Development of dentogingival unit

•conclusion

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Introduction

The periodontium is simply defined as the tissues supporting

and investing the tooth - consists of cementum, PDL, bone

lining the alveolus & that part of the gingiva facing the tooth.

The tissues supporting the tooth are developmentally derived

from the dental follicle proper, whereas those investing the

tooth, that is the gingiva, are an adaptation of the oral mucosa.

- Richard Ten Cate

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The widespread occurrence of periodontal diseases & the realization that

periodontal tissues lost to the disease can be repaired has resulted in

considerable effort to understand the factors & cells regulating the

formation, maintenance, & regeneration of the periodontium.

- Ten Cate et al Periodontology 2000, Vol. 13

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Reciprocal induction

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Tooth germ

Ectodermal cells of dental lamina divide more rapidly & form little knobs

that grow into underlying mesenchyme.

These little down growths from the dental lamina represents the beginning

of enamel organ of the tooth bud.

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As cell proliferation continues...

Each enamel organ increase in size & sink deeper into the

ectomesenchyme, & due to differential growth shape also changes.

First it takes a shape that resembles a Cap, with an outer convex facing

the oral cavity & inner concavity.

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Three basic parts of Tooth germ

◦ Enamel organ (ectodermal component)

◦ Dental papiila

◦ Dental sac or dental follicle

Ectomesenchymal

component

Enamel organ

• Enamel

Dental papilla

• dentin

• pulp

Dental follicle

• Cementum

• PDL

• Alveolar bone

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Hertwig’s epithelial root sheath

(HERS) & root formation

Schour & Massler suggested that the major function of the

Hertwig’s epithelial root sheath is to induce and regulate root

formation, including the size, shape and number of roots

Characteristics of HERS:

1. HERS consists of inner & outer enamel epithelia only.

2. Cells of inner layer remain short in size and induce the

differentiation of radicular dental papilla cells into

odontoblast which lays down first layer of radicular dentin.

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3. Some outer layer cells in coronal root region induce cells of dental follicle to

differentiate into cementoblasts similar to osteoblasts which give rise to acellular

cementum.

4. Slavkin suggests, since the epithelial cells of the inner layer of Hertwig’s epithelial root

sheath are analogous to the preameloblasts, it is suggested that they might secrete

enamel matrix proteins over the newly deposited root dentin.

5. In addition to these matrix proteins there are also the components of the epithelial

basement membrane, such as laminin and collagen type IV are sectreted by root sheath.

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Development of cementum

Process of cementum development is called as “cementogenesis”.

Cementum is calcified, avascular mesenchymal tissue that forms outer coverings of the

anatomic root.

It was first demonstrated microscopically in 1835 by two pupils of purkinje.

It is a specialized connective tissue that shares some physical, chemical & structural

characteristics with compact bone.

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Cementogenesis (Briefly)

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[Hertwig’s epithelial root sheath is broken up &

separated from root, and differentiation of

cementoblasts lead to formation of cementum] Dr.Jignesh

Varieties of cementum

Two basic types of cementum, hence they are usually classified on the basis of presence of

cementocyte (cellular cementum) or absence of it (acellular cementum).

It can also be classified on the basis of the types of fibers (extrinsic/intrinsic) presence or

their absence (afibrillar cementum).

acellular extrinsic fiber cementum is regarded as Primary cementum as it forms first.

Cellular cementum is regarded as secondary cementum because it forms later than

primary cementum.

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Growth factor families involved in the differentiation of

cemetoblasts from dental follicle

TGFβ 1-5

BMP2-8

EGF & IGF

PGE2 & PGF2α enhance differentiation byactivating protein kinase cell signallingpathway

Fibroblast growth factor promotes proliferation, migration & angiogenesis

CAP, BSP and osteopontine helps inattachment of differentiated cells to newlyforming tissue

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Development of acellular cementum

Development of acellular cementum is associated with secretion of

enamel matrix protein (EMP) by HERS after mineralization of first

layer of dentin adjacent to the root.

Enamel proteins including amelogenin & certain basement membrane

constituents are reported to be involved in epithelial-mesenchymal

reaction.

HERS also secrete cementum related proteins like Bone sialoprotein

(BSP), osteopontin & fibrillar collagen.

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At the same time fibroblast precursors cells from dental follicle come in contact with

predentine matrix and start depositing bundle of collagen fibrils to form a thin layer of

perpendicularly oriented “Sharpey’s fibers” or “fringe fibers”.

Sharpey’s fibers interdigitate with unmineralized dentin at one end and into extracellular

compartment of acellular cementum at another end.

As the mineralization front advances, it contacts the sharpey’s fibers and they undergo slow

mineralization to complete the process of acellular extrinsic fiber cementum formation.

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Development of cellular cementum(a more rapidly formed & less mineralized variety of cementum)

Formation occurs after at least half the root is formed.

Development can be divided into 2 stages:

An early stage in which extrinsic sharpey’s fibers produced by fibroblasts are few &

traces of intrinsic fibers produced by cementoblasts are randomly arranged

Later stage of cementogenesis- it closely resembles bone formation. Cementoblasts and

cementocytes are involved in the secretion of intrinsic fibers.

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Rapid and multipolar mode of mineralization

takes place

Entrapped cementoblasts are called as –

cementocytes

Cementcytes have processes that lodge in

canaliculi that communicate but do not form a

syncitium that extends all the way to the surface,

as is the case with in the bone.

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Cementoid tissue & calcification of matrix

The uncalcified matrix is called as cementoid.

the growth of cellular cementum is a rhythmic process, and as a new layer of cementoid is formed, the old calcifies.

Gla proteins – osteocalcin & osteonectin acts as neucleators for mineralization due to their strong affinity for calcium & BSP.

Alkaline phosphatase promotes mineralization.

Osteopontine regulates growth of apatite crystals.

Major proteoglycan located in non-mineralized cementum is keratan sulfates- lumican & fibromodulin.

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The periodontal ligamment (PDL) is composed of a complex

vascular & highly cellular connective tissue that surrounds the tooth

root & connects it to the inner wall of the alveolar bone.

Over the years it has been described by number of terms:

• Desmodont

• Gomphosis

• Pericementum

• Dental periosteum

• Alveodental ligament

• Periodontal membrane

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Development

Development of PDL begins with root formation, prior to tooth eruption.

The dental follicle cells located between the alveolar bone & HERS are composed of two

subpopulations:

Mesenchymal cells of dental follicle proper

Perifollicular mesenchyme

Perifollicular mesenchymal cells bounded by mesenchymal cells of dental follicle proper.

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As the root formation continues, cells in the perifollicular region and follicle proper are gain their

polarity & the cellular volume & synthetic activity increases.

These cells obtain long & thin, elongated cytoplasm with increased amount of mitochondria,

RER & active Golgi complex.

As a result, these cells actively synthesize & deposits collagen fibrils & glycoprotein in the

developing periodontal ligament.

progenitors for periodontal ligament, osteoblast and cementoblast cells adopt a paravascular

location in the periodontal ligament, and these cells, which exhibit some features of stem cells,

can regenerate functional tissues when the need arises.

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Developmet of principal fibers

Immediately before tooth eruption & for sometimes there

after, active fibroblasts adjacent to the cementum of the

coronal 1/3 of the root, aligne in oblique direction to the

long axis of the tooth.

Soon, thereafter, first collagen fiber bundles of the

ligament become discernible. These are the precursors of

the alveolar crest fiber bundle group.

Later, similar fibers are observed on the adjacent osseous

surface of the developing alveolar bone.

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Both set of fibers, alveolar & cemental, continue to elongate toward each other, ultimately to meet,

intertwine & fuse, & cross linking of individual collagen molecules occur.

By the time of first occlusal contact of the tooth with its antagonist, the principle fibers around the

coronal 1/3 of the root, the horizontal group are almost completely developed

Oblique fibers in middle third of the root are still being formed.

After complete root apex is formed, apical group of fibers are developed.

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PDL homeostasis

A remarkable capacity of PDL is that it maintains its width more or less, despite the fact, it is

squeezed in between two hard tissues.

Various molecules have been proposed, which play a role in maintaining an unmineralized PDL.

Msx2

Bone sialoprotein

Matrix Gla proteins

(Inhibitors of mineralization)

• Inhibit mineralized bone tissue

• Prevents osteogeningdifferentiation of PDLfibroblasts by repressingcbfa1 activity

• osteopontin

Prostaglandins

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Synthetic cells

Fibroblasts

Osteoblasts

cementoblasts

Resorportivecells

Fibroblasts

Osteoclasts

cementoclasts

Progenitor cells

Undifferentiated stem cells

Epithelial cells

Epithelial cell rests of

malassez

Defense cells

Mast cells

Macrophages

eiosinophils

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Epithelial cell rests of malassez

Roles attributed to the Epithelial Rest of Malassez cells

range from bad to good.

Bad Role

Malassez cells are held responsible for the formation of

periodontal cysts and tumours as a result of peri-apical

inflammation associated with pulpal necrosis.

Epithelial Rest of Malassez cells contribute to the

formation of the periodontal pocket because of their

continuum with the junctional epithelium.

-Ohshima M, Nishiyama T, Tokunaga K, Sato S, Maeno M, Otsuka K.

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Good Role

The cells of the Epithelial Rest of Malassez may protect the root from resorption

- Wallace JA, Vergona K.

Epithelial cells Rest of Malassez secrete hyaluronic acid, which contributes to the formation

of the loose connective tissue characteristics of the periodontal ligament & react to

mechanical stress, like that associated with orthodontic tooth movement, by increasing their

proliferation rate and cell size. - Brunette DM & Merrilees MJ, Sodek J, Aubin JE

Epithelial Rest of Malassez - help in cementum repair because of their ability to activate

matrix proteins, such as amelogenin, which are also expressed during tooth development -

Hamamoto Y, Nakajima T, Ozawa H, Uchida T.

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The alveolar process is the portion of the maxilla & mandible that forms & supports the tooth

sockets.

It forms when the tooth erupts to provide the osseous attachment to the forming PDL & it

disappears gradually after the tooth is lost.

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Intramembranous ossification

Formation of bone matrix

Formation of woven bone

Appositional growth & formation of harvesian

system (osteon)

Endochondral bone formation

Formation of cartiagenous model

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Development of alveolar process

An alveolar bone in the strict sense of words develops only during the eruption of the teeth.

As the root & its covering of primary cementum form, new bone is deposited against the

crypt wall.

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Crystal form coalescing bone nodules with fast

growing, non oriented collagen fibers- is the

substructure of woven bone, first bone formed in the

alveolus.

Later, through bone deposition, remodelling &

secretion of oriented collagen fibers in sheets, mature

lamellar bone is formed.

Subsequently, a tissue may develop at alveolar crest

that combines characteristics of cartilage & bone.

It is called as chondroid bone.

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Structure of alveolar bone

(a thin lamella of the bone that surrounds the rootof the tooth & gives attachment to the

principle fibers of the PDL)

• Inner socket wall of thin compact bone

• Bundle bone

• Cribriform plate

• Lamina Dura (radiographically)

supporting alveolar bone

(the bone that surrounds the alveolar bone proper

& gives support to the socket)

• External plate of cortical bone

• Spongy bone/ cancellous trabeculae

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Bone remodeling

Bone is a highly dynamic connective tissue with continuous remodeling.

Process of bone formation & bone breakdown go on simultaneously, thus the bone represents

the net results of a balance between the two processes

This phenomena is called as coupling of bone resorption & bone formation.

The main function of the remodeling are to prevent the accumulation of damaged & fatigued

bone by regenerating new bone & to facilitate mineral homeostasis.

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Mediators of bone remodeling

Mechanical factors : when stress is applied on the alveolar bone, two sites are formed, bone is

resorbed at compression site & bone is deposited at tension site.

Parathyroid hormone

Vita. D metabolites

Growth factors

Bacterial products

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Markers of bone turnover

• Alkaline phosphatase

• osteocalcin

• Procollagen I extension peptide

• Urine calcium

• Urine hydroxyproline

• Collagen crosslink fragments

• Urine N-telopeptides

• Urine C-telopeptides

• Urine pyridinoline

• Urine free deoxypyridinoline

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Gingiva is an adaptation of oral epithelium in areas involved in mastication of food

The gingiva is a part of the oral mucosa that covers the alveolar processes of the jaws &

surrounds the neck of the teeth.- McCall

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Dentogingival junction

(junctional epithelium)

The epithelium of the gingiva which gets

attached to the tooth is called as junctional or

attachment epithelium.

It consists of collar like band of stratified

squamous non keratinizing epithelium, located at

CEJ in healthy tissue

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JE resembles reduced enamel epithelium (RER) in its structure in that they have a basal

layer & few layers of flattened cells & express CK 5, 14, 19, which is typical of

nondifferentiating tissue like RER.

Highest turnover rate of 5-6 days

JE is highly permeable & it has large intracellular spaces, so that neutrophils can easily pass

in & out of the epithelium.

Permits easy flow of GCF

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Development of dentoginval junction &

gingival sulcus

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Formation of JE & Gingival sulcus

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Shift of dentogingival junction

A. The actual movement of crown towards the occlusal plane is called as a active eruption

B. The separation of primary attachment epithelium from the enamel is termed as passive

eruption

crown exposure involving passive eruption & further recession has been described in

four stages

firsr two stages may be physiologic but last two are probably pathologic.

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Shift of dentogingival junction

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Reciprocal induction between oral ectoderm & mesenchymal cells derived from neural crest

cells form the major pathway for the development of periodontal tissues.

Various histochemical molecules favours the differentiation of fibroblasts, cementoblasts &

osteoblasts from the inner cells of the dental sac, which are also secreted at the time of

periodontal regeneration or repair by PDL

PDL contains both formative & resorptive cells for cementum, A.bone & PDL itself.

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Based on the information presented, it appears that the developed or adult

periodontium retains its potential for repair/regeneration in the form of

cells of the Epithelial Rest of Malassez, progenitor cells and stem cells,

which can be induced to differentiate into cementoblast, osteoblast or

periodontal ligament cells to regenerate periodontal tissues.

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References:

1. Textbook of Orban’s Oral histology & Embryology, 12th Ed.

2. Textbook of TenCate’s Oral histolgy & Embryology, 8th Ed.

3. Margarita zeichner-david, Regeneration of periodontal tissues: cementogenesis revisited,

Periodontology 2000, Vol. 41, 2006, 196–217.

4. A. Richard ten cate, The development of the periodontium - a largely ectomesenchymally derived

unit, Periodontology 2000, Vol. 13, 1997, 9-19.

5. Thomas HF, Kollar EJ. Differentiation of odontoblasts in grafted recombinants of murine epithelial

root sheath and dental mesenchyme. Arch Oral Biol 1989; 34: 27-35.

6. Textbook of clinical periodontology, F.A.Carranza, 10th Ed.

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