DEVELOPMENT ANG GROWTH OF TEETH

Dr.bhavna tyagi

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IntroductionThe primitive oral cavity is lined by stratified

squamous epithelium called oral ectoderm or primitive oral epithelium.

The oral ectoderm contacts the endoderm of foregut to form the buccopharyngeal membrane.

At the 27th day of gestation this membrane ruptures and the primitive oral cavity establish a connection with the Foregut.

Connective tissue cells underlying the oral ectoderm instruct or induce the ectoderm to start tooth development.

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Diagram showing Buccopharyngeal membrane

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Primary epithelial band2 or 3 week after the rupture of

buccopharyngeal membrane.

Certain area of basal cells of oral ectoderm proliferate more rapidly.

Leads to formation of Primary epithelial band .(These band of epithelium are roughly horseshoe shaped structure. These correspond in position to the future dental arches)

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The position of the maxillary and mandibular primary epithelium band shown by shaded areas

Maxillary primary epithelial band

Mandibular primary epithelial band

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Dental laminaAt about 7th week of the primary epithelium band divides into Dental lamina Vestibular lamina(inner/lingual process) (outer/buccal process) The dental lamina serves as the primordium

for the ectodermal portion of the deciduous teeth .

Permanent molars arise directly from the distal extension of the dental lamina.

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The successors of the deciduous teeth develop from a lingual extension of the free end of the dental lamina opposite to the enamel organ of each deciduous teeth.

The lingual extension of the dental lamina is named the successional lamina & develops from the 5th month in utero ( permanent central incisor) to the 10th month of age (second premolar).

Diagram showing Dental lamina and vestibular lamina

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Fate of dental lamina It is evident that total activity of dental lamina

exceeds over a period of at least 5 yrs. The dental lamina may still be active in the third

molar region . As the teeth continue to develop, they loose their

connection with the dental lamina. They later break up by mesenchymal invasion, which is at first incomplete and does not perforate the total thickness of the lamina.

Remnant of dental lamina persist as Epithelial pearls or islands with in the jaw as well as in gingiva called as cell rest of serres.

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Epithelium rests of serres

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Degenerating dental lamina

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Vestibular lamina

Labial and buccal to the dental lamina in each dental arch, another epithelial thickening develops independently called as Vestibular Lamina also termed as lip furrow band.

Subsequently hollows and form the oral vestibule between the alveolar portion of the jaws and the lips and cheeks.

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Initiation of Tooth How dental development initiated?Mouse 1st arch epithelium + caudal or cranial neural

crest in the anterior chamber of eye =tooth formation.Epithelium from any other source like 2nd arch ,does

not elicit the response.After 12th day of development, 1starch epithelium loses

this odontogenic potential, assumed by Ectomesechyme.

Eg.late 1st arch ectomesenchyme +embryonic plantar epithelium =enamel organ.

Epithelial dental organ + skin mesenchyme =organ loses dental characteristics.

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What these experiments indicate?Odontogeneis is initiated first by factor

resident in the 1st arch epithelium influencing ectomesenchyme but that with time this potential is assumed by ectomesenchyme.

Intraocular recombination of neural crest and dental epithelium. A, Tooth formed from the combination of cranial neural crest and mandibular epithelium. B, Tooth formed from the combination of trunk neural crest and mandibular epithelium. (Courtesy A.G.S. Lumsden.)

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What are the signals mediating the initial steps in tooth development? The earliest mesenchymal markers for tooth

formation are the Lim- homeobox domain genes (transcription factors),Lhx-6 and Lhx-7.

Experimental data demonstrate that the expression of Lhx-6 and Lhx-7 results from a signaling molecule originating from the oral epithelium of the 1st branchial arch.

2nd arch mesenchyme recombinant with the 1st branchial arch of oral epithelium=Lhx-6,7 induced.

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If 1st branchial arch mesenchyme (which expresses Lhx-6 and Lhx-7) + 2nd branchial arch epithelium = expression of both genes down regulated.

A prime candidate for the induction of Lhx genes is Fgf-8(fibroblast growth factor).

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What controls the position and number of tooth germs along the oral surface?The Pax-9(paired box homeotic gene) gene is

one of the earliest mesenchymal gene that define the localization of tooth germ.

Pax -9 gene co - localizes with the exact sites where tooth germs appear.

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Tooth Type Determination

The determination of specific tooth type at their correct positions in the jaw is referred as patterning of the dentition.

Two hypothetical models have been proposed

Field theory Clone theory

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Field model

Proposes that the factors responsible for tooth shape reside with in the ectomesenchyme.

The fact that each of the fields expresses differing combinations of patterning homeobox genes supports this theory.

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The homeobox genes Msx-1, Msx-2, Dlx-2, and Barx-1 expressed in mandibular mouse ectomesenchyme associated with differing tooth families. Because the mouse does not develop a canine, the code given is speculative, but the overlap is present in the presumptive canine region. (Information derived from Thomas BL, Sharpe PT: Patterning of the murine dentition by homeobox genes, Eur J Oral Sci 106[suppl 1]:48, 1998.)

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Oodontogenic homeobox code model of dental patterning. A, Domains of Barx-1 and Dlx-1/-2 expression overlap in the mesenchyme of the presumptive molar region, whereas

domains of Msx-1, Msx-2, and Alx-3 overlap in presumptive incisor mesenchyme. B, Mouse dental pattern. Incisors deriving from Msx-1/Alx-3 expressing cells; molars

deriving from Barx-1/Dlx-1/-2 expressing cells. C, Human dental pattern. Premolars and canines can be derived from the same odontogenic code as that observed in mice by

virtue of the overlapping domains of gene expression. Thus canines and premolars may be derived from cells expressing DLX-1/-2 and MSX-1, for example. (From McCollum MA, Sharpe PT: Developmental genetics and early hominid craniodental evolution, Bioessays

23:481, 2001.)

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Clone modelProposes that each tooth class is derived from a

clone of ectomesenchymal cell programmed by epithelium to produce teeth of a given pattern.

Clone theory. A, The molar clone has induced the dental lamina to begin tooth development. At its posterior border the clone and dental lamina grow posteriorly by means of the progress zone. B, When a clone reaches the critical size, a tooth bud is initiated at its center. A zone of inhibition surrounds the tooth bud, and the next tooth bud, C, is not initiated until the progress zone of the clone has escaped its influence. (From Osborn JW,Ten Cate AR: Advanced dental histology, ed 3, Oxford, UK, 1983. Reprinted by permission of Elsevier Ltd.)

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Tooth developmentIntroductionTooth formation is a continuous process.Characterized by series of stages. Each tooth develops through successive bud,

cap and bell stages.During these stages, the tooth germs grow

and develop into specialized cell which form enamel, dentin, cementum.

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Ectodermal cells multiply rapidly & little knobs grow into the underlying mesenchyme.

Representing the location of one of the 10 mandibular and 10 maxillary deciduous teeth.

Each of these little down growths from the dental lamina represents the beginning of the enamel organ of the tooth bud of a deciduous tooth.

NOTE: Not all of these enamel organs start to develop at the same time, First to appear are those of anterior mandibular region.

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Developmental stages

Bud stageCap stageBell stageRoot formation

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Stages in tooth growth

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Bud stage / Proliferation stage

This is the initial stage of tooth formation where enamel organ resembles a small bud.

During the bud stage, the enamel organ consists of peripherally located low columnar cells & centrally located polygonal cells.

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Bud stage

a

b

c

d

e

f

g

a.Oral ectoderm b.Dental laminac.Enamel organ d.central polyhedral cell e.Peripheral columnar cellf.Condensation of ectomesenchymeg.Developing bone

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The surrounding mesenchymal cells proliferate, which results in their condensation in two areas.

The area of condensation immediately below the enamel organ is the dental papilla.

The ectomesenchymal condensation that surrounds the tooth bud & the dental papilla is the tooth sac.

The cells of the dental papilla form the dentin and pulp while the dental sac forms cementum & periodontal ligament.

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Bud to Cap Transition

The transition from bud to cap marks the onset of morphologic differences between tooth germs that give rise to different type of teeth.

Msx-1 expressed with Bmp-4 in mesenchymal cell that condense around tooth buds.

Pax-9 is also expressed in bud stage mesenchyme.Msx-1-/- and Pax-9 -/- embryos have tooth

development arrested at the bud stage.

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Cap stage / proliferation

a

b

cde

f

g

a.Oral ectoderm b.Dental laminac.Outer enamel epitheliumd.Dental folliclee.Stellate reticulumf.Inner enamel epetheliumg.Dental papilla

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As the tooth bud continues to proliferate, it does not expand uniformly into a large sphere.

Instead unequal growth in different parts of the tooth bud leads to the cap stage which is characterized by a shallow invagination on the deep surface of the bud

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OUTER & INNER ENAMEL EPITHELIUM

The peripheral cells of the cap stage are cuboidal cover the convexity of the cap &are called the outer enamel epithelium.

The cells in the concavity of the cap become tall columnar cells & represent the inner enamel epithelium.

The outer enamel epithelium is separated from the dental sac, & the inner enamel epithelium from the dental papilla, by a delicate basement membrane

Hemidesmosomes anchor the cells to the basal lamina

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Stellate reticulumPolygonal cells located between the outer and

the inner enamel epithelium, begin to separate due to water being drawn into the enamel organ from the surrounding dental papilla as a result of osmotic force exerted by glycosaminoglycan contained in the ground substance

As a result the polygonal cells become star shaped but maintain contact with each other by their cytoplasmic process

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As the star shaped cells form a cellular network, they are called the stellate reticulum

This gives the stellate reticulum a cushion like consistency

Acts as a shock absorber that may support and protect the delicate enamel forming cells

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Dental PapillaUnder the influence of proliferating epithelium of

enamel organ, the ectomesenchyme that is partially enclosed by the invaginated portion of inner enamel epithelium proliferates.it condense to form Dental Papilla.

It is the formative organ of the dentin and the primordium of the pulp.

The dental papilla shows active budding of capillaries and mitotic figures.

Peripheral cells adjacent to IEE enlarge and later differentiate into the odontoblasts.

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Dental Sac

Marginal condensation in the ectomesenchyme surrounding the enamel organ and the dental papilla called as Dental sac.

Diagram showing: Dental follicleDental papilla Enamel organ

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Temporary structures Enamel knotEnamel cordEnamel septumEnamel niche

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The cells in the center of the enamel organ are densely packed and form the enamel knot.

This knot projects toward the underlying dental papilla.

At the same time a vertical extension of the enamel knot , called the enamel cord occurs.

When the enamel cord extends to meet the OEE called as enamel septum.

Fgf-4 and slit-1 may be the best molecular markers for enamel knot formation

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Diagram showing enamel knot

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Enamel cord or septum

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function of enamel knot & cordMay act as a reservoir of the dividing cells for

the growing enamel organ.The enamel knot act as a signaling centers as

many important growth factors are expressed by the cells of the enamel knot & thus play an important role in determining the shape of the tooth.

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Enamel niche

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Bell stage/HISTODIFFERENTIATION Due to continued uneven growth of the enamel organ

it acquires a bell shape.In bell stage crown shape is determined. It was thought that the shape of the crown is due to

pressure exerted by the growing dental papilla cells on the inner enamel epithelium.

This pressure however was shown to be opposed equally by the pressure exerted by fluid present in the stellate reticulum.

The folding of enamel organ to cause different crown shapes is shown to be due to different rates of mitosis & difference in cell differentiation time

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Bell stage

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INNER ENAMEL EPITHELIUM

consists of a single layer of cells that differentiate prior to amelogenesis into tall columnar cells called ameloblasts.

These elongated cells are attached to one another by junctional complexes laterally &to cells in the stratum intermedium by desmosomes

The cells of the inner enamel epithelium exert a strong influence on the underlying mesenchymal cells of the dental papilla, which later differentiate into odontoblasts

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STRATUM INTERMEDIUM A few layers of squamous cells form the stratum

intermedium , between the inner enamel epithelium & the stellate reticulum.

These cells are closely attached by desmosomes & gap junctions.

The well developed cytoplasmic organelles, acid mucopolysaccharides ,and glcogen deposites indicate high degree of metabolic activity.

This layer seems to be essential to enamel formation.The cells of this layer soon are characterised by an

exceptionally high activity of the enzyme alkaline phosphatase.

Absent in part of tooth germ that outlines root portion.

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STELLATE RETICULUMThe stellate reticulum expands further due to

continued accumulation of intra-cellular fluid.The star shaped cells, having a large processes

anastomose with those of adjacent cells.As the enamel formation starts., the Stellate

reticulum collapses to a narrow zone thereby reducing the distance between centrally placed ameloblast and the nutrient capillaries near the outer enamel epithelium

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OUTER ENAMEL EPITHELIUM

The cells of the outer enamel epithelium flatten to form low cuboidal cells

The outer enamel epithelium is thrown into folds which are rich in capillary network, this provides a source of nutrition for the intense metabolical active of the avascular enamel organ

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DENTAL LAMINA

Dental lamina is seem to extend lingually and is termed successional dental lamina as it gives rise to enamel organs of permanent successors of deciduous teeth

The enamel organs of deciduous teeth in the bell stage show successional lamina &their permanent successor teeth in the bud stage

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Diagram showing Permanent tooth bud

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DENTAL PAPILLA Dental papilla is enclosed in the invaginated portion

of the enamel organ Before the inner enamel epithelium begins to

produce enamel. Peripheral cells of the dental papilla differentiate into odontoblasts

These cuboidal cells later assumes a columnar form & produce dentin

The basement membrane that separates the enamel organ ant the dental papilla just prior to dentin formation is called the MEMBRANA PERFORMATIVA

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DENTAL SAC The dental sac exhibits a circular arrangement

of fibres & resembles a capsule around the enamel organ.

The fibres of the dental sac form the periodontal ligament fibres that span between the root & the bone.

The junction between the inner enamel epithelium & odontoblasts outlines the future dentino – enamel junction.

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Two Important Events Occur During BELL STAGE

The dental lamina joining the tooth germ to the oral epithelium breaks up into discrete islands of epithelial cells , thus separating the developing tooth from the oral epithelium.

The IEE completes its folding ,making it possible to recognize the shape of the future crown pattern of the tooth.

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Advanced Bell stage / MORPHODIFFERENTIATION

Characterized by the commencement of mineralization & root formation.

The boundary between the inner enamel epithelium & odontoblasts outline the future dentino enamel junction

Formation of dentin occurs first as a layer along the future dentino enamel junction in the region of future cusps &proceeds pulpally & apically

After the first layer of dentin is formed , the ameloblasts lay down enamel over the dentin in the future incisal & cuspal areas

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Advanced bell stage

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The enamel formation then proceeds coronally & cervically in all the regions from the dentino enamel junction toward the surface

The cervical portion of enamel organ gives rise to Hertwig Epithelial Root Sheath (HERS)

This HERS outlines the future root &thus responsible for the size, shape ,length & number of roots

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Collapsed stellate reticulum

Ameloblast Enamel

Dentin OEE

Dental sac

odontoblast

Developing pulp

Advanced bell stage

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FORMATION OF ENAMEL & DENTIN MATRIX ( APPOSITION)

Apposition is the deposition of the matrix of the hard enamel structures

Appositional growth of the enamel & dentin is a layer like deposition of an extracellular matrix.

This type of growth is therefore additive Appositional growth is characterised by regular &

rhythmic deposition of the extracellular matrix, which is of itself incapable of further growth

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ROOT FORMATION

The development of roots begin after enamel & dentin formation has reached the future cementoenamel junction.

The enamel organ plays an important role in root development by forming HERS, which models the shape of the root.

HERS consists of outer & inner enamel epithelium only (cervical loop /zone of reflexion)

The cell of the IEE remains short and normally do not produce enamel.

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Hertwig’s root epithelium

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As the first layer of the dentin has been laid down, the epithelial root sheath loses its structural continuity and is close relation to the surface of the root.

Its remnants persists as an epithelial network of strands or clumps near the external surface of the root.

These epithelial remnants are found in the periodontal ligament of erupted teeth and are called as rests of malasez

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Prior to the beginning of root formation , the root sheath forms the epithelial diaphragm.

The outer & the inner enamel epithelium bend at the future cementoenamel junction into a horizontal plane, narrowing the wide cervical opening.

The proliferation of the cells of the epithelial diaphragm is accompanied by the proliferation of the cells of the connective tissues of the pulp, adjacent to the diaphragm.

The free end of diaphragm does not grow into the connective tissue but the epithelium proliferates coronal to the epithelial diaphragm

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The differentiation of odontoblast and the formation of dentin follow the lengthening of root sheath.

Connective tissue of the dental sac surrounding the root sheath proliferates& invades the continuous double epithelial layer dividing it into network of epithelial strands.

The epithelium is moved away from the surface of the dentin so the connective tissue come in contact with the outer surface of the dentin and differntiate into cementoblast that deposit layer of cememtum over dentin.

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Root formation

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The rapid sequence of proliferation &destruction of Hertwig’s root sheath explains the fact that it cannot be seen as a continuous layer on the surface of developing root.

In the last stages of the root development, the proliferation of the epithelium in the diaphragm lags behind that of the pulpal connective tissue.

The wide apical foramen is reduced first to the width of the diaphragmatic opening itself & later is further narrowed by opposition of dentin & cementum to the apex of the root

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Fragmentation of the root sheath and the initial formation of cementum. Follicular cells migrate through a break in the epithelium (arrow) to lie against the surface of newly

formed dentin.

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Differential growth of the epithelial diaphragm in the multirooted teeth causes the division of root trunk into 2 or 3 roots.

During the general growth of enamel organ, expansion of its cervical opening occurs in such a way that long tongue like extensions of the horizontal diaphragm develop.

Before division of the root trunk occurs ,free ends of the horizontal epithelial flaps grow towards each other & fuse. The single cervical opening is divided into2 or 3 openings.

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Root formation as seen on the undersurfaces of developing tooth germs: A, two-rooted tooth, and B, three-rooted tooth. C, Section of a tooth with developing root. The roots have not finished forming, and the division into two roots is clearly visible. (Redrawn from Oöe T: Human tooth and dental arch development, Tokyo, 1981, Ishiyaka.)

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On the pulpal surface of the dividing epithelial bridges, dentin formation starts.

On the periphery of each opening, root development follows in the same way as described for single rooted teeth.

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Clinical considerations

A lack of initiation results in the absence of either a single tooth or multiple teeth (partial anodontia),most frequently the permanent upper lateral incisors , 3rd molars and lower 2nd premolar.

May be a complete lack of teethAbnormal initiation may result in the

development of single/multiple supernumerary teeth

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In vit A deficiency the ameloblast fails to differentiate properly , so osteodentin is formed.

Hypopituitarism and Hypothyroidism results in a small clinical crown.

Disturbances in morphodifferentiation may affect the form and size of the tooth without impairing the function of the ameloblasts or odontoblast.

FUSION The phenomenon of tooth fusion arises through union of two normally separated tooth germs, and depending upon the stage of development of the teeth at the time of union, it may be either complete or incomplete.

GEMINATION arises when two teeth develop from one tooth bud and, as a result, the patient has an extra tooth

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Congenital syphilis : peg shapes teeth with the permanent central incisor showing a notched incisal edge condition is known as Hutchinson’s incisor.

Genetic and environmental factors may disturb the normal synthesis and secretion of the organic matrix of enamel leading to a condition called Enamel hypoplasia

Organic matrix is normal but its mineralization is defective, then the dentin or enamel is said to be hypocalcified or hypomineralized.

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References

• Orban’s, Textbook of oral histology & embryology.

• Ten cates, Textbook of oral histology.• James k. avery,oral development and

histology.• Mazi jose, Mannual of oral histology and oral

pathology