DEVELOPMENT OF TOOTH

Piyush vermaMDS 1st yearDept of Pedodontics & Preventive Dentistry

CONTENTS

1. Introduction2. Dental Lamina3. Vestibular Lamina4. Tooth development5. Developmental stages• Bud stage• Cap stage• Bell stage• Advanced bell stage6. Hertwig’s epithelial root sheath and root formation7. Conclusion8. References

INTRODUCTION

Tooth formation occurs in the 6th week of intrauterine life with the formation of primary epithelial band. At about 7th week the primary epithelial band divides into a lingual process called dental lamina & a buccal process called vestibular lamina. All deciduous teeth arises from dental lamina, later the permanent successors arise from its lingual extension & permanent molars from its distal extension

• The primitive oral cavity, or stomodeum, is lined by stratified squamous epithelium called the oral ectoderm

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

• Membrane ruptures at about 27th day of gestation and the primitive oral cavity establishes a connection with the foregut

• Most of the connective tissue cells underlying the oral ectoderm are of neural crest or ectomesenchyme in origin

• These cells instruct the overlying ectoderm to start the tooth development, which begins in the anterior portion of the future maxilla & mandible and proceeds posteriorly

3= Connective tissue cells

DENTAL LAMINA

• 2- 3 weeks after the rupture of buccopharyngeal membrane, certain areas of basal cells of oral ectoderm proliferate rapidly, leading to the formation of primary epithelial band

• The band invades the underlying ectomesenchyme along each of the horse-shoe shaped future dental arches.

• At about 7th week the primary epithelial band divides into an inner (lingual) process called Dental Lamina & an outer ( buccal) process called Vestibular Lamina

• The dental lamina serves as the primordium for the ectodermal portion of the deciduous teeth

• Later during the development of jaws, permanent molars arise directly from the distal extension of the dental lamina

• 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)

FATE OF DENTAL LAMINA

• It is evident that total activity of dental lamina exceeds over a period of atleast 5 yrs

• 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

• Fragmentation of the dental lamina progresses toward the developing enamel organ

• Any particular portion of the dental lamina functions for a much briefer period since only a relatively short time elapses after initiation of tooth development before the dental lamina begins to degenerate

• However the dental lamina may still be active in the third molar region after it has disappeared elsewhere, except for occasional epithelial remnants

CLINICAL CONSIDERATIONS

ANODONTIA

• Anodontia, also called anodontia vera, is a rare genetic disorder characterized by the congenital absence of all primary or permanent teeth

• It is of following types1. Complete anodontia/ total anodontia2. Partial anodontia/ sub-Total anodontia

• Forms-1. True anodontia2. Psuedo anodontia3. False anodontia

COMPLETE

PARTIAL

SUPERNUMERARY TEETH

Supernumerary teeth can be classified by shape and by position. The shapes include:• Supplemental(where the tooth has a normal shape for the teeth in that series);• Tuberculate (also called "barrel shaped");• Conical (also called "peg shaped");• Compound odontome (multiple small tooth-like forms);• Complex odontome (a disorganized mass of dental tissue)

Hyperdontia is the condition of having supernumerary teeth, or teeth which appear in addition to the regular number of teeth

When classified by position, a supernumerary tooth may be referred to as a mesiodens, a paramolar, or a distomolar.

VESTIBULAR LAMINA

• Labial and buccal to the dental lamina in each dental arch, another epithelial thickening develops independently • It is 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.

TOOTH DEVELOPMENT

• At certain points along the dental lamina each representing the location of one of the 10 mandibular & 10 maxillary teeth, ectodermal cells multiply rapidly & little knobs that grow into the underlying mesenchyme

• 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

• First to appear are those of anterior mandibular region

• As the cell proliferation occurs each enamel organ takes a shape that resembles a cap

DENTAL PAPILLA

On the inside of the cap, the ectomesenchymal cells increase in number. The tissue appears more dense than the surrounding mesenchyme and represents the beginning of the dental papilla

B = Dental Papilla

DENTAL SAC/ DENTAL FOLLICLE

Surrounding the combined enamel organ or dental papilla, the third part of the tooth bud forms. It is known as dental sac/follicle and it consists of ectomesenchymal cells and fibres that surrounds the dental papilla and the enamel organ.

C= Dental sac

• Thus the tooth germ consists of ectodermal component- the enamel organ, the ectomesenchymal components- the dental papilla & the dental follicle

• The enamel is formed from the enamel organ, the dentin and the pulp from the dental papilla and the supporting tissues namely the cementum, periodontal ligament & the alveolar bone from the dental follicle

• During & after these developments the shape of the enamel organ continues to change

• The depression occupied by the dental papilla deepens until the enamel organ assumes a shape resembling a bell

• The dental lamina becomes longer, thinner & finally loses its connection with the epithelium of the primitive oral cavity

DEVELOPMENTAL STAGES

MORPHOLOGICAL1. Dental lamina2. Bud stage3. Cap stage4. Early bell stage5. Advanced bell stage6. Formation of enamel and dentin matrix

PHYSIOLOGICALInitiation

ProliferationHistodifferentiationMorphodifferentiationApposition

BUD STAGE / PROLIFERATION

• 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

• 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 dental papilla as well as the dental sac are not well defined during the bud stage, they become more defined during the subsequent cap & bell stages

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

CAP STAGE / PROLIFERATION

• 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

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

STELLATE RETICULUM

• Polygonal 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 the polygonal cells become star shaped but maintain contact with each other by their cytoplasmic process

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

• 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

• The function of enamel knot & cord may 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

• The ectomesenchymal condensation i.e the dental papilla & the dental sac are pronounced during this stage of dental development

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

INNER ENAMEL EPITHELIUM

• The 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

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

• This layer seems to be essential to enamel formation

STELLATE RETICULUM

• The stellate reticulum expands further due to continued accumulation of intra-cellular fluid

• These 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 the outer & inner enamel epithelium

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 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

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

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

CLINICAL CONSIDERATIONS

DENTINOGENESIS IMPERFECTA

• Dentinogenesis imperfecta (hereditary Opalescent Dentin) is a genetic disorder of tooth development.

• This condition causes teeth to be discolored (most often a blue-gray or yellow-brown color) and translucent. Teeth are also weaker than normal, making them prone to rapid wear, breakage, and loss.

• These problems can affect both primary (baby) teeth and permanent teeth.

• This condition is inherited in an autosomal dominant pattern, which means one copy of the altered gene in each cell is sufficient to cause the disorder.

ADVANCED BELL STAGE / MORPHODIFFERENTIATION Characterized by the commencement of mineralization & root formation

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

Formation of dentin occurs first as a layer along the future dentinoenamel 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

The enamel formation then proceeds coronally & cervically in all the regions from the dentinoenamel 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

CLINICAL CONSIDERATIONS

HUTCHINSON’S INCISOR

MULBERRY MOLARS

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.

• However, fusion can also be the union of a normal tooth bud to a supernumerary tooth germ. In these cases, the number of teeth is fewer if the anomalous tooth is counted as one tooth.

GEMINATION

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

FORMATION OF ENAMEL & DENTIN MATIX ( 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

CLINICAL CONSIDERATIONS

ENAMEL HYPOPLASIA

Enamel hypoplasia is the defect of the teeth in which the tooth enamel is hard but thin and deficient in amount This is caused by defective enamel matrix formation with a deficiency in the cementing substance

AMELOGENESIS IMPERFECTA

• Amelogenesis imperfecta presents with abnormal formation of the enamel or external layer of teeth. Enamel is composed mostly of mineral, that is formed and regulated by the proteins in it. Amelogenesis imperfecta is due to the malfunction of the proteins in the enamel: ameloblastin, enamelin, tuftelin, amelogenin

• People afflicted with amelogenesis imperfecta have teeth with abnormal color: yellow, brown or grey. The teeth have a higher risk for dental cavities and are hypersensitive to temperature changes. This disorder can afflict many number of teeth.

Dens- In- Dente ( DENS INVAGINATUS)

• Represents a defect of tooth in which a focal area on the tooth surface is folded or invaginated pulpally to a variable extent

• Defect in generally localized to a single tooth & interestingly maxillary lateral incisors are more commonly affected

• Bilateral involvement is often seen & sometimes defect can involve multiple teeth involving the supernumeraries

• In case of pulp involvement with or without apical pathology, endodontic treatment should be attempted. However in more severe form extraction should be done

DENS EVAGINATUS

• Dens evaginatus is a condition found in teeth where the outer surface appears to form an extra bump or cusp.

• Premolars are more likely to be affected than any other tooth. This may be seen more frequently in Asians

• The pulp of the tooth may extend into the dens evaginatus.

• There is a risk of the dens evaginatus chipping off in normal function

• Hence this condition requires monitoring as the tooth can lose its blood and nerve supply as a result and may need root canal treatment.

TALON CUSP

• A talon cusp, also known as an "eagle's talon", is an extra cusp on an anterior tooth.

• Of all cases, 55% occur on the permanent maxillary lateral incisor, and 33% occur on the permanent maxillary central incisor. They are found rarely in primary teeth

• Whenever the lingual pits are present restorative treatments should be done to prevent caries

• When talon cusp interferes with normal occlusion preventive care should be taken by performing endodontic treatment

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

• 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 mallasez

• 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

• 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 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

• 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 into 2 or 3 openings

• 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

CLINICAL CONSIDERATIONS

DILACERATION

• Dilaceration refers to an angulation or a sharp bend or curve anywhere along the root portion of a tooth

• Condition probably occurs subsequent to trauma or any other defect of development which alters the angulation of the tooth germ during root formation

• Can easily be detected by radiographs

• Care should be taken during extraction since these teeth are more prone to fracture

CONCRESCENCE

Concrescence is a condition of teeth where the cementum overlying the roots of at least two teeth join together. The cause can sometimes be attributed to trauma or crowding of teeth.

Radiographic diagnosis is mandatory before attempting tooth extraction

CONCLUSION

Since development of tooth forms the base of dentistry, a thorough understanding and a sound knowledge is required by a dentist regarding the development stages of tooth & the anomalies related to it, so as to identify & treat them in a proper fashion.

REFERENCES

• Orban’s, Textbook of oral histology & embryology

• Ten cates, Textbook of oral histology

• Shaefer’s. Textbook of oral pathology

• Avery JK: Embryology of the tooth. J Dent Res 30:490,1951

• Avery JK: Primary induction of tooth formation. JDent Res 33:702,1954