DEVELOPMENT OF TOOTH

Dr. Muznah SultanLecturerDepartment of Oral Biology, DIDC-DUHS

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 ofintrauterine life with the formation of primaryepithelial band.

•At about 7th week the primary epithelial banddivides into a lingual process called dental lamina &a buccal process called vestibular lamina.

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

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 developingenamel organ

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

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 ectodermalcomponent- the enamel organ, theectomesenchymal components- the dentalpapilla & the dental follicle

• The enamel is formed from the enamel organ,the dentin and the pulp from the dental papillaand the supporting tissues namely thecementum, periodontal ligament & the alveolarbone from the dental follicle

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

• The depression occupied by the dental papilladeepens until the enamel organ assumes a shaperesembling a bell

• The dental lamina becomes longer, thinner &finally loses its connection with the epitheliumof 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 formationwhere enamel organ resembles a small bud

• During the bud stage, the enamel organconsists of peripherally located lowcolumnar cells & centrally locatedpolygonal cells

• The surrounding mesenchymal cellsproliferate, which results in theircondensation in two areas

• The area of condensation immediatelybelow the enamel organ is the dentalpapilla

• The ectomesenchymal condensation thatsurrounds the tooth bud & the dentalpapilla is the tooth sac

• The dental papilla as well as the dental sac are not well defined during the budstage, 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 formscementum & periodontal ligament

CAP STAGE / PROLIFERATION

• As the tooth bud continues to proliferate, it does not expand uniformly intoa large sphere

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

OUTER & INNER ENAMEL EPITHELIUM

• The peripheral cells of the cap stage arecuboidal , cover the convexity of the cap& are called the outer enamel epithelium

• The cells in the concavity of the capbecome tall columnar cells & representthe 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 stellatereticulum

• 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 ectomesenchymalcondensation 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 fibresthat span between the root & the bone

• The junction between the inner enamel epithelium & odontoblasts outlines the future dentino-enamel junction

ADVANCED BELL STAGE/MORPHODIFFERENTIATION

Characterized by the commencement of mineralization & root formation

The boundary between the inner enamel epithelium & odontoblastsoutline the future dentinoenameljunction

Formation of dentin occurs first as a layer along the future dentinoenameljunction 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 proceedscoronally & cervically in all the regionsfrom the dentinoenamel junctiontoward the surface

The cervical portion of enamel organgives rise to Hertwig Epithelial RootSheath (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

ROOT FORMATION

• The development of roots begin afterenamel & dentin formation has reachedthe future cementoenamel junction

• The enamel organ plays an importantrole in root development by formingHERS, which models the shape of theroot

• HERS consists of outer & inner enamelepithelium only

• As the first layer of the dentin has beenlaid down, the epithelial root sheathloses its structural continuity and isclose relation to the surface of the root

•Its remnants persists as anepithelial network of strands orclumps near the external surfaceof the root

• These epithelial remnants arefound in the periodontalligament of erupted teeth and arecalled as rests of mallasez

• Prior to the beginning of root formation,the root sheath forms the epithelialdiaphragm

• The outer & the inner enamel epitheliumbend at the future cementoenameljunction into a horizontal plane,narrowing the wide cervical opening

• The proliferation of the cells of theepithelial diaphragm is accompanied bythe proliferation of the cells of theconnective tissues of the pulp, adjacent tothe diaphragm

• The free end of diaphragm does notgrow into the connective tissue but theepithelium proliferates coronal to theepithelial 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

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 angulationof 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