DEVELOPMENT OF TEETH

By Shilpa Maria1st yr pgdepartment of oral and maxillofacial pathology

Contents

Introduction Formation of epithelial band Vestibular lamina Dental lamina Bud stage Cap stage Bell stage Hertwigs epithelial root sheath Transitory structures Histophysiology Clinical considerations

introduction

The primitive oral cavity is called the stomatodeum

It is lined by stratified squamous epithelium

Also called oral ectoderm or primitive oral epithelium

oral ectoderm contacts the endoderm of the foregut to form -buccopharyngeal membrane

At about the 27th day of gestation this membrane ruptures

Primitive oral cavity establishes a connection with the foregut

Most of the connective tissue cells underlying the oral ectoderm are of neural crest origin

These cells are thought to instruct or induce the overlying ectoderm to start tooth development

FORMATION OF PRIMARY EPITHELIAL BAND

1st histological sign of tooth development is the appearance of a condensation of mesenchymal tissue and capillary networks beneath the presumptive dental epithelium of the primitive oral cavity

By the 6th week of development the oral epithelium thickens and invaginates into the mesenchyme to form a PRIMARY EPITHELIAL BAND

Primary epithelial band

By the 7th week the primary epithelial band divides into 2 processes

1) VESTIBULAR LAMINA 2) DENTAL LAMINA

Vestibular lamina is buccaly located Dental lamina is lingually located

Vestibular lamina

contributes to the development of the vestibule of the mouth

Delineates the lips and cheeks from the tooth bearing regions

To form the vestibule of the oral cavity the cells of the vestibular lamina proliferate

subsequently there is degeneration of the central epithelial cells to produce the sulcus of the vestibule

Dental lamina contributes to the development of the teeth Further development of the dental lamina is

characterized by an increase in the length By the 8th week a series of swellings

develop on the deep surface of the dental lamina

Each epithelial swelling is almost completely surrounded by a mesenchymal condensation

Epithelial swellings indicate early developing tooth germs

For descriptive purposes tooth germs are classified into

Bud stage Cap stage Bell stage According to the degree of

morphodiffrentiation and histodifferentiation of their epithelial components

Bud stage

enamel organ in the bud stage appears as a simple spherical to ovoid epithelial condensation

Poorly morphodifferentiated and histodifferentiated

surrounded by mesenchyme Successful development of the tooth

germ relies upon a complex interaction of the mesenchymal and epithelial components

Bud stage

Cap stage

By the 11th week morphogenesis has progressed

Deeper surface of the enamel organ invaginates to form a cap shaped structure

greater distinction develops between the more rounded cells in the central portion of the enamel organ and the peripheral cells which become arranged to form the external and internal enamel epithelia

Cap stage

Late cap stage

By the 12th week the central cells of the enlarging enamel organ become separated

intercellular spaces contain significant quantity of glycosaminoglycans

The resulting tissue formed is the STELLATE RETICULUM

cells of the external enamel epithelium remain cuboidal

internal enamel epithelium becomes columnar

The part of the mesenchyme lying beneath the internal enamel epithelium is termed the DENTAL PAPILLA

The mesenchyme surrounding the tooth germ becomes the DENTAL FOLLICLE

Early bell stage

By the 14th week further morphodifferentiation

and histodifferentiation leads to the early bell stage

configuration of the internal enamel epithelium broadly maps out the occlusal pattern of the crown of the tooth

This folding is related to differential mitosis along the internal enamel epithelium

Bell stage

During the bell stage bone resorption defects that restrict the space for development of the tooth germ are associated with the increased folding pattern of the internal enamel epithelium leading to changes in tooth shape

dental lamina breaks down and the enamel organ loses connection with the oral epithelium

At the same time the dental lamina between the tooth germs also degenerates

Remnants of dental lamina are called CELL RESTS OF SERRE

Interposed between the enamel organ and the wall of the developing bony crypt is the mesenchymal tissue of the dental follicle

It has 3 layers: Inner investing layer Outer layer Between the 2 layers is lose

connective tissue with no marked concentration of blood vessels

The enamel organ shows four distinct layers

External enamel epithelium Stellate reticulum Stratum intermedium Internal enamel epithelium

External enamel epithelium This forms the outer layer of cuboidal

cells which limits the enamel organ separated from the surrounding

mesenchymal tissue by a basement membrane

cells of the external enamel epithelia contains large centrally placed nuclei

Contact each other via desmosomes and gap junctions

It is involved in the maintenance of the shape of the enamel organ

Exchange of substances between the enamel organ and the environment

The cervical loop at which there is considerable mitotic activity lies at the growing margin of the enamel organ

Here the external enamel epithelium is continuous with the internal enamel epithelium

Stellate reticulum

Most fully developed at the bell stage Intercellular spaces become fluid filled Cells are star shaped Conspicuous nuclei and many branching

processes

The cell contains: glycosaminoglycans Alkaline phosphatase Small amounts of RNA glycogen

Collagens type I II III are expressed in stellate reticulum

Possess few mitochondria minimal endoplasmic reticulum Well developed Golgi complex Microvilli are present on the cell

surface Numerous tonofilaments present in

cytoplasm Desmosomes and gap junctions

present between the cells

Functions: protection of the underlying dental

tissue against physical disturbance

maintenance of the tooth shape

Contribute to the secretion of extracellular material

Stratum intermedium

First appears at the bell stage

Consists of 2 or 3 layers of flattened cells

lying over the internal enamel epithelium

Resemble the cells of the stellate reticulum

Intercellular spaces are much smaller

Functions :

Concerned with the synthesis of proteins

Transport of materials to and from the enamel forming cells in the internal enamel epithelium

Internal enamel epithelium The cells are columnar at the bell stage At the regions associated with the

future cusp tips the cells become elongated

Cells are rich in RNA

Do not contain alkaline phosphatase

Desmosomes connect the internal enamel epithelium cells and link this layer to the stratum intermedium

The internal enamel epithelium is separated from the peripheral cells of the dental papilla by a basement membrane and a cell free zone 1-2 micrometer wide

Late bell stage

Associated with the formation of dental hard tissues

Commences at about 18th week Dentin formation always precedes

enamel formation Down growths of the external

enamel epithelia appear from the lingual sides of the enamel organ

The boundry between the inner enamel epithelium and odontoblasts outlines the future dentinoenamel junction

The formation of dentin occurs first as a

layer along the future dentinoenamel junction in the region of future cusps and proceeds pulpally and apically

After the first layer of dentin is formed

the ameloblast which has differentiated from the inner enamel epithelial cells lay down enamel over the dentin in the future incisal and cuspal areas

Enamel formation then proceeds coronally and cervicaly in all regions from the dentinoenamel junction towards the tooth surface

The cervical portion of the enamel organ gives rise to the epithelial root sheath of hertwigs

Hertwigs epithelial root sheath The development of the roots begins

after enamel and dentin formation has reached the future cementoenamel junction

Hertwigs root sheath consists of outer and inner epithelia only

Does not include stratum intermedium and stellate reticulum

Cells of the inner layer remain short and donot produce enamel

Once the first layer of dentin is laid down , the epithelial root sheath loses its structural continuity and its close relationship with the surface of the root

Remnants are found in the periodontal ligament of erupted teeth – rests of malassez

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

The outer and inner enamel epithelial bend at the future cementoenamel junction into a horizontal plane

This narrows the wide cervical opening of the tooth germ

The proliferation of the cells of the epithelial diaphragm is accompanied by proliferation of cells of the connective tissue of the pulp

The free end of the diaphragm does not grow into the connective tissue

Epithelium proliferates coronal to the epithelial diaphragm

The differentiation of odontoblasts and the formation of dentin follow the lengthening of the root sheath

At the same time the connective tissue of the dental sac surrounding the root sheath proliferates and invades the continuous double epithelial layer dividing it into a network of epithelial strands

The epithelium is moved away from the surface of the dentin so that the connective tissue cells come into contact with the outer surface of dentin and differentiate into cementoblasts that deposit a layer of cementum onto the surface of the dentin

The rapid sequence of proliferation and destruction of hertwigs root sheath explains the fact that it cannot be seen as a continuous layer on the surface of the developing root

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

Wide apical foramen is reduced first to the width of the diaphragmatic opening itself

Later narrowed by apposition of dentin and cementum to the apex of the root

Transitory structures

During the early stages of tooth development 3 transitory structures maybe seen

Enamel knot Enamel cord Enamel niche

Enamel knot

A localized mass of cells in the center of the internal enamel epithelium

The enamel knot forms a bulge into the dental papilla at the center of the enamel organ

It soon disappears and seems to contribute to the enamel cord

Disappearance of the enamel knot by the bell stage maybe associated with apoptosis

Enamel knot

Enamel cord

A strand of cells seen at the early bell stage

Extends from the stratum intermedium to the stellate reticulum

When present the enamel cord overlies the incisal margin of a tooth or the apex of the 1st cusp to develop

When it completely divides the stellate reticulum into 2 parts , reaching the external enamel epithelium – it is termed the enamel septum

Where the enamel cord meets the external enamel epithelium a small invagination termed the enamel naval maybe seen

Cells of the enamel cord are distinguished from their surrounding stellate reticulum cells by their elongated nuclei

It has been suggested that the enamel cord maybe involved in the process by which cap stage is transformed into the bell stage

Or that it is a focus for the origin of stellate reticulum cells

Enamel niche

It is seen where the tooth germ appears to have a double attachment to the dental lamina [lateral and medial enamel strands]

These strands enclose the enamel niche It appears as a funnel shaped depression

containing connective tissue

Functional significance is unknown

Nerve fibres

Present in the immediate vicinity of presumptive dental epithelium at the very earliest stage of tooth induction and subsequently form a plexus below the dental papilla at the cap stage

From such plexus the nerves spread into the dental follicle as it develops

Penetration of nerves into the dental papilla occurs with the onset of dentinogenesis

The nerve fibres associate with blood vessels are presumed to be autonomic

Others lying free within the papilla are presumed to be sensory

The innervation of the dental papilla remains rudimentary until after birth

Maybe fully developed only after the tooth has erupted

Blood supply

Small blood vessels invade the dental papilla at the early bell stage

They are also evident in the dental follicle in close association with the external enamel epithelium

Histophysiology

Stages in tooth growth

Initiation Proliferation Histodifferentiation Morphodifferentiation Apposition

INITIATION: Sites of the future teeth are established

with the appearance of tooth germs along an invagination of the oral epithelium called the dental lamina

MORPHOGENESIS: The shape of the tooth is determined by

a combination of cell proliferation and cell movement

HISTOGENESIS: Differentiation of cells started during

morphogenesis proceeds to give rise to fully formed dental tissues

initiation

The dental lamina and associated tooth buds represent those parts of the oral epithelium that have the potential for tooth formation

Specific cells within the dental lamina have the potential to form the enamel organ of certain teeth

Different teeth are initiated at different times

Initiation induction requires ectomesenchymal-epithelial interaction

It has been demonstrated that dental papilla mesenchyme can induce or instruct tooth epithelium and even non tooth epithelium to form enamel

Proliferation

Enhanced proliferative activity ensues at the points of initiation and results successively in the bud cap and bell stages

Proliferative growth causes regular

changes in the size and proportion of the growing tooth germs

Histodiffereniation

Succeeds the proliferative stage The formative cells of the tooth

germs developing during the proliferative stage undergo definite morphologic as well as functional changes and acquire their functional assignment

The cells become restricted in their functions

They differentiate and give up their capacity to multiply as they assume their new function

This phase reaches its highest development in the bell stage of the enamel organ

Organising influence of the inner enamel epithelium on the mesenchyme is evident in the bell stage

Leads to differentiation of the adjacent cells of the dental papilla into odontoblasts

With the formation of dentin the cells of the inner enamel epithelium differentiate into ameloblasts

And enamel matrix is formed opposite the dentin

Dentin formation preceeds and is essential to enamel formation

Morphodifferentiation

Basic form and relative size of the future tooth is estalished by morphodifferentiation

Morphodifferentiation is impossible without proliferation

Advanced bell stage marks not only active histodifferentiation but also an important stage of morphodifferentiation in the crown, outlining the future dentinoenamel junction

Apposition

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

Appositional growth of enamel and dentin is a layer like deposition of an extracellular matrix

It is characterized by regular and rhythmic deposition of the extracellular matrix

Periods of activity and rest alternate at definite intervals during tooth formation

Clinical considerations

Disturbances of epithelial and mesenchymal interaction can disturb tooth development

Splitting of tooth germ or joining of adjacent germs can be responsible for variations in tooth number and shape

Trauma and infection of the deciduous predecessors has been implicated in the malformation of permanent teeth

Malformations of teeth can be

Variations in size shape number structure

Variations in size include : Macrodontia

Microdontia

Variations in shape include: Peg shaped laterals

Gemination: Partial cleavage of tooth germ

Fusion:Union of two adjacent tooth germs

Concresence:Fusion of teeth at the roots

Abnormal tooth shapes: Hutchinson's incisors Mulberry molars

Odontomes: Are irregular masses or a large

number of irregular denticles found in place of a developing tooth

Hypodontia: Reduction in the number of teeth

Anodontia: Complete absence of teeth

Hyperdontia: Increase in tooth number

REFERENCES

Ten Cates oral histology, development, structure and function – 7th edition

Oral anatomy histology embryology – 3rd edition -Berkovitz , Holland , Moxham

Orbans oral histology and embryology-12th edition

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