Tooth development

Enamel knot

A localised mass of cells in the centre of the inner enamel epithelium

Bulges into the dental papilla

Represents an important signalling centre during tooth development

Disappears in the bell stage

What is epithelium derived from?

Ectoderm

What is ecto-mesenchyme derived from?

Neural crest cells

Mesenchyme

A connective tissue found in organisms during development (gives rise to most body connective tissues)

Derived from mesoderm

What does the tooth germ consist of?

Enamel organ, dental follicle, dental papilla

What is the enamel organ derived from?

Ectoderm

What is the dental follicle and dental papilla derived from?

Ectomesenchyme

What does the enamel organ turn into?

Enamel

What does the dental follicle turn into?

Cementum, periodontal ligament, alveolar bone

What does Dental papilla turn into?

Dentine, pulp

what happens in the 6th week of intra uterine life?

Oral epithelium proliferates and forms primary epithelial band

What happens in the 7th week of intra uterine life?

Primary epithelial band divides into two processes: dental lamina and vestibular lamina

Formation of dental lamina signals the beginning of tooth development

What occurs in the bud stage?

Initial stage of definitive tooth development

Localised proliferation of the epithelial cells of the dental lamina results in the formation of bud shaped enamel organ

Proliferation of ectomesenchymal cells surround the bud

What occurs in the cap stage?

The deeper surface of the enamel organ invaginate to form a cap shaped structure

The dental ectomesenchyme that surrounds the enamel organ is called dental papilla (embryonic dental pulp)

In the 11th week of intra uterine life

What occurs in the late cap stage?

12th week of intra uterine life

Central cells of the enamel organ becomes stellate reticulum

Inner cells become inner enamel epithelium

Outer cells become outer enamel epithelium

What occurs in the bell stage?

In the 14th week of intra uterine life further development leads to the bell stage or differentiation stage

Bell stage has two characteristics: shape of the future crown is defined, inner enamel epithelial cells elongate and differentiate to pre-ameloblasts

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

Dental lamina between tooth germs also degenerates

Remnants of dental lamina may remain in the adult mucosa

What are the cell layers of the enamel organ?

Outer enamel epithelium, stellate reticulum, stratum intermedium, inner enamel epithelium

Outer enamel epithelium

Cuboidal cells

Separated from surrounding mesenchyme by a basement membrane

Cells contain large, centrally placed nuclei

Contact each other via desmosomes and gap junctions

Involved in the maintenance of the shape of enamel organ, exchange of substance

Joins with the inner enamel epithelium at the cervical loop

Stellate reticulum

Fully developed at the bell stage

Star shaped cells

Intra cellular spaces are filled with fluid

Function relates to the protection

Stratum intermedium

2-3 layers of flattened cells

Lying over the inner enamel epithelium

Contains alkaline phosphatase

Concerned with protein synthesis and its transport to and from the ameloblasts

Inner enamel epithelium

Columnar at the bell stage

Rich in RNA

No alkaline phosphatase

Link with stratum intermedium by desmosomes

Separated from the peripheral cells of dental papilla by a basement membrane

Late bell stage (appositional stage)

Formation of dental hard tissues

18th week of intra uterine life

Dentin and enamel are produced as a result of reciprocal induction

Dentine formation always precedes enamel formation

Enamel and dentin formation begins at future cusp tips/ incisal edges

Reciprocal induction in the appositional stage

Reciprocal induction between IEE and dental papilla leads to histodifferentiation. IEE turn into pre-ameloblasts. Pre-ameloblasts induce dental papilla cells to differentiate into pre-odontoblasts. Pre-odontoblasts lay the first layer of Dentine. First formed Dentine induces the pre-ameloblasts to change to ameloblasts. Ameloblasts lay down enamel.

Enamel cord

A strand of cells seen at the early bell stage

Extends from the stratum intermedium to stellate reticulum

It is called enamel septum when it completely divides the stellate reticulum

Cells have elongated nuclei

Enamel niche

Seen when the tooth germ appear double attachment to the dental lamina (strands)

Strands enclose the niche - contain connective tissue

Functional significance unknown

Root formation

Begins when crown formation is completed

Initiated through the contributions of the cells originating from the enamel organ, dental follicle and dental papilla

Cells proliferate where inner enamel epithelium and outer enamel epithelium met to form cervical loop

Cell proliferation at the cervical loop creates hertwig’s epithelial root sheath

Epithelial root sheath bends inwards to dental papilla to create epithelial diaphragm

Root development continued…

Inner layers of hertwig’s epithelial root sheath induce papilla cells to form odontoblasts - form Dentine in the root

After onset of dentinogenesis, hertwig’s epithelial root sheath lose their continuity

Dental follicle cells adjacent to root Dentine differentiate into cementoblasts to produce cementum

As cementum forms, dental follicle fibroblasts produce collagen fibres and insert into cementum forming the periodontal ligament

Remnants of hertwig’s epithelial root sheath trap in the periodontal ligament - may lead to development of cysts.

Hypodontia

Refers to the absence of fewer than 6 teeth (not including third molars)

Oligodontia

Refers to the absence of 6 or more teeth (not including third molars)

Anodontia

Complete absence of teeth

Microdontia

Small tooth

Gemination

Partial division of tooth germ

Macrodontia

Large tooth

Hutchinson’s incisors

Congenital syphilis

What are the stages of amelogenesis

Pre-secretory, secretory, transitional, maturational, post-maturational

What happens in the pre-secretory stage?

IEE cells turn into pre-ameloblasts

pre-ameloblasts Change from cuboidal to columnar

pre-ameloblasts Induce dental papilla to differentiate into pre-odontoblasts

Pre-odontoblasts release enzymes that degrade the basal lamina

Odontoblasts begin to lay down dentin matrix

First formed Dentine induces the preameloblasts to change to ameloblasts

Ameloblasts first lay down enamel at the future cusp tips/ incisal edges

What happens in the secretory stage?

Tomes process: cone shaped process forms at the secretory end of the ameloblast

All secretions and modifications occurs via tomes processes

Responsible for the prismatic form of the enamel

Ameloblasts are joined to each other by a terminal bar apparatus (terminal web distally at the base of the tomes process

Enamel matrix protein synthesis takes place in the rER

Enamel crystals appear as hexagons when viewed in cross section

Enamel crystals that elongate around the tip of the tomes process form the prism core

Enamel crystals extending from where the ameloblasts are joined to each other form at the prism boundary

What occurs in the Transition stage?

Enamel secretion stops

Ameloblasts reduce in height and its number is reduced by 50%

All organelles of ameloblasts are reduced

Blood vessels invaginate into the enamel organ

Developing enamel has two groups of proteins

-amelogenins 90-95%

-non-amelogenins 5-10%

What occurs in the maturation stage?

New enamel has: water(65%), organic material(20%), inorganic material(15%)

Enamel is changed into its final form

Crystals increase in width and thickness and there is reduction in the crystal space

Functions of ameloblasts at this stage

-move calcium, phosphate and carbonate ions into the matrix

-remove water and degraded enamel matrix protein from enamel

Ameloblasts undergo further morphological changes

Organelle content reduced

Final maturation leads to lowering the molecular weight of enamel proteins and increase mineral content

Increase in mineral density begins over the cusp tips and proceed cervically

What occurs in the post-maturation stage?

Ameloblasts become flat ended

A thin layer of protein separates from enamel (primary enamel cuticle)

Remnants of enamel organ merge with flattened ameloblasts to form reduced enamel epithelium (REE)

Primary enamel cuticle + REE = nasmyth’s membrane : protects enamel during eruption

Dentine

Produced by odontoblasts (dentinogenesis)

Forms the bulk of the tooth

Covered in the crown by enamel and the root by cementum

Forms the protective covering for dental pulp

Support for overlying enamel

Vital tissue- contain odontoblasts processes and nerves

Tubular structure

What are the physical properties of Dentine?

Pale yellow

Harder than bone and cementum but softer than enamel

Organic matrix and tubular structure provides greater compressive, tensile and flexural strength

Permeable

Inorganic (70%), organic (20%), water(10%)

When does dentinogenesis start?

Begins at the bell stage

Starts at future cusp tips and continue along the cusp slopes, walls of the crown to the cervical loop (coronal)

Root Dentine forms when hertwig’s epithelial root sheath is formed

Continues until a predetermined thickness is reached and then slows down

What are the stages of dentinogenesis?

Odontoblast differentiation, matrix deposition, mineralisation and modification

What occurs during odontoblasts differentiation? (Dentinogenesis)

Epithelial - ectomesenchymal interactions induce IEE cells to differentiate into pre-ameloblasts

Pre-ameloblasts induce the peripheral layer of the dental papilla to form pre-odontoblasts

Mitosis of one dental papilla cell produces a pre-odontoblast and undifferentiated mesenchymal cell

What changes occur in the pre-odontoblasts? (Dentinogenesis)

Increase in size

nucleus move to the basal part of the cell

golgi complex become prominent and positioned above nucleus

rER increase in size and arrange parallel to the long axis

Small cell processes extend and directed towards the basement membrane of the IEE

One large process will remain and others disappear

Cell to cell junctions increase in number

Deposition of Dentine matrix

Fully developed odontoblasts produce matrix

Matrix is composed of: collagen (type 1) fibrils, Dentine phosphoprotein

Collagen is laid down initially and lie at right angle to future DEJ

First formed Dentine is called mantle Dentine

Second formed Dentine is called circumpulpal Dentine

Newly formed uncalcified Dentine is called pre-dentine

What occurs in mineralisation? (Dentinogenesis)

The matrix vesicles form in the mineralisation stage

Matrix vesicles contain alkaline phosphatase and metalloproteinases

These matrix vesicles later rupture

There is deposition of calcium in the ruptured vesicles, further leading to crystal growth

What is periodontium composed of?

Cementum, periodontal ligament, alveolar bone, gingiva, dento-gingival junction

Cementogenesis

Starts at the cervical margin after crown completion

hertwig’s epithelial root sheath loses its continuity and perforate to expose the root Dentine

Cementoblasts develop from the dental follicle

-secrete collagen fibrils into unmineralised Dentine

-secrete non-collagenous matrix

-migrate towards the PDL secreting cementum matrix until the forming PDL fibres (extrinsic fibres) become incorporated to the cementum

After onset dentinogenesis, hertwig’s epithelial root sheath lose their continuity

Dental follicle cells adjacent to root Dentine differentiate into cementoblasts to produce cementum

Where is acellular and cellular cementum located?

Acellular: upper 2/3 of the root

Cellular: lower 1/3 of the root

Cementogenesis

First layer of cementum has extrinsic fibers perpendicular to the root surface

These extrinsic fibers are produced by fibroblasts and few cementoblasts and they are seen most commonly in the acellular cementum

Cementoblasts become trapped in the forming cementum and become cementocytes

The cementoblasts secrete intrinsic fibers and these are seen in cellular cementum. These fibres have no role in attachment.

Cemento-enamel junction (CEJ)

Where the cementum meets enamel

Three configurations

-cementum overlaps enamel for a short distance (60%)

-butt joint between enamel and Dentine (30%)

-Dentine between enamel and cementum is exposed (10%)

Periodontal ligament

Derived from dental follicle

A dense fibrous connective tissue

Occupies the periodontal space

~0.25mm and hourglass in shape

Composed of: fibres, cells, ground substance