UP TO DATE IN

ORAL BIOLOGY AND EMBRYOLOGY

Prof. Dr. Nahed A. KhalilHead of Oral Biology Department

&Contributing Staff Members

Faculty of Oral &Dental Medicine Cairo University

At the age of 5-6 W.I.U.L. the primitive oral cavity is lined by ectoderm composed of 2 -3 layers, basal

columnar & superficial flat cells.

The mesoderm containing ectomesenchymal cells is separated from the oral ectoderm by a basment

membrane

At 5-6 w.i.u.l Embryo

Head Head

Stomodeum Stomodeum

Heart

Basement membrane

MESODERM

ECTODERM

The Morphological stages of tooth development are:

1- dental lamina formation

2- Bud stage

3- Cap stage

4- Bell stage : which is divided into :

-* Early bell stage (before dentin formation).

-* Advanced (Late) bell stage (after formation of the 1st layer of dentin).

Under the influence of the ectomesenchyme (neural crest cells), ectodermal proliferation in the underlying connective tissue takes place in separate islands that coalesce to form a band exhibiting a horse shoe shape in the upper & lower jaws. This band is called the tooth band or the dental lamina.

1- Dental lamina formation

11--Stage of dental laminaStage of dental lamina

Flat cells

Columnar cells

At 6-7 W.I.U.L

Ectomesenchymal tissue

Ectoderm

Basement membrane

Condensation of ectomesenchymal cells occurs around epithelial proliferation at certain sites on the labial surface of the dental lamina. Each site represents one of the deciduous teeth.

From this point tooth development proceeds in 3 stages describing the morphology of the developing tooth germ: Bud, Cap & Bell.

2-Bud stageThe cells of the dental lamina proliferates on the labial side of dental lamina forming rounded buds called the dental or enamel organ.

The ectomesenchyme condenses below the enamel organ & is called the dental papilla.

The ectomesenchyme that encircle both the enamel organ & the dental papilla is called dental sac

22--BUD STAGE BUD STAGE (a(at 8- WIU)t 8- WIU)

DENTAL ORGAN

DENTAL PAPILLA

DENTAL SAC

TOOTH GERM

The vestibular lamina is formed as ectodermal proliferation in the underlying ectomesenchyme buccal to the dental lamina. Epithelial degeneration occurs inside the vestibular lamina to form the oral vestibule separating the cheek & lip from the teeth bearing area. Oral

vestibuleTeeth buds

Differential growth results in increase in the size of the enamel organ & change in its shape to exhibit a cap shape with short & broad connection to the dental lamina.

The cap- shaped enamel organ has a convex outer surface & concave inner surface.

Cap stage

33--CAP STAGECAP STAGETOOTH GERM At 9-10 WIU

1-DENTAL ORGAN

2- DENTAL PAPILLA

3- DENTAL

SAC

Cap stageCap stage1- Outer enamel epithelium:

A single layer of cuboidal cells with deeply stained rounded nuclei arranged on the convex surface of the enamel organ

2- Inner enamel epithelium:

A single layer of columnar cells with deeply stained rounded nuclei arranged on the concave surface of the enamel organ.

A basement membrane separates the inner & outer enamel epithelia from the dental papilla & the dental sac respectively.

Cap stageCap stage

3- Stellate reticulum: Star shaped cells with long processes anastomse with each other by desmosomes.4- Enamel Knot: A condensation of ectodermal cells in the central region of the inner enamel epithelium that may bulge towards the dental papilla. It extends towards the outer enamel epithelium forming a strand of cells (Enamel cord). They are both transient structures They may have a role in determining the cusp position in molars, or may give the stratum intermedium in the bell stage

33 ( (CAP STAGECAP STAGE

Enamel knot

Inner D. E.

Outer D. E.

Stellate R.Enamel cord

Dental Papilla.Dental Sac

The bell stage Differential growth accompanied by Histodifferentiation & Morphodifferentiation leads to increase in size & shape of the enamel organ to exhibit a bell shape.

It is divided into:

Early bell stage: before any hard tissue formation

Advanced or late bell stage: which begins by formation of the first layer of dentin.

44--Early bell stageEarly bell stage At 11-12 WIU

1- The dental lamina:*Break up of the dental lamina by mesenchymal invasion divides it into:

Main dental lamina: (carrying permanent successors)

Lateral dental lamina : which become long & narrow connecting the E. Org. of deciduous teeth to the main dental lamina.

Early bell stageEarly bell stage

2- The enamel organ:

* The outer enamel epithelium:

the cells decrease in height & become low cuboidal.

Early bell Early bell stagestage

* The inner enamel epithelium: (IEE) (Preameloblasts) * The columnar cells elongate towards the dental papilla at the expense of the cell free zone to exhibit 40 microns length & be on contact with the un-differentiated M Cs. of the dental papilla causing an organizing influence on them to differentiate into Odontoblasts a process called induction.*They show alteration in functional polarity in which the nucleus & mitochondria migrate towards the proximal end while the Golgi & centerioles migrate towards the distal end.

* Cervical loop: The outer & inner enamel epithelium meet at the cervical part of the enamel organ

The inner enamel epithelium reflects on the outer E.E. for a short distance.

It may contain small amount of stellate reticulum.(rare)

•The stellate reticulum:* The star shaped cells of the st. reticulum are connected to each other, to the cells of the O.E.E. & to the st. interm. by desmosomes

* The glucosaminoglycans are hydrophilic & attract water into the center of the enamel organ to keep space for the developing enamel.

Early bell stage

Stratum intermedium

* 2-3 layers of flat cells between inner E.E. & the stellate reticulum & are connected to both of them by desmosomes

* they are rich in alkaline phosphatase enzyme essential for enamel mineralization.

*With the ameloblasts, they are considered as a functional unit in enamel formation & mineralization.

* They control fluid diffusion into & out of the ameloblasts.

Early bell stage

44--Early bell stageEarly bell stage

Cervical loop

At 11-12 WIU

ODE

Preameloblasts

Stratum Intermedium

Stellate Reticulum

44--Early bell stageEarly bell stage

Dental papilla

At 11-12 WIU

Odontoblasts

Dental sac

Inner Enamel Epithelium

Odontoblasts

Dental Sac

44--Early bell stageEarly bell stage

44--Early bell stageEarly bell stage

SR

SI

Preameloblast

OB

DP

44--Early bell stageEarly bell stage

SUCCESSIONAL LAMINASUCCESSIONAL LAMINADENTAL LAMINA

PROPER

LATERAL DENTAL LAMINA

SUCCESSIONAL LAMINA

Dental Organ

SUCCESSOR PRIMORDIUM

Dentin Matrix FormationDentin Matrix Formation

Odontoblasts differentiation Early dentin formation

5-Advanced (Late) bell stage

The Advanced Bell StageDeposition of the first layer of

dentin by the differentiated odontoblasts, causes many

changes in the different layers of the tooth germ:

1- The lateral dental lamina: breaks up into islands of epithelial cells called Serres pearls or epith. Rests of Serrs which separates the developing tooth from the oral epith.

2- The outer enamel epithelium: Formation of dentin breaks up the nutrition coming via the dental papilla. Therefore: * Folding of the outer E.E. will increase the surface area for increased nutrition. The vascular inner layer of the dental sac brings its Capillary loops very close to the cells of the enamel organ, providing it with a rich source of nutrition. * The cells of the outer E.E. become flat, develop microvilli & show increased mitochondria

The Advanced bell stage

3-The inner E.E. (Ameloblasts):

* The newly formed dentin exerts an influence on the inner enamel epithelium to differentiate into ameloblasts, a process called reciprocal induction.

* The ameloblasts are 40 microns in length & 4-5 microns in diameter & in cross section they are hexagonal in shape. They are attached to each other by junctional complexes & to the stratum intermedium by desmosomes.

4-Stellate reticulum:

Undergo shrinkage

The intercellular fluid is utilized in nutrition & the space is utilized by

the formed enamel

5-Stratum intermedium:

*Its cells show high alkaline phosphatese enzyme activity that is important for enamel mineralization.

55--Advanced (Late) bell stageAdvanced (Late) bell stageSTRATUM

INTER.

STELLATE RETICULUM

PREDENTIN

ODONTOBLASTS

AMELOBLAST

ENAMEL MATRIX

DENTIN

PREDENTIN

OUTER DENTAL EPITHELIUM

CAPILLARY LOOP

SR

AB

SI

5-Advanced (Late) bell stage

Dental papilla

OB

PD

DE matrix

SR

SI

AB

5-Advanced (Late) bell Stage

DENTAL LAMINADENTAL LAMINAVESTIBULAR LAMINA

Stages of tooth development are also described according to physiologic changes

& function during development & are called histophysiological stages.

These include:

1- Initiation 2- Proliferation 3- Histodifferentiation

4- Morphodifferentiation

5- Apposition

Root FormationRoot FormationCervical loop

Hertwig’s epith.root sheath

Odontoblast differentiation

Dentin formation

Disintegration of root sheath

Cementoblast differentiation

Root formationBegins when the enamel & dentin formation reaches the future cemento-enamel junction.

The epithelial cells of inner & outer enamel epithelium proliferate from the cervical loop to form a double epith. membrane known as Epithelial root sheath of Hertwig.

* It bends forming obtuse angle to the enamel organ at the future cemento-enamel junction to form the epithelial diaphragm.

Root formation

* The inner enamel epithelium in the epithelial root sheath of Hertwig, will induce U.D.M.Cs of the dental papilla to differentiate into Odontoblas which form the 1st layer of root dentin.

* The Epithelial root sheath of Hertwig degenerates forming epithelial rests of Mallassez

* The formed dentin that come in contact with the dental follicle, induces the differentiation of U.M.Cs. in the dental follicle in to cementoblasts which lay down cementum.

* This occurs in the formation of single rooted tooth.

Proliferation zone

cementoblasts

Cervical loop Hertwig’s epith.root sheath

Odontoblast differentiation

Dentin formation

Disintegration of root sheath

Cementoblast differentiation

EPITHELIAL DIAPHRAGMEPITHELIAL DIAPHRAGM

Epithelial diaphragm

NEWLY FORMED ROOTNEWLY FORMED ROOT

MULTI-ROOTED TOOTHMULTI-ROOTED TOOTH

Formation of multi- rooted teeth

* The root trunk of molars is formed like single rooted tooth.

* At the bifurcation area, the epithelial diaphragm produces 2 or 3 tongue- like processes in case of 2 rooted & 3 rooted teeth.

* The processes grow towards each other & fuse dividing the wide root trunk into 2 or 3 roots.

* Each one of these roots proceeds in development as in single rooted tooth.

MULTI-ROOTED TOOTHMULTI-ROOTED TOOTH

Clinical consideration

Formation of accessory root canals : Causes: 1) When the Hertwig’s epithelial root sheath looses its continuity before odontoblastic differentiation & dentin formation.

2) Failure in fusion of the tongue –like processes of the epithelial diaphragm in the pulpal floor.

3) If a large blood vessel is present and disturb the continuity of the epithelial root sheath.

* If the continuity of the epithelial root sheath of Hertwig is broken after odontoblastic differentiation & before dentin formation, intermediate cementum will be deposited.

* if the epithelial root sheath of Hertwig remains adherent to the dentin surface, its inner cells may differentiate into ameloblasts & produce enamel, known as Enamel pearls.

ENAMEL PEARLENAMEL PEARLENAMEL PEARL

Test yourselfTest yourselfStellate reticulum

Stratum intermedium

Ameloblasts

Enamel matrix

Dentin

PredentinOdontoblasts

Dental papilla