dev of tooth

8/3/2019 Dev of Tooth

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the primitive oral cavity or stomodeum is lined by stratified squmaous

epithelium called-oral ectoderm.

the oral ectoderm contacts the endoderm of the foregut to form the

buccopharyngeal membrame. at about 27day of gestation, this membrane

ruptures and the primitive oral cavity establishes a connection with theforegut.

two or three weeks after the rupture of this membrane, when the embryo

is about 6 weeks old, certain areas of basal cells of the oral ectoderm

proliferate at more rapid rate than do the cells of the adjacent areas. this

leads to the formation of the dental lamina, which is the band of

epithelium that has invaded the underlying ectomesenchyme along each

of the horseshoe-shaped future dental arches.

functions of dental lamina:

1. serves as the primordium for the ectodermal portion of thedecidious teeth

2. the permanent molars arise directly from a distal extension of the

dental lamina.

3. the successors of the deciduous teeth develop from lingual

extension—successsional lamina.

fate of dental lamina:

- total activity of the dental lamina extends about for 5 years.

- after initiation of tooth development, the dental lamina begins to

degenerate

- however it may still be active in the third molar region after it has

disappeared elsewhere

- exceptionally sometimes remnants of dental lamina persists as

epithelial pearls or islands within jaw as well as in the gingiva-

known as cell rests of cerez.

labial and buccal to the dental lamina in each arch, another epithelial

thickening develops independently—known as Vestibular lamina or Lip

furrow band--- which later forms the oral vestibule between alveolar portion of the jaws and the lips and cheeks.

tooth development

at certain points along the dental lamina, the ectodermal cells multiply

and form little knobs, known as tooth bud, that grow into the underlying

mesenchyme.



Although tooth development is a continuous process, the developmental

history of a tooth is divided into several morphologic "stages" for

descriptive purposes.

these stages are named after the shape of the epithelial part of the tooth

germ and thus they are termed as Bud stage, Cap stage, Bell stage andAdvanced bell stage.

bud stage:

- starts from 8th week of IUL.

- development of tooth germs initiated.

- some part of dental lamina proliferate in rapid rate to give tooth

bud.

- around tooth bud, condensation of ectomesenchymal cells occurs

- the main function of tooth bud is to form the tooth enamel whichform the enamel organ.

- 2 stages:

- Early: - in 8th week of IUL.

increased oxidative enzyme activity.

decreased glycogen.

increased RNA.

Late:- seen around 9th/10th week of IUL.

- dental lamina elongates giving Proper tooth bud.

- in this bud, peripheral cells are –cuboidalcentral cells are—Polygonal.

- same time, more ectomescenchymal condensation around tooth

bud occurs.

- now the tooth bud is known as Enamel organ or Dental organ.

- the area of condensation immediately below the enamel organ is

called—Dental papilla.

- the ectomesenchymal condensation that surrounds the tooth bud

and the dental papilla is called – Dental sac.

- the cells of the dental papilla form the dentin and pulp.

- while the dental sac forms cementum and periodontal ligament.

Cap stage:

- also consists Early and Late part

Early stage: - occurs in 11th week of IUL.

- not all the cells proliferate in same rate but the central part

proliferate more rapidly than the peripheral.

- thus there occur shallow invagination on the deep surface of the

bud.



- the ectomesncymal condensation below the invasination of enamel

organ known as Dental Papilla.

- peripheral cells are more cuboidal covering the convexity of the

cap and are called ---Outer enamel epithelium.

- whereas the central cells are polygonal.Late stage:

- occurs in 12 ½ week of IUL.

- invasination deepens to form proper cap like structure.

- the cells in the concavity of the cap become low columnar—known

as Inner enamel epithelium.

- the outer enamel epithelium is separated from the dental sac, and

the inner enamel epithelium by a delicate basement membrane.

- the central cells between inner and outer enamel epithelium begin

to separate as more intercellular fluid is produced -- known asStellate reticulum.

Bell stage:

- occurs in 14th week of IUL.

- as the invagination of the epithelium deepens and its margins

continue to grow, the enamel organ assumes a bell shape.

- inner enamel epithelium becomes tall columnar cells.

- there occur appearance of a layer inbetween inner enamel

epithelium and stellete reticulum—known as Stratum intermedium.

- 4 different types of epithelial cells can be distinguished:

o Inner enamel epithelium

o Stratum intermedium

o Stellate reticulum

o Outer enamel epithelium

- intercellular substances of polygonal cells are rich in

glycoaminoglycan and thus absorb water and become star shaped.

- those polygonal cells are attached with one another by desmosomes

having Tonofilment and Plaque and with basement membrane by

hemidesmosomes.

Inner enamel epithelium:

- is a single layer of columnar cells

- 4-5 microm. in diameter.

- 40 microm. in height.

- also called Preameloblast.

- hemidesmosome attachment with basement membrane for proper

attachment.Stratum Intermedium:



- seen as 2-3 layers of flattened cells.

- not much separated from one another and are closely attached by

desmosomes and gap junctions.

- rich in alkaline phosphate.

- functions:- synthesize protein

- transport of material to and from the preameloblast.

- essential for enamel formation.

Stellate reticulum:

- rich in water content.

- star shaped with long processes that anastomose with those of

adjacent cells.

- before enamel formation begins, this layer collapses, reducing the

distance between the centrally situated ameloblasts and the nutrientcapillaries near the outer enamel epithelium.

- functions:

- help as a mechanical protection for newly from tooth/enamel.

- determine crown pattern.

Outer enamel epithelium:

- flattened to a low cuboidal form.

- desmosomes are inbetween cells and hemidesmosomes inbetween

basement membrane and cells.

- transport of material to and from dental sac to Inner enamel

epithelium.

- help growth of Preameloblast.

Cervical loop:

- is the point where Outer and Inner enamel epithelium meet.

- is devoid of stratum intermedium and stellate reticulum.

- helps in formation of root in later stage.

Transient Structures: these structures disappear in advanced bell stage.they are:

1. Enamel knot

2. Enamel cord

3. Enamel septum

4. Enamel navel

5. Enamel niche

Enamel knot:

- are the cluster of cells in inner enamel epithelium due tocondensation of cells.



- present at the deepest point of bell stage.

- determine the cusp tip.

Enamel cord:

- run from enamel knot to outer enamel epithelium.- sometimes it reaches completely to outer enamel epithelium

dividing enamel organ into 2 parts. then it called—Enamel septum.

The function of the enamel knot and cord is to act as a reservoir of

dividing cells for the growing enamel organ.

Enamel navel:

- is the enamel cord touching point at outer enamel epithelium where

small depression occur.

Enamel niche:

- appearance of 2 dental lamina connecting and the connective tissue

present inbetween them.

- it is an artifact ie cutting defect.

Cell Free Zone:

- 1-2 microm. inbetween dental papilla and basement membrane.

- also known as Membrana preformativa.- helps to convert cuboidal cells into columnar cells.

Advanced Bell Stage:- seen in 18th week of IUL.

- features:

o determination of crown portion.

o onset of mineralization.

o root formation starts.

- detachment of dental lamina from tooth germ occur and have

connection only to permanent tooth germ.

- Dental papilla adjacent to basement membrane give Odontoblasts

which produce Dentin.

- similarly the inner enamel epithelium gives the Ameloblasts which

produce Enamel.

- the boundary between inner enamel epithelium and odontoblasts

outline the future dento-enamel junction.

- there wont be cell free zone due to increase in size of papillarycells.



- here, Reciprocal induction occur ie dentin and ameloblasts are

necessary for differentiation of each other.

- initially, odontoblasts lay down the predentin.

- once predentin layer form, it induces formation of enamel by

differentiation of inner enamel epithelium into ameloblast and produce layer of enamel.

- the ameloblasts move away from dentin leaving behind an ever

increasing thickness of enamel.

- once layer of dentin and enamel form, they loose connection blood

circulation from dental papillae.

- the part of crown where first enamel is produced develops into 1st

cusp, the occurrence of 2nd point of enamel formation assume 2nd

cusp and 3rd zone leads to 3rd cusp.

- as the internal dental epithelium is constrained at the cervical loopand because there is continuous proliferation of cells on each side

of zone of maturation(enamel formation), the epithelium bends and

forms a cuspal outline.

- thus the future cusp is pushed toward the external dental

epithelium.

Hertwig's epithelial root sheath(HERS) and Root formation:

- the development of root begins after enamel and dentin formation

has reached the future CEJ.

- the cervical portion of the enamel organ gives to the HERS.

o it molds the shape of the roots and

o initiates radicular dentin formation.

- HERS consists of inner and outer enamel epithelium only.

- the odontoblasts develop from dental papilla and lay down

dentin.There is no reciprocal induction. thus no production of

enamel by inner enamel epithelium.

- once the first layer of dentin has been laid down, the epithelial root

sheath loses its structural continuity. the remaining epithelia ie

remnants are found in the periodontal ligament of erupted teeth— are known as Rests of Malassez.

- once the dental sac comes in contact with newly formed dentin, the

ectomesenchymal cells differentiate into cementoblasts which

forms cementum.

-

- Dental sac has 3 types of cells:

o cells near dentin –form cementum.

o cells near bone – form alveolar bone.

o cells between bone and dentin – form periodontal ligament.



Epithelial diaphragm:

- prior to the beginning of root formation, the root sheath bend at the

future CEJ into a horizontal plane to form epithelial diaphargm.

- it narrows the cervical opening of the tooth germs.- it remains relatively fixed.

- the proliferation of cells of the epithelial diaphragm is

accompanied by proliferation of cells of the connective tissue of

the pulp in the area adjacent to the diaphragm.

- the free end of the diaphragm doesnot grow into the connective

tissue but the epithelium proliferate coronally to the epithelial

diaphragm.

- the differentiation of odontoblast and the formation of dentin

follow the lengthening of the root sheath.- at the same time disintegration of HERS occurs and cementum is

formed.

- in this way a single rooted tooth is formed.

- in case of multirooted tooth, roots develop essentially in the same

way but the tongue like extention of horizontal diaphragm--- 2 in

double rooted, 3 in 3rooted tooth grow each other and fuse.

- thus the single cervical opening of coronal enamel organ is divided

into 2 or 3 openings.- on the pulpal floor of the dividing epithelial bridges,dentin

formation start.

- if the cells of HERS remain adherent to the dentin surface, they

may differentiate into fully functioning ameloblast and produce

enamel.- such droplets of enamel are called—Enamel pearls. – found in the

area of furcation of root of permanent molars in same cases.

Morphologic stages Physiologic processes

Dental lamina Initiation

Bud stage Proliferation

Cap stage Histodifferentiation

Bell stage Morphodifferentiation

Advanced bell stage

Formation of enamel and dentin matrix Apposition

dev of tooth

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