Development And Growth Of Teeth

Structure of the primitive oral cavity:

= Primitive oral cavity of a fetus is lined by ectodermal tissue

(epithelium) covering an ectomesenchymal tissue (mesoderm).

I- Ectoderm: is composed of 3-4 layers of epithelial cells:

1-Basal cell layer: is the first layer toward mesoderm and composed

of cuboidal cells separated from mesoderm by basement membrane.

2- Superficial layers: composed of two or three rows of flat cells lie

superficial to the basal cell layer.

II=Ectomesenchymal tissue: lie beneath the ectoderm and composed of

condensation of ectomesenchymal cells (UMC).

! Figure 1: Oral Ectoderm

Origin of the tooth structure: 1. Enamel is developed from ectoderm.

2. Dentin, pulp, cementum, and periodontal ligament are development from ectomesenchymal tissue (mesoderm).

Stages of Tooth Development 1. Development of dental lamina.

2. Bud stage.

3. Cap stage.

4. Bell stage: a- Early bell stage.

b- advanced bell stage.

5- Development of the root.

Dental Lamina Definition of dental lamina:

Dental lamina is an arch shaped band of epithelium at the future dental

arch, derived from oral ectoderm and give the germ of development of

the tooth.

!

Figure 2: Future dental arch.

Origin of dental lamina: The dental lamina was originated from the basal cell layer of oral

ectoderm.

Time of origin of dental lamina:

= Dental lamina begins to develop at 35 days of intrauterine life.

= the length of the embryo at this time is about 13 mm.

Development of dental lamina: •The first sign of tooth development is the condensation of

ectomesenchymal cells (UMC) beneath the epithelium at the future

dental arch. •These ectomesenchymal cells induce the basal cells of oral ectoderm to

make more cell division toward the ectomesenchymal tissue leading to

the formation of band of epithelium termed main dental lamina.

Types of dental lamina:

1. Main dental lamina = Mother dental lamina:

2. Lateral dental lamina: • Time of origin: 2 MIUL. • Origin: from the labial and buccal side of the main dental

lamina. • Gives rise to: Deciduous tooth germs.

3. Successional lamina: • Time of origin: 5 MIUL. • Located lingual to deciduous tooth germs. • Gives rise to: tooth germs of permanent successors.

4. Distal extension of dental lamina: • Time of origin: 4 MIUL. For first permanent molar.

: 1 year for second permanent molar.

: 4 years for third molar.

!

Figure 3. Lateral dental lamina for deciduous tooth germ.

! Figure 4: Successional dental lamina.

Bud StageDevelopment of bud stage:

The cells of lateral dental lamina at certain areas related to deciduous

tooth sites proliferate and give more mitosis than the adjacent cells

leading to the formation of a ball of cells termed bud stage.

Number of tooth buds: • 10 mandibular and 10 maxillary, each one related to deciduous

tooth. • Not all buds develop at the same time. • The first to develop is the lower deciduous central incisor.

Sites of deciduous tooth buds: • The tooth buds are located facial to the main dental lamina.

Structure of tooth buds.

Bud stage composed of:

1. Dental organ: which has two types of cells:

a.Peripherally located cells: low columnar cells.

b.Centrally located cells: polygonal in shape.

2. Ectomesenchymal condensation.

! Figure 5: Bud Stage.

Cap stage

The tooth bud continue to grow, it does not expand into a lager sphere

ball, but this growth leading to the formation of cap shaped enamel organ

termed Cap Stage.

Factors leading to the formation of cap shape:

1- Unequal growth of the cells of the enamel organ in which the outer

cells develops more rapidly then the cells facing the dental papilla.

2- Pressure exerted from the dental papilla leading to a shallow concavity

of the ventral surface of the enamel organ.

Structure of cap stage:

1. Enamel organ (dental organ): a. Outer enamel epithelium.

b. Inner enamel epithelium.

c. Stellate reticulum

2. Dental papilla.

3. Dental sac (dental follicle).

1- Dental organ: a- Outer enamel epithelium (Outer dental epithelium):

- It is a single layer of epithelial cells that cover the convexity of the

cap.

- This layer is composed of cuboidal cells with round nuclei.

- This layer is rested on basement membrane, which separate them from

the dental follicle.

- The cells are attached with each other by desmosomes .

b- Inner enamel epithelium (Inner dental epithelium):

- It is a single layer of epithelial cells which line the inferior concave

surface of the enamel organ.

- This layer is composed of columnar cells with large oval nuclei that

approximately fill the cell.

- This layer is rested on basement membrane, which separate them from

the dental papilla.

- This cells are attached with each other by desmosomes.

c- Stellate reticulum:

- Stellate reticulum is a several layers of star shaped cells found in the

center of enamel organ between the outer and inner enamel epithelium.

- Why the stellate reticulum as star shaped cells?

- Because:

- The cells of stellate reticulum are attached to each other by

desmosome.

- These cells secrete a mucoid material into the intercellular space

which is hydrophilic.

- The mucoid material press on the cell membrane of the stellate

reticulum, in which this cell became compressed except at the area

of desmosomes, so,, the cells appear as a star shaped.

Functions of stellate reticulum:

1. Shock-absorber from the pressure generated by the surrounding

mesoderm or shock from outside.

2. Act as store house for nutrition needed for the actively

functioning cells.

3. Keep a room for the developing enamel.

Enamel knote and Enamel cord:

-Enamel knote is a condensation of stellate reticulum at the center of the

enamel organ.

- Enamel cord is a strand stellate reticulum cells running from the knote

to reach the outer enamel epithelium.

- Enamel knote and enamel cord are transient structure and disappear

before beginning of bell stage.

- Function of knote and cord is unknown, but may be act as balance

enamel organ to develop in a normal situation.

2- Dental Papilla:

- Dental papilla is an ectomesenchymal condensation found within the

concavity of the enamel organ.

- Structure: a- Condensation of ectomesenchymal cells.

b- Active budding of capillaries = blood vessels.

c- Numerous mitotic cells.

- Acellular zone (cell free zone):

The peripheral part of the dental papilla adjacent to the inner enamel

epithelium appears free from the cells. This area was termed acellular

zone.

3- Dental Follicle:

- Dental sac is an ectomesenchymal condensation that surrounds both

enamel organ and dental papilla.

- Structure:

a- Ectomesenchymal cells.

b- Fibers.

- The dental sac appears denser and fibrous than the dental papilla.

! Figure 6: Cap Stage.

Bell stage.

- The enamel organ continue to enlarge and the dental papilla continue to grow leading to the formation of bell shaped enamel organ.

- This stage is termed bell stage. - During this stage the enamel organ take the crown shape of the

developing tooth.

- Bell stage is divided into two subdivisions: 1. Early bell stage, before formation of dentin. 2. Advanced bell stage, after beginning of dentin formation.

Early Bell stage.

- Structure of bell stage: 1. Enamel organ (dental organ):

a. Outer enamel epithelium.

b. Inner enamel epithelium.

c. Stellate reticulum.

d. Stratum intermedium.

2. Dental papilla.

3. Dental sac (dental follicle).

1. enamel organ:a- Outer enamel epithelium (Outer dental epithelium):

b- Inner enamel epithelium (Inner dental epithelium):

- The cells of inner enamel epithelium elongated to become tall columnar.

- This elongation is toward the dental papilla, leading to disappearance of

cell free zone.

- The IEE Become facing the dental papilla cells.

- Induction: The cells of IEE induce the ectomesenchymal cells of

dental papilla to differentiate into odontoblast cells and arranged in one

layer.

c- Stellate reticulum:

d- Stratum intermedium: (imp)

- Definition: Stratum intermedium is a new layer of cells found between

IEE and stellate reticulum.

- Structure: this layer is consists of two or three raws of flat cells

arranged perpendicular to the IEE. Each cell is attached to the adjacent by

desmosome.

- Functions: Stratum intermedium is high metabolic cells that contain:

1- Glycogen needed for formation of enamel matrix.

2- Alkaline phosphatase enzyme needed for enamel calcification.

Note: Absence of Stratum intermedium means no enamel formation.

e- Cervical loop: Definition: Cervical loop formed at the periphery of enamel organ, consists of two types of cells only IEE and OEE. Function: Cervical loop is responsible for the formation of root.

! Figure 7: Early bell stage

Advanced Bell Stage

Definition: Advanced bell stage is marked by beginning of dentin

formation and calcification.

1. Enamel organ:

Inner enamel epithelium: -The inner enamel epithelium induces the cell of dental papilla to

differentiate into odontoblast cells, arranged in one layer and begin to

form dentin.

- Note: The dentins begin to form before enamel.

- Once the first layer of dentin is formed, it induce the inner enamel

epithelium to elongated and differentiated into ameloblast cells (cells

forming the enamel).

- This type of induction between odontoblast and ameloblast cells is

termed reciprocal induction.

Cervical loop

Lateral dental lamina

Successional lamina

Cell free zone

Permanent bud

- Once the first layer of dentin is formed, the dental papilla is termed

dental pulp

Change of nutritional source:

- Once the first layer of dentin is formed, it leads to cutting of the

nutrition of enamel organ from dental papilla, so, the nutrition source

changed into the dental follicle through outer enamel epithelium.

- So, the outer enamel epithelium become flat and the stellate reticulum

collapsed.

Fate of the dental lamina -At the advanced bell stage, the dental lamina that attached to the enamel

organ break up and separate.

- The dental lamina cells become arranged in clusters and must be

degenerated.

- Remnant of dental lamina may be persist and termed epithelial rest of

serre's and become found in gingiva and may develop into cyst or tumor.

Root formation 1. Single Rooted Teeth:

After complete formation of crown, the cervical loop grow

vertically to small distance, then grow horizontally to form

Epithelial Diaphragm.

The cells of cervical loop coronal to epithelial diaphragm

proliferate leading to the formation of epithelial root sheath of

Hertwig's.

The inner enamel epithelial cells of root sheath induce the adjacent

dental papilla cells to differentiate into odontoblast and begin to

form the root dentin on the inner surface of cervical loop.

The root sheath breakdown and degenerate leading to exposure the

dentin to dental follicle.

Persistence of epithelial root sheath without degeneration leads to

the formation of epithelial rest in the periodontal ligament termed

epithelial rest of Malassez.

The cells of the dental follicle migrate toward the dentin and

differentiate into cementoblast and begin to form cementum on the

dentin.

Then other part of root sheath proliferates to form other part of root

as described.

2. Multirooted teeth:

Formation of root trunk was performed as in single rooted teeth.

At the area of bifurcation in two rooted teeth: Two tongue like processes arise from the epithelial diaphragm and grow

horizontally to reach to each other and fused together which

divide the opening (apical foramen) into two openings. Each one opening performs one root as the way of single root

formation.

At the area of trifurcation in two rooted teeth: Three tongue like processes arise from the epithelial diaphragm and grow

horizontally to reach to each other and fused together which

divide the opening (apical foramen) into three openings.

Each one opening performs one root as the way of single root formation.