DEVELOPMENT OF TEETH

Development and growth of the crown initially and the

not later results in the crown occupying an intraoral position

and root occupying an intraosseous position in which it is

anchored in a bony socket.

Six stages are explained for the tooth development and

growth process namely:

1. Lamina or initiation stage – in which germinal

tissue is formed (dental).

2. Bud stage – Here the analge responsible for enamel

development forms.

3. Cap stage.

4. Bell stage.

5. Apposition and calcification stage.

6. Tooth eruption.

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Further it is to be noted that the dental tissues not only

vary in size and shape but also in their development pattern as

well as period of growth and development.

The development of tooth is a study of prime importance

since many of clinical conditions are associated with

disturbances in developmental process. The study of

development of teeth involves many complex biological

processes. This includes epithelial mesenchymal interactions,

differentiation, morphogenesis, fibrillogenesis and

mineralization.

It is at around 6.5 weeks of gestation or when embryo is

13-14 mm in length stomatodeum i.e. primitive oral cavity

when is examined under light electron microscopy shows a

primitive 4 to 5 layered epithelial cells covering a band of

connective tissue. This connective tissue based on its site of

origin from neural crest is termed as ectomesenchyme. The

ectomesenchyme here consists of a few spindle shaped cells

separated by a gelatinous ground substance.

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Primary Epithelial Band(Nevy 1970, Ruch J.V. 1984)

It is around the 6.5 weeks of embryonic life, there

appears a continuous band of thickened epithelium around the

mouth in place of future upper and lower jaws. It occurs by

fusion of separate plates of thickened epithelium.

These epithelial thickenings are roughly hose shoe

shaped and correspond in position to the future dental of upper

and lower jaws (Nevy et al 1970).

This resulting thickening of epithelium is not mainly

because of increased proliferative activity of the cells but

rather is because of change in the orientation of cleavage plane

of dividing cells.

These bands of epithelium (upper and lower) are termed

as primary epithelial bands. Hence formed primary and band

very quickly given rise to two subdivisions, the vestibular

lamina and dental lamina. The division of band in to these two

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layers is so fast that majority of investigators consider them

(laminae) as separate entities.

Vestibular Lamina

(Nevy et al 1970) If coronal section of a developing

embryo at 6 weeks is examined at head region, no sulcus or

vestibule are seen clinically. It is the vestibular lamina that

proliferates within the mesenchymal tissues and forms a

vestibule or depression between developing jaws and cheeks.

The formation of vestibule occurs by enlargement and

degeneration of cells of vestibular lamina.

Dental Lamina

It’s the first stage of tooth development increased mitotic

activity in a specific portion of stomatodeal ectoderm of both

upper and lower arches produces prominent thicknenings that

dips into underlying ectomesenchyme. The epithelial

thickening seen now produces two horse shoe shaped bands

defining the prospective upper and lower dental arches in the

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stomatodeum. This germinal band of ectodermal epithelium

circumscribing the future maxillary and mandibular arches is

termed as Dental Lamina.

At certain points along dental lamina each representing

the location of one of the 10 mandibular and 10 maxillary

deciduous teeth the ectodermal cells that grow into underlying

mesenchyme. Each of these little down growth represents the

beginning of enamel organ of the tooth bud of a deciduous

tooth.

It is found that the mitotic index as well as epithelial

replication rate is slower when compared to that in underlying

ectomesenchyme. Thus the down growth of epithelium is also a

result of a growth of ectomesenchyme (Tencate 1970).

Once the cells from tooth bud proliferating, they assume

different types of -----. Now the bud resemble cap shape. The

ectomesenchymal cells in the cap proliferative more rapidly

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and appear more dense than surrounding structures. This

represents beginning of formation of dental papilla.

Now surrounding dental papilla and enamel organ a third

layer of cells develop. This third layer is called as dental sac.

This consists of ectomesenchymal cells and fibers that

surrounds the dental papilla and enamel organ.

The function of individual structures will be discussed

later.

Successional Laminae

The portion of dental lamina adjacent to developing tooth

anlage retains its connection with the lingual aspect of tooth

primordium via Lateral Lamina. The free terminal end of dental

lamina begins to proliferate at the end of forth month IU. This

newly established growth center is known as successional

lamina. It provides anlage for permanent teeth replacing

deciduous teeth.

Parent Dental Laminae

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In the 8 t h week of tooth development tooth anlagen for 10

primary teeth is produced by the dental lamina. This lamina

also provides germ for the permanent molars which does not

have any predecessors. Because of this the dental lamina

providing for the formation of I, II and III molars is called as

Parent Lamina or Lamina for Permanent Molars.

The mechanism involved simply is one of the continued

distal growth i.e. the dental lamina after having established the

growth centers for 10 primary molars on either jaws keep

growing on distal aspect. The growth of this keeps in pace with

growth of the dental arches.

The buds of first permanent molars appear at 4 month of

time. That of II and III molars appear after birth (9 month and

1 year post natally respectively).

Thus activity of dental lamina begins at 6.5 weeks IV and

lasts for about 4-5 years postnatally.

Stage II or Bud stage:

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Within days after formation of dental lamina knoblike

structures start appearing on the vestibular aspect of dental

lamina in ten specific approximating areas in maxillary and

mandibular arches. These are the primordial for the deciduous

teeth and are variously referred to as tooth buds, tooth

primordial or dental anlagen.

The first primordial to form are those of incisors of

mandibular arch (at the end of seventh week ambryo 17mm in

length). Then the maxillary incisor buds start appearing. At the

8 t h week when embryo is about 25mm in length, the buds for all

primary teeth are formed.

Since the main function of certain epithelial cells of

tooth bud is to form enamel those cells constitute enamel organ

which is critical to normal tooth development.

In the bud stage, the enamel organ consists of a

peripherally arranged low columnar cells and centrally located

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polygonal cells. Now many of the cells of tooth bud and the

surrounding ectomesenchyme undergo mitosis.

As a result of this mitosis and the migration certain cells

from neural crest into the area of ectomesenchyme, cells

surrounding the tooth bud condense. The area of

ectomesenchymal condensation immediately adjacent to enamel

organ is called Dental Papilla.

Now the ectomesenchymal condensation that surrounds

both dental papilla and enamel organ is known as Dental Sac.

Dental Papilla and sac form (pulp and dentin) and

(cementum and periodontium) respectively. At this stage the

cells of dental papilla start invaginating in the center of bud

and next stage i.e. cap stage begins.

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Bud Stage

Stage II or Cap Stage

The cells of this dental primordium starts proliferating and

growing. As these proliferation continues the intercellular spaces

are enlarged particularly those of the core.

This proliferation is not uniform in all the areas of tooth bud.

Instead an unequal growth is seen in the bud. Like more amount of

growth is seen in the inferior border of bud. This leads to an inward

growth of mesenchyme below the bud with expanding growth of

mesenchyme more and more amounts of mesenchyme is

accumulation in the enlarging concavity and the epithelial bud is

rapidly transformed in to a cap or cup shaped structure.

The connective tissue bordering the lining epithelium of the

cap is dental papilla.

Continued cell movements or rearrangements effected by

growth forces result in a change in the shape of the cap. Now the

cells in the tooth forming structure are no more of uniform size or

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shape. The cells lining the concavity of the cap become low

columnar and is called inner enamel epithelium. The cells lining the

convea portion of the cap are cuboidal in shape and are called as

outer enamel epithelium. The polygonal cells that are present

between inner and outer enamel epithelium begin to separate as

more amount of fluid starts collecting between them and form a

cellular network called as stellate reticulum.

This cells in stellate reticulum assume a branched reticular

pattern. The spaces between these reticular forms are filled with a

mucoid fluid rich in albumin, which gives the stellate reticulum a

cushion like consistency that may support and protect, delicate

enamel forming cells.

Enamel knot and Cord

The cells in the center of enamel organ are densely packed

and form enamel knot. The knot projects in part towards dental

papilla, so that the center of epithelial invagination shows a

knoblike structure that is bordered by the labial and lingual enamel

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grooves. At the same time in the increasingly high enamel organ a

vertical extension of the enamel organ occurs called as enamel

cord. Both these structures are temporary that disappear before

enamel formation begins. The functions of enamel knot and used

may be to act as reservoirs of dividing cells for growing enamel

organ.

Dental Papilla

The changes in the dental papilla occurs concomitantly with

the development of epithelial enamel organ.

Although epithelization exerts a deminating influence over

the adjacent connective tissue condensation of connective tissue is

not a passive crowding by proliferating epithelium.

D.P. shows active budding of capillaries and mitotic figures

and its peripheral cells adjacent to inner enamel organ proliferative

and differentiate into odontoblasts.

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Dental Sac

Concomitant with the development of enamel organ and

dental papilla there is a marginal condensation in the

ectomesenchyme surrounding the enamel organ and D.P. gradually

a more fibrous and dense layer develops which terms in to dental

sac (D.S.).

The cells of D.P., D.S., and E.O. are formative structures for

entire tooth and supporting structures.

Bell stage or IV stage

As the invagination of epithelium deepens and margins

continue to grow, four different types of epithelial cells can be

distinguished and it assumes a bell shape.

The fourth layer comes from differentiation of stellate

reticulum into another layer of cells called stratum intermedium.

Hence the four layers of the tooth bud include:

1. Outer enamel epithelium.

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2. Stratum intermedium.

3. Stellate reticulum and

4. Inner enamel epithelium.

Inner Enamel Epithelium

The inner enamel epithelium consists of a single layer of

epithelium that differentiate into tall columnar cells prior to

amelogenesis. These full columnar cells are called as ameloblasts.

These cells are 4-5µm in diameter and 40µm in length. These

elongated cells are attached to one another by junctional complexes

laterally and by means of desmosomes with cells of stellate

reticulum.

The cells of inner enamel epithelium has a organizing

influence over cells of (etcomesenchyme of D.P.) which gets

converted into odontoblasts.

Stratum Intermedium

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A few layers of squamous cells form stratum intermedium

between inner enamel epithelium and stellate reticulum. These cells

are closely attached by desmosomes and gap junctions.

They possess a high degree of metabolic activity which is

indicated by increased cytoplasmic organelles acid

mucopolysaccharides and glycogen deposits.

This layer is essential for enamel formation and it is absent

in the part which takes part in root formation.

Stellate Reticulum

The stellate reticulum expands further mainly by an P.D.

accumulation of intracellular fluid. These cells are star shaped with

pointed ends in contact with one another. This layer collapses prior

to amelogenesis thereby reducing the distance between nutrient

capillaries outside the outer enamel expansion and ameloblasts. At

this stage these cells are very difficult in distinguishing from those

of it intermedium. The change begins at the height of cusp tips or at

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incisal edges, and proceeds cervically as the formation of enamel

progresses.

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Outer Enamel Epithelium

The cells of outer enamel epithelium flatten to a low cuboidal

form. At the end of bell stage preparatory to and during

amelogenesis, the O.E. epithelization which was smooth becomes

folded into papillae which extend into stratum intermedium. In

these papillary extensions blood capillaries carrying rich nutrients

from dental sac are found. Thus nutrition is provided to avascular

enamel organ by this modification for its intense metabolic activity.

Dental lamina

In all these areas except in the areas of permanent molars, the

dental lamina proliferates at its deep end contributing for

permanent successors.

Dental papilla

At this stage, before the enamel organ begins to produce

enamel, the peripheral cells of mesenchyme proliferate and under

the organizing influence of ameloblasts convert into odontoblasts.

These odontoblasts are cuboidal in form in the initial stage. Later

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they assume a columnar form and acquire specific potential to

produce dentin.

The basement membrane that separates enamel organ from

dental papilla just prior to dentin formation is called as membrane

preformation.

Dental Sac

Before formation of dental tissues begins the dental sac

shows circular arrangement of its fibers and resembles a capsular

structure with the development of root, the fibers get embedded

into cementum to alveolar bone and converted into periodontal

ligament.

The transformation of cap stage to bell stage occurs when the

embryo is about 100-160mm in length (Provenza 1986).

Advanced Bell Stage:

During advanced bell stage, the boundary between inner

enamel epithelization and odontoblasts outlines the future dentino-

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enamel junction. In addition, the cervical portion of enamel organ

gives rise to epithelial sheath of Heartwig.

Hertwig’s epithelial root sheath and root formation:

The development of root formation begins after the enamel

and dentin formation has reached the future cemento-enamel

junction. The enamel organ plays an important role in root

development by forming Hertwig’s epithelial root sheath which

molds the shape of the roots and initiates radicular dentin

formation.

The Hertwigs root sheath includes only outer and inner

enamel epithelium and thus does not involve of intermedium and st.

reticulum. The cells of inner enamel epithelium remain short and

normally do not produce enamel.

These cells will induce radicular cells to get converted into

odontoblasts. These odontoblasts shorts laying down dentin. When

the first layer if dentin has been laid down, the epithelium and

Hertwigs root sheath looses its continuity and its close relation to

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the root surface. Its epithelial remnants may persist on the surface

of root as group or clusters of cells or tubules which are found in

periodontal ligament as cell rests of malasses.

There is a pronounced difference in the development of

Hertwig’s epithelial root sheath in teeth with single root and in

teeth with two or more roots. Prior to the beginning of root

formation the sheath bonds into a horizontal plane forming

epithelial diaphragm. This horizontal bonding causes narrowing of

wide cervical opening.

The plane of diaphragm remains relatively fixed during

growth and development of root. The proliferation of epithelial

diaphragm is accompanied by proliferation of cells of connective

tissue of the pulp which occurs in the area adjacent to pulp

diaphragm.

The free end of diaphragm does not itself grow into the

connective tissue but the epithelium present coronally to diaphragm

proliferates. Now the differentiation of odontoblasts and formation

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of dentin follows lengthening of root sheath. At the same time, the

connective tissue from dental sac proliferates and divides the root

sheath into epithelial strands. These by the D.S. connective tissue

comes in contact with underlying dentin. These cells from D.S.

once they come in contact with dentin differentiates into

cementoblasts and deposit cementum on the dentin.

In the last stage of epithelial development proliferation of

epithelium lags behind that of pulpal connective tissue. The apical

foramen is at first established to the width of Hertwigs epithelial

root sheath. Later the dentin deposition at a rapid rate causes

narrowing of apical foramen to its normal size.

Differential growth of epithelial diaphragm in multirooted

teeth causes division of root trunk into two or three roots. During

the general growth of enamel organ expansion of cervical opening

occurs in such a way that small tongue like extensions develop

from horizontal diaphragm. Two such extension occurs in lower

molars and 3 extensions from upper molars. Before the division of

root trunk occurs the free ends of these extensions grow towards

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each other and fuse. The cervical opening of these teeth is hence

divided into two or 3 openings. On the pulpal surface the dividing

epithelium starts dentin formation and on the periphery of each

opening root development follows as in case of single rooted teeth.

Histopathology and Clinical Considerations:

A number of growth processes participate in the progressive

development of teeth. Except for initiation which is a momentary

process each of these processes overlap each other and many are

continuous throughout the various morphologic stages of

odontogenesis.

These processes include:

1. Initiation.

2. Proliferation.

3. Histodifferentiation.

4. Morpho differentiation.

5. Apposition.

Initiation:

Dental laminae and associated tooth buds represent those

parts of oral epithelium that have the potential for tooth formation.

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Specific cells in horse shoe shaped lamina have the potential to

form enamel organ of certain teeth by responding to certain factors

that initiate or induce tooth development. Different teeth are

initiated at definite times.

Initiation or induction requires ectomesenchymal epithelial

interaction. The mechanism of such as interaction is not clearly

understood. However it has been demonstrated that dental papilla

mesenchyme can induce or instruct tooth epithelium and even non

tooth epithelium to form enamel.

The lack of initiation results in loss of a single tooth or

multiple teeth (Anodontia). On the other hand abnormal initiation

may lead to development of single or multiple supernumerary teeth.

Proliferation

In this stage enhanced proliferative activity from initiation

stage results successively in bud, cup and bell stages. The

proliferative growth causes changes in proportions and size of

growing enamel organ (tooth germ).

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Any disturbance during these stages may result in defective

dental problems ranging from absence of tooth to disturbed tooth

formation.

Histo-differentiation

This succeds the proliferative stage. The cells of tooth germ

in proliferative stage undergo definitive functional and

morphologic changes and acquire their functional assignment. They

differentiate and give up their capacity to multiply as they assume

new functions. This law governs all differentiating cells.

This stage is seen at its peak in bell stage when cells of

developing enamel organ differentiate into cells of enamel and

dentin formation.

In the process of histodifferentiation under the organizing

influence of inner enamel epithelisation cells the cells of adjacent

connective tissue differentiate to form odontoblasts. These

odontoblasts lay down dentin. This process of dentin formation

causes differentiation of cells of I and E into ameloblasts. These

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ameloblasts now lay down a thin layer of enamel adjacent to dentin

formed. Thus the formation of dentin proceeds and essential to

enamel formation.

In vitamin A deficiency the ameloblasts fail to differentiate

properly. Consequently their organizing influence on adjacent

mesenchymal tissue is disturbed . this may result in formation of a

typical dentin known as osteodentin.

Morpho differentiation:

The morphologic pattern or basic form and size of the tooth

is established by morphodifferentiation i.e. by differential growth.

It is therefore proliferation is very important stage in tooth

development.

The dentino-enamel junction and dentino-enamel are

different and characteristics of particular teeth and act as blue print

pattern. In conformity with those pattern ameloblasts, odontoblasts

and cementoblasts deposit enamel, dentin and cementum

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respectively and thus give the completed tooth a particular shape

and size.

Any disturbances during the process of morphodifferentiation

may result in size and shape disturbances. Example:

- Supernumerary cusps or roots.

- Twinning of teeth.

- Loss of cusps.

- Disturbed shape – peg shaped incisors, Hutchinsons incisors.

Apposition:

Apposition is the process of deposition of matrix of hard

dental structures.

Appositional growth of enamel and dentin is a layer like

deposition an extracellular matrix which later gets calcified. This

type of growth is additive. The appositional growth is characterized

by regular and rhythmic deposition of extracellular matrix which is

of itself incapable of further growth. This process of apposition

occurs in periods of alternating rest and activity.

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Disturbances during the process of apposition and

calcification may result in enamel hypoplasia, enamel

hypocalcification etc. These result in formation of teeth.

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