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1. 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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2. 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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3. 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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4. 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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5. 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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6. 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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7. In the 8th
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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8. 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
8th
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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9. 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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10. 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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11. 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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12. 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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13. 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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14. 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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15. 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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16. incisal edges, and proceeds cervically as the formation of enamel
progresses.
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17. 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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18. 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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19. 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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20. 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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21. 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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22. 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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23. 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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24. 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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25. 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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26. 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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27. 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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