Oral Histology and

embryology

Lec. 9

tooth development
The primitive oral cavity, or stomodeum, is lined by stratified
squamous epithelium called the oral ectoderm or primitive oral
epithelium. The oral ectoderm contacts the endoderm of the foregut to
form the buccopharyngeal membrane. Most of the connective tissue cells
underlying the oral ectoderm are of neural crest or ectomesenchyme in
origin. These cells are thought to be the anterior portion of what will
instruct or induce the overlying ectoderm to start tooth development,
which begins in the future maxilla and mandible and proceeds
posteriorly.
Primary epithelial band:
Two or three weeks after the rupture of the buccopharyngeal
membrane, when the embryo is about 6 weeks old, certain areas of basal
cells of the oral ectoderm proliferate more rapidly than do the cells of the
adjacent areas. This leads to the formation of the Primary epithelial
band which is a band of epithelium that has invaded the underlying
ectomesenchyme along each of the horseshoe-shaped future dental
arches .At about 7th week the primary epithelial band divides into an
inner (lingual) process called Dental lamina and an outer (buccal)
processcalled Vestibular lamina.
Dental lamina:
The dental lamina serve as the primordium for the ectodermal portion
of the deciduous teeth. Later, during the development of the jaws, the
permanent molars arise directly from a distal extension of the dental
lamina.
Along the leading edges of the lamina 20 areas of enlargement next
appear, which are the forming buds of the 20 primary teeth, and the
leading edge of the lamina continues to develop the 32permanent tooth
buds.
Successional tooth buds form the permanent dentition lingual to the
buds of the primary predecessors. Permanent molars develop posterior to
the primary molars, and the general dental lamina grows posteriorly to
form the permanent molar buds. The last teeth to arise from dental lamina
are the third molars, which developed in about 4 years after birth.
The dental lamina is thus functional in developing the 52 teeth from the
sixth prenatal week until 4 years after birth. So the activity of dental
lamina extends over a period of about five years and disintegrates
completely or remains as remnants in the gingiva and the jaw and they
are called epithelial rests of Serres( Serres’ pearls).
Vestibular lamina:
Labial and buccal to the dental lamina in each dental arch, another
epithelial thickening develops independently and somewhat later. It is the
vestibular lamina, also termed the lip furrow band. It subsequently
hollows and forms the oral vestibule between the alveolar portion of the
jaws and the lips and cheeks.
Physiological phases in tooth development
Tooth development can be divided into the following overlapping phases,
which are:
1- Initiation : During this phase, the sites of the future teeth are
established with the appearance of tooth germs along an invagination of
the oral epithelium called dental lamina.
2- Prolifertion: During this phase, Proliferative growth causes regular
changes in the size and proportions of the growing tooth germ
3- Histodifferentiation: During this phase, differentiation of cell (begun
during morphogenesis) proceed to give rise to the fully formed dental
tissues, both mineralized ( such as enamel, dentin and cementum) and
unminaralized ( such as pulp and periodontal ligament).
4- Morphodifferentiation : During this phase, the shape of the teeth are
determined by a combination of cell proliferation and cell movement.
5- Apposition: During this phase, the deposition of dental hard tissue
occur in the tooth like dentin and enamel.
Morphological stages of tooth development
For descriptive purposes, tooth germs are classified into bud, cap and
bell stages according to the degree of morphodifferentiation and
histodifferentiation of their epith. components (enamel organs). Leading
up to the late bell stage, the tooth germ changes rapidly both in its size
and shape, the cells are dividing and morphogenetic processes are taking
place.
Bud stage
Round or ovoid swellings at ten different points arise from dental
lamina in each jaw, corresponding to the future position of deciduous
teeth. They are the primordial of enamel organs (the tooth buds), thus the
development of the tooth germ is initiated, proliferation of cells is still
faster than adjacent ectomesenchymal cells. These epith. condensation is
poorly morphodifferentiated and histodifferentiated. The cells of the tooth
bud have a higher RNA content than those of the overlying oral epith., a
lower glycogen content and an increased oxidative enzyme activity.
Enamel organ histology in this stage consist of peripherally located
low columnar cells and centrally located polygonal cells. The enamel
organ is separated from the adjacent ectomesenchyme by basement
membrane. Many cells of the tooth bud and the surrounding mesenchyme
undergo mitosis. As a result of the increased mitotic activity and the
migration of neural crest cells into the area the ectomesenchymal cells
surrounding the tooth bud condense. The area of ectomesenchymal
condensation immediately subjacent to the enamel organ is the dental
papilla. The condensed ectomesenchyme that surrounds the tooth bud and
the dental papilla is the dental sac. Both the dental papilla and the dental
sac become more well defined as the enamel organ grows into the cap
and bell shapes
Cap stage
As the epith. bud continues to proliferate into the ectomesenchyme,
morphogenesis has progressed, the deeper surface of the enamel organ
invaginating to form a cap-shaped structure. Although, enamel organ
appearing relatively poorly histodifferentiated, a greater distinction
develops between the more rounded cells in the central portion of the
enamel organ and the peripheral cells which are becoming arranged to
form the outer and inner dental epith..
In the late cap stage of tooth development, the central cells of the
enlarging enamel organ have become separated (through maintaining
contact by desmosomes), the intercellular spaces containing significant
quantities of glycosaminoglycans. The resulting tissue is termed the
stellate reticulum, although it is not fully developed until the late bell
stage. The cells of the outer enamel epith. remain cuboidal, where as
those of inner e. epith. become more columnar and show an increase in
RNA content, and hydrolytic and oxidative enzyme activity.
The adjacent ectomesenchymal cells are continue to prolipherat and
surround the E. organ. The part of the ectomesenchyme lying beneath the
inner E. epith.. is called dental papilla. The ectomesenchymal tissue
surrounding both enamel organ and dental papilla is called dental sac or
dental follicle.
Transitory structures:
During the early stages of tooth development, three transitory
structures may be seen, which are E. knot, E. cord and E.niche.
1- Enamel knot: It’s a localized mass of cells in the center of the
inner E. epith. Once thought that E. knote played a role in the formation
of crown pattern by outlying the central fissure.
2- Enamel cord: It’s avertical extension of the
E. knot. Its termed E. septum when E. cord extend from E. knot to the outer E.
epith.

3- Enamel niche: Its an apparent structure in the histologic section, created because
the dental lamina is a sheet rather than a single strand and often contains a concavity
filled with C.T. A section through this arrangement creates the impression that the tooth
germ has a double attachment to the oral epithelium by two separate strands.

Bell stage
The dental organ is bell-shaped during this stage, the majority of its cells are
called stellate reticulum because of their star-shaped appearance. The bell stage is
known for the histodifferentiation and morphodifferentiation that takes place. The
bell stage is divided into the early bell stage and the late bell stage .
Characteristic features of bell stage:
1- Tooth crown assumes its final shape (Morphodifferentiation), and the cells that
will make the hard tissues of crown (Ameloblasts and odontoblasts) acquire their
distinctive phenotype (Histodifferentiation).

6
2- The enamel organ is bell-shaped and it composed of 4 distinctive cell layers.
Cuboidal cells on the periphery of the enamel organ are known as outer enamel
epithelium (OEE). The columnar cells of the enamel organ adjacent to the enamel
papilla are known as inner enamel epithelium (IEE). The cells between the IEE and
the stellate reticulum(SR) form a layer known as the stratum intermedium(SI).
3- The rim of the enamel organ where the outer and inner enamel epithelium join is
called the cervical loop.
4- Other events occur during the bell stage, the dental lamina disintegrates, leaving
the developing teeth completely separated from the epithelium of the oral cavity.

A-early bell stage

High degree of histodifferentiation is achieved in the early bell stage. The enamel organ
show four distinict layers :
1- Inner enamel epithelium(IEE):
It consists of a single layer of cells that differentiate prior to amelogenesis into
columnar cells, the ameloblasts. These elongated cells are attached together by
junctional complex laterally and by desmosomes to stratum intermedium .The IEE
cells rich in RNA. The IEE. is separated from the peripheral ells of dental papilla
by a basement membrane and cell free zone about 1-2 µm wide.
The function of cells of this layer first is exert an organizing influence on the
undelying ectomesenchymal cells of dental papilla to differentiate into
odontoblasts, and then it differentiate into ameloblasts which form enamel. Also
when the enamel formation of the crown completed, the IEE with OEE formed the
structure from cervical loop which called Hertwig’s epithelial root sheath which
formed root.
2- Stratum intermedium (SI):
This first appears at the early bell stage and as layers of flattened cells lying
7
between the IEE and stellate reticulum. The cells of this layer consists of several
layer of squamous cells, closely attached by desmosomes and gab junction.
The function of cells of stratum intermedium is important in the mineralization of
the enamel during amelogenesis because it characterized by high degree of alkaline
phosphatase enzyme. Also the cells of this layer concern in the transport of
materials to and from the IEE which later differentiate to ameloblasts.
3. The stellate reticulum(SR):
The intercellular spaces become fluid filled, presumably related to osmotic
effects arising from the high concentration of glycos-aminoglycans. In addition ,
the cells also contain alkaline phosphatase. But have only small amounts of RNA
and glycogen.
The cells are star-shaped with bodies containing prominent nuclei and many
branching processes. The cells of this layer have numerous tonofilaments and few

8
endoplasmic reticulum and mitochondria which present within the cytoplasm. The
desmosomes and gap junctions are present between the cells.
The main function of stellate reticulum is a mechanical one. This relates to the
protection of the underlying IEE against physical disturbance and to the
maintenance of tooth shape. The hydrostatic pressure generated within the stellate
reticulun is in equilibrium with that of the dental papilla, allowing the proliferative
pattern of the IEE cells to determine crown morphogenesis, however, a change in
either of these pressures might lead to change in the outline of the IEE and this
could be important for crown morphogenesis.
4- The outer enamel epithelium (OEE)
As its name suggests, this forms the outer layer of cuboidal cells which limits
the enamel organ. It is separated from the surrounding ectomesenchymal tissue of
dental sac or follicle by a basement membrane 1-2µm thick, which , at the
ultrastructural level(under electron microscope), corresponding to basal lamina and
hemidesmosomes.
The OEE cells contain large, centrally placed nuclei. Ultrastructurally, they
contain small amounts of the intracellular organelles associated with protein
synthesis (e.g. endoplasmic reticulum, Golgi complex, mitochondria) and they
contact each other via desmosomes and gap junctions.
The function of OEE is thought to be involved in the maintenance of the shape of
the enamel organ and in the exchange of substances between the enamel organ and
the environment. At the advance bell stage, when dentin lay down, the formerly
smooth surface of the outer enamel epithelium is laid in folds. Between the folds,
adjacent mesenchyme of the dental sac forms papillae that contain capillary loops
and thus provide nutritional supply for the intense metabolic activity of the
avascular enamel organ during enamel formation.

9
Dental papilla:
The dental papilla is enclosed in the invaginated portion of the enamel organ.
Before the inner enamel epithelium begins to produce enamel, the peripheral cells
of the mesenchymal dental papilla differentiate into odontoblasts under the
organizing influence of the epithelium. The dental papilla ultimately gives rise to
dental pulp, once the dentin formation begins at the cuspal tip of the bell stage
tooth germ.
The basement membrane that separates the enamel organ and the dental papilla just
prior to dentin formation is called the membrana preformativa.
Dental sac (d.S.):
Before formation of dental hard tissue begins, the d.S. shows a circular
arrangement of its fibers and it resemble a capsular structure. With the
development of the root, the fibers of d.S. differentiate into periodontal ligament
fibers that become embedded in the cementum and alveolar bone.

B -Advance (late) bell stage or crown stage:

Hard tissues, including enamel and dentin, develop during advance bell stage.
This stage is also called the crown, or apposition stage, by some researchers.
Important cellular changes occur at this time. In prior stages, all of the IEE cells
were dividing to increase the overall size of the tooth bud, but rapid dividing,
called mitosis, stops during the crown stage at the location where the cusps of the
teeth form. The first mineralized hard tissues form at this location is dentin. At the
same time, the IEE cells change in shape from cuboidal to columnar and become
pre-ameloblasts. During this stage the boundary between inner E. epithelium and
odontoblasts outlines the future D.E.J. (dentino enamel junction). In addition, the

10
basal margin of the E. organ (cervical loop) gives rise to the Herwig’s epithelial
root sheath.

Function of dental lamina:

First the functional activity of the dental lamina is concerned with the initiation of
the entire deciduous dentition that begins at the 6 weeks of intra-uterine life of
embryo.
Second it deals with the initiation of the successors of the deciduous teeth.. It is
preceded by the growth of the free end of the dental lamina (successional lamina),
lingual to the enamel organ of each deciduous tooth, and occurs from about the 5th
month in embryo for the permanent central incisors to 10 months of age for the
second premolar.
Third is the extension of the dental lamina distal to the enamel organ of the second
deciduous molar and the formation of permanent molar tooth germs.
Fate of dental lamina
1- Dental lamina is functional in developing 52 teeth from 6 prenatal weeks until 4
years after birth (development of third permanent tooth).
2- The dental lamina degenerates by mesenchymal invasions in late bell stage.
3- Developing tooth lose its connection with dental lamina.
4- Sometimes remnants of dental lamina remains in the jaws as epithelial rests of
Serres( Serres’ pearls) .

Nutrition and tooth development

Nutrition has an effect on the developing tooth. Essential nutrients for a healthy
tooth include calcium, phosphorus, and vitamins A, C, and D.

Calcium and

11
phosphorus are needed to properly form the hydroxyapatite crystals(minerals), and
their levels in the blood are maintained by Vitamin D.
Vitamin A is necessary for the formation of keratin, and Vitamin C is for collagen.
Fluoride is incorporated into the hydroxyapatite crystal of a developing tooth and
makes it more resistant to demineralization and subsequent decay.
Deficiencies of these nutrients can have a wide range of effects on tooth
development:
 In situations where calcium, phosphorus, and vitamin D are deficient, the
hard structures of a tooth may be less mineralized.
 A lack of vitamin A can cause a reduction in the amount of enamel
formation.
 Fluoride deficiency causes increased demineralization when the tooth is
exposed to an acidic environment, and also delays remineralization.
 Furthermore, an excess of fluoride while a tooth is in development can lead
to a condition known as fluorosis.

Developmental disturbances of teeth:

1- Anodontia: is a complete lack of tooth development. Anodontia is very rare,
most often occurring in a hereditary condition called ectodermal dysplasia.
2- Hypodontia (congenital missing tooth or teeth): It is one of the most common
developmental abnormalities. The absence of third molars is very common,
followed in prevalence by the second premolar and lateral incisor.
3- Hyperdontia(supernumerary teeth): It is believed to be associated with the
remanent of dental lamina or epithelial rest of Serres.
4- Dilaceration is an abnormal bend found on a tooth, and is nearly always
associated with trauma that moves the developing tooth bud.

12
5- Regional odontodysplasia is rare, but is most likely to occur in the maxillary
anterior teeth. The enamel, dentin, and pulp of teeth are affected, so the teeth are
very brittle. On radiographs the teeth appear more radiolucent than normal, so they
are often described as "ghost teeth".
6- Amelogenesis imperfect: is hereditary condition characterized by a defect in
dental enamel formation. Teeth are often free of enamel, small, misshapen, and
tinted brown.
7- Dentinogenesis imperfecta: is hereditary condition characterized by a defect in
either dentin matrix formation or mineralization of dentin.
8- Natal and neonatal teeth : Natal teeth are present at the time of birth. Neonatal
teeth will erupt during the first 30 days after birth. Natal teeth are three times more
common than neonatal teeth.
9- Gemination: arises when 2 teeth developed from one tooth bud, as a result
patient have extra tooth.

13
14
 Bell stage

Vestibular lamina Dental lamina disintegra

Stellate reticulum Permanent successor

Stratum intermedium OEE

IEE Cervical lope

Cervical lope

Dental papilla

15
 IEE
SR
OEE SI

DS

DP

16