Development of face

DEVELOPMENT OF FACE AND
ORAL CAVITY
-1-
Oral Histology
Dr. Ibtisam Briek SENUSSI, BDS, Msc Orthodontist,
Msr & PhD Physiology and Physiopathology
Department of Oral Biology
Faculty of dentistry - Misurata

INTRODUCTION
The human somatic cell contains 46 chromosomes, called as the diploid
number. Out of which 44 are autosomal and the remaining 2 are sex
chromosomes, designated as X and Y.
The sex chromosomes in females are XX and in males are XY.
There are two series of division of somatic cells- MITOSIS and MEIOSIS.
MITOSIS produces the same number of chromosomes in the resulting
daughter cell while MEIOSIS produces half the number i.e. 23 designated as
haploid, with resultant formation of gametes .
Development begins with FERTILIZATION, the process in which the male
gamete- the sperm, and the female gamete- the oocyte, unite to form a
ZYGOTE.

GROWTH IS BROADLY SUBDIVIDED AS
a. Prenatal growth
1. Period of ovum: From time of fertilization till 1 week.
2. Period of embryo: from 2nd week till 8th week
3. Period of fetus: from 9th week onwards till birth
b. Postnatal growth
c. Maturity
d. Old age

GROWTH IS BROADLY SUBDIVIDED AS

PRENATAL GROWTH
The fertilized ovum, undergoes cleavage as it moves toward the uterine
cavity.
The cells formed are called blastomeres, which soon begin to rearrange
themselves in order to differentiate into various groups and layers.
 By the 4th day, when the zygote reaches the uterus, it is a many celled mass
called a MORULA

DIVISION OF FERTILIZED OVUM
MORULA

MORULA
As the cell mass divides, it enlarges and gains a
fluid filled cavity termed the blastocele (5th day).
 The blastocoele separates the cell into
2 parts:
 An outer cell layer, the trophoblast, and
 An inner cell mass, the embryoblast.
Blastocoels

Blastocyst
 During the second week, the cells of
the inner cell mass of the growing
blastocyst differentiate into 2 cell types:
1. Columnar shaped ectodermal cells &
2.Cuboidal shaped endodermal cells
adjacent to blastocele.
 The amniotic cavity appears between
the ectodermal cells and the overlying
trophoblast.
 Later in the developmental process,
the amnion expands, filling the entire
extra embryonic coelom.
Thus in its final form, the amnion is a free
membrane enclosing a fluid-filled space
around the embryo.
 Again, cells grow from the trophoblast
and the embryonic disc, to form a
primitive yolk sac

Notochord
 On day 15, a groove, called
the primitive streak , appears on
the surface of the midline of the
dorsal aspect of the ectoderm
of the embryonic disc.
 By day 16, a primitive knot of
cells, the Henson’s node,
appears at the cephalic end of
the primitive streak.
 This knot gives rise to the cells
that form the notochordal
process.

PROCHORDAL PLATE
 Cells from the primitive streak and the notochordal process migrate laterally between the
ectodermal and endodermal layers of the embryonic shield.
 These cells form the third germ cell layer called the mesodermal layer.
 By the end of the third week, the mesoderm migrates in a lateral direction between the
ectoderm and the endoderm, except at the anterior prochordal plate and posterior cloacal
membrane.

PROCHORDAL PLATE
The anterior plate forms the future oro-pharyngeal membrane.
 Finally, mesodermal cells of the embryonic disc migrate peripherally to join the extra
embryonic mesoderm on the amnion and yolk sac.
Anteriorly, mesodermal cells pass on either side of the prochordal plate to meet each other
in front of this region.
Mesodermal cells spread sideward's
from Primitive streak
A
K L

Formation of neural tube and neural groove
Neural tube undergoes massive expansion to form the forebrain, midbrain and
hindbrain

Folding of the Embryo & Neural Crest Cells
Head fold forms a primitive stomatodeum or oral cavity; leading to ectoderm lining the
stomatodeum and the stomatodeum separated from the gut by buccopharyngeal membrane.
Onset of folding is at 24 days and continues till the end of week 4.
Avian neural crest cells

FATE OF GERM LAYERS
 Ectodermal cells will give rise to the nervous system; the epidermis and its appendages (hair,
nails, sebaceous and sweat glands); the epithelium lining the oral cavity, nasal cavities and
sinuses; a part of the intraoral glands, and the enamel of the teeth.
 Endodermal cells will form the epithelial lining of the gastrointestinal tract and all associated
organs.
 The mesoderm will give rise to the muscles and all the structures derived from the
connective tissue(e.g., bone, cartilage, blood, dentin, pulp, cementum and the periodontal
ligament).
 The embryonic disc will soon become altered by bends and folds necessary for further
development.
Mesodermal cells spread sideward's
from Primitive streak
A
K L

DEVELOPMENT OF OROPHARYNX
 The primitive oral cavity or stomodeum
appears late in the third prenatal week as a pit
or invagination of the tissues underlying the
forebrain. This invagination appears as a result of
the growth of the forebrain anteriorly and of the
enlargement of the developing heart.
 At the deepest end of the stomodeum, the
oral ectoderm lies in close contact with the
foregut endoderm.
 The wall between the oral and pharyngeal
cavity is termed the oropharyngeal membrane,
as it separates the stomodeum from the first part
of the foregut.
 During the fourth week of intrauterine life, the
oropharyngeal membrane disintegrates to
establish continuity between the two cavities.

Head Formation
Rhombomeres
one of the first are the
occipital somites
Neural Crest Cell Migration

ORAL CAVITY
-2-
Oral Histology

DEVELOPMENT OF EARLY FACE
 The face develops from 5 mesenchymal prominences (swellings or processes) that appear in the
4th week. One Fronto-nasal prominence (formed by proliferation of mesenchyme and ectoderm
ventral to the forebrain). Two maxillary swellings and two mandibular swellings (from 1st pharyngeal
(Brachial) arch).
 These 5 prominences surround the stomodeum (primitive mouth) , cranially , laterally and
caudally
 The face develops during the 5th to 7th week of intrauterine life from 4 primordia that surround a
central depression called the central pit.
 These include the frontal process (a single cranially located process), the 2 bilaterally located
maxillary process, and the mandibular process derived from the first branchial arch.

fronto - nasal process
Stomodeum
Eye
Mandibular
Arch
Max P
Hyo-mandibular cleft

 Mouth: Primitive oral cavity: develops from: a)An ectodermal depression between fronto-nasal
prominence and the first pharyngeal arch: the stomodeum. Its floor is closed by the bucco-
pharyngeal membrane. b) An endodermal part: is the cranial end of the pharynx. The buccal
membrane degenerates during the 4 th week, i.e. the 2 parts continue together.
 Lips & gingivae: They develop as a linear ectodermal thickenings around the stomodeum, labio-
gingival laminae. They grow into mesenchyme, then degenerate forming labio-gingival grooves
separating lips from gingivae. A small area of laminae persists in median plane forming sulcus of the
lip.

 Salivary glands: Appear as epithelial buds from oral cavity.
 Parotid gland: The first to appear, early in 6th week, from oral ectoderm, near angle of
stomodeum. It forms a tube, extends into cheek’s mesoderm.
Its Proximal part forming the parotid duct;
Its distal end breaks to form the glandular alveoli.
Capsule & connective septae develop from surrounding mesoderm.
 The duct opening is carried to open inside the cheek.
 Submandibular gland: Appear late in 6th week, from an endodermal bud in floor of stomodeum
(alveolo- lingual groove). Develops in same way as parotid gland.
 Sublingual gland: appear in 8th week, from multiple endodermal buds in the alveolo-lingual
groove.

 Frontonasal process : Bilateral ectodermal thickenings above the lateral angle of stomodeum form
the nasal placodes. By the 5th weak, the nasal placodes are invaginated to form the nasal pits, thus the
nasal placodes are divided into medial and lateral nasal folds (prominences). The two medial nasal
folds fuse to form median nasal fold.
 In the 6th week, the nasal pits deepen ➪ nasal sacs, grow dorsally, separated from oral cavity by
oronasal membrane, which soon ruptures. Both cavities continue together via primitive choanae, dorsal
to 1ry palate. Later, choanae lies between nasal cavity & pharynx. Lateral wall develops 3 shelf-like
projections: Superior, middle & inferior conchae.
Development Of Nasal cavities

 In the roof of each cavity, the ectoderm shows thickened patch, olfactory epithelium, forming
receptor cells, they are ciliated bipolar neurons. Their axons form the olfactory nerves.
 Para-nasal air sinuses: -They develop after birth, except maxillary sinus which appears late in fetal life.
-They develop as lateral nasal walls, extend into their bones. -They reach mature size during puberty.
DEVELOPMENT OF Nose

DEVELOPMENT OF Nose
•  The point of contact of the epithelial covered medial nasal and maxillary processes is termed the
nasal fin.
• This vertically positioned epithelial sheet under each nostril separates the medial nasal and
maxillary processes; and when the fin disappears, the lip will fuse.
• On each side, the lateral nasal process is separated from the maxillary process by a groove
called the naso-lacrimal groove.
• This groove will eventually disappear , but before it disappears, the epithelium at its depth will
canalize , and form the naso-lacrimal duct.

Fate of the fronto-nasal process
 The lateral nasal folds form the alae of the nose.
 The nasal pits get deeper and they form the primitive nasal cavities.
The median nasal fold forms:
1. Middle of the nose and nasal septum
2. philtrum of the upper lip
3. Premaxilla

DEVELOPMENT OF UPPER LIP AND MAXILLA
 During the 6th week, the 2 medial nasal processes merge in the midline to form the intermaxillary
segment.
 This will give rise to the centre of the upper lip, the primary palate, and the part of the alveolar
process carrying the incisor teeth.
 Each maxillary process grows medially and fuses, first with the lateral nasal processes and then with
the medial nasal process.
The medial and lateral nasal processes also fuses with each other ;thus closing the nasal pits to the
stomodeum.
 Maxillary processes: The maxillary process develops as mesodermal proliferation from the 1st
pharyngeal arch. It grows ventrally and medially, compressing the medial nasal folds towards the
middle line and converting them into one median nasal fold. The maxillary processes are separated
from the lateral nasal folds by the naso-lacrimal groove.
 The lower part of the groove will form the naso-lacrimal duct while its upper part will form the
lacrimal sac.
 A palatine shelf arises from the medial aspects of each maxillary process.
 Both shelves are approximated towards each other and fuse together and with the premaxilla
forming the hard and soft palate. Thus the nasal cavity becomes separated from the oral cavity.

DEVELOPMENT OF UPPER LIP AND MAXILLA
 Fate of maxillary process
1. Cheek
2. Upper lip except the philtrum
3. Palate except the premaxilla Fronotnasal process

DEVELOPMENT OF MANDIBLE
The mandibular process appears initially as a partially divided bilateral structure but soon merges at
the median line. This process will give rise to the mandible, the lower part of the face and the body
of the tongue.
Fate of the mandibular process :
1. Lower jaw
2. Lower lip
3. Floor of the mouth

Development Of Palate
 Medial growth of the two maxillary processes leading to fusion of the two medial nasal
folds in midline ➪ intermaxillary segment ➪ philtrum of upper lip, 4 incisors & 1ry palate
(premaxilla).
 Early in 6th week, Two medial outgrowths from maxillary processes called, palatine
shelves, fuse along palatine raphe forming Secondary palate. It fuses with the 1ry palate at
the incisive foramen ➪ definitive palate. Anterior part of definitive palate ➪ hard palate,
while post part ➪ soft palate.
 Nasal septum develops from fronto-nasal prominence & medial nasal folds; fuses with
definitive palate.
 By the 6th week of development, the primitive nasal cavities are separated by a primitive
nasal septum and partitioned from the stomodeum by a primary palate.
 The formation of secondary palate starts between 7 and 8 weeks and is completed
around the 3rd month of gestation.
 Three outgrowths appear in the oral cavity: the nasal septum grows downwards from the
fronto-nasal process along the midline, and 2 palatal shelves or processes , one from each
side, extend from maxillary process towards the midline.
 The shelves are directed first downward on each side of the tongue.

 After the 7th week of development, the tongue is withdrawn from between the shelves, which now
elevate and fuse with each other above the tongue and with the primary palate.
 The septum and 2 shelves converge and fuse along the midline, thus separating the oro-nasal
cavity into oral and nasal cavities.
Development Of Palate
Mice

PALATAL SHELF ELEVATION
 This process has been presumed to take place rapidly, about as fast as the act of
swallowing, as it has never been precisely recorded.
 Several mechanisms have been proposed to account for the movement of the palatal
shelves from vertical to the horizontal position.
 The closure of the secondary palate may involve an intrinsic force in the palatine shelves
the nature of which has not been determined yet.
 The extrinsic forces derived from the tongue and jaw movements may be responsible for
this.

 For the fusion of palatine shelves to occur, elimination of the epithelial covering of the
shelves is necessary. To achieve this fusion, DNA synthesis ceases within the epithelium
some 24 to 36 hours before the epithelial contact.
 Surface epithelial cells are sloughed off as they undergo physiologic cell death to
expose the basal epithelial cells.
 These cells have the carbohydrate rich surface coat that permits rapid adhesion and
the formation of the junctions to achieve fusion of the processes.
 A midline steam is thus formed of two layers of the epithelial cells. This midline must be
removed to permit ectomesenchymal continuity between the fused process.
 The growth of the steam fails to keep pace with the palatal growth so that the steam
first thins and then breaks down into discrete islands of epithelial cells.
 The basal lamina surrounding these cells is lost and the epithelial cells transforms into
mesenchymal cells.
PALATAL SHELF FUSION

ORAL CAVITY
-3-
Oral Histology

DEVELOPMENT OF THYROID GLAND
 In the 4th week, the thyroid gland develops as a depression and epithelial thickening in
the floor of the pharynx. the thyroid gland develops between the median tongue bud
(tuberculum impar) and the copula (on day 24 of intrauterine life). It is the first endocrine
gland to develop.
 The thyroid primordium descends in the neck as a bilobed diverticulum to reach in front
of the trachea in the 7th week.
The thyroid diverticulum remains connected to the embryonic tongue by a thyroglossal
duct. This descends into the developing neck anterior to the hyoid bone and trachea. The
definitive shape and position of the thyroid gland is complete by the 7th week of intra-
uterine life.
On the fully formed tongue, the site of origin of the thyroid gland is demarcated by a small
pit, the foramen caecum.
 The thyroid gland begins to function at the beginning of the 3rd month when colloid
containing follicles appear.

Development of the tongue and thyroid gland
Foramen Caecum

DEVELOPMENT OF TONGUE
 The tongue is composed of the body which is the movable oral part and the posterior (attached)
base or pharyngeal part.
 The tongue develops from the tissues of the 1st, 2nd 3rd and 4th branchial arches and from the
occipital myotomes.
 The body of the tongue develops from 3 elevations on the ventromedial aspect of the 1st arch: a
tuberculum impar and paired lateral lingual swellings. These lateral lingual swellings rapidly enlarge,
merge with each other , and overgrow the tuberculum impar to form the oral part of the tongue.
A U-shaped sulcus develops in front and on both sides of this oral part, which allows it to be free and
highly mobile except at the region of the lingual frenum.

DEVELOPMENT OF TONGUE
 The base of the tongue develops mainly from the 3rd branchial arch. Initially, it is indicated by 2
midline elevations that appear caudal to the tuberculum impar.
 These are the copula of the 2nd arch and the large hypobranchial eminence (hypopharyngeal
eminence) of the 3rd and 4th arches.
 Later the hypopharyngeal eminence overgrows the 2nd branchial arches to become continuous
with the body of the tongue.
 The site of union between the base and body of the tongue is delineated by a V-shaped groove
called sulcus terminalis. The tip of the V present the foramen caecum.
 The occipital myotomes migrate anteriorly into the tongue during the 5th to 7th weeks.
 Later, various types of papillae differentiate in the dorsal mucosa of the body of the tongue,
whereas lymphatic tissue develop into the base of the tongue.

Tooth development
Tooth development can be divided into three overlapping phases: initiation,
morphogenesis and histogenesis. During initiation, the sites of the future teeth are
established, with the appearance of tooth germs along an invagination of the oral
epithelium called the dental lamina. During morphogenesis, the shape of the tooth is
determined by a combination of cell proliferation and cell movement. During
histogenesis, differentiation of cells (begun during morphogenesis) proceeds to give rise
to the fully formed dental tissues, both mineralized (i.e. enamel, dentine and
cementum) and unmineralized (i.e. dental pulp and periodontium).
Tooth development is characterized by complex interactions between epithelial and
mesenchymal tissues. The first histological sign of tooth development is the appearance
of a condensation of mesenchymal tissue and capillary networks beneath the
presumptive dental epithelium of the primitive oral cavity. The mesenchymal cells are
ectomesenchymal (neural crest) in origin, having migrated into the jaws from the
margins of the neural tube. Recent research on amphibians and mammals suggests
that, in addition to oral ectoderm and neural crest mesenchyme, foregut endoderm
plays a role in tooth initiation. in fact, there is evidence that the specification of dental
epithelium takes place in the oral epithelium adjacent to foregut endoderm and
above midbrain neural crest cells.

Tooth development
By the 6th week of development, the oral epithelium thickens and invaginates into the
mesenchyme to form a primary epithelial band (Fig1-A). By the 7th week, the primary
epithelial band divides into two processes: a buccally located vestibular lamina and
a lingually situated dental lamina (Fig1-B). The vestibular lamina contributes to the
development of the vestibule of the mouth, delineating the lips and cheeks from the
tooth-bearing regions. The dental lamina contributes to the development of the
teeth. To form the vestibule of the oral cavity, the cells of the vestibular lamina
proliferate, with subsequent degeneration of the central epithelial cells to produce
the sulcus of the vestibule (Fig1-C).
1. Primary epithelial band (arrowed) at the 6th week of intra-uterine life/B. The vestibular lamina and dental lamina seen at the 7th week of
intra uterine life/C. Diagram illustrating the formation of the vestibule of the oral cavity/D. The developing dental lamina (*)/ E. Model showing
the stage of tooth development by the 8th week of intra-uterine life when a series of swellings representing developing tooth germs (arrows)
develops on the deep surface of the dental lamina. (**) = vestibular fold.
*
**

Development of teeth
Further development of the dental lamina is characterized by an increase in length,
although it is not known whether this results from active invagination of the lamina or upward
proliferation of the mesenchyme. By the 8th week, a series of swellings develops on the deep
surface of the dental lamina.
It is important to appreciate that the dental lamina appears as an arch-shaped band of
tissue, which follows the line of the vestibular fold. Although, each epithelial swelling is almost
completely surrounded by a mesenchymal condensation. For descriptive purposes, tooth
germs are classified into bud, cap and early bell stages according to the degree of
morphological differentiation and histological differentiation of their epithelial 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. At
the late bell stage, hard tissues are forming and further growth of the crown is related mainly
to the deposition of enamel, the rate of cell division being reduced.
*
**
Fig -1

BUD STAGE
The enamel organ in the bud stage (Fig. 2) appears as a simple, spherical to ovoid, epithelial
condensation that is poorly morpho-differentiated and histo-differentiated. It is surrounded by
mesenchyme. The cells of the tooth bud have a higher RNA content than those of the overlying
oral epithelium, a lower glycogen content and increased oxidative enzyme activity. It would
appear that the epithelium is instructive in tooth initiation (Nevertheless, the successful
development of the tooth germ relies upon a complex interaction of the mesenchymal and
epithelial components since, should these components be separated and cultured individually,
neither will differentiate further. The epithelial component is separated from the adjacent
mesenchyme by a basement membrane.
2-Bud stage of tooth development. A = enamel organ;
B= mesenchymal condensation (Masson’s trichrome)
A
B

CAP STAGE
By the 11th week, morphogenesis has progressed, the deeper surface of the enamel organ
invaginating to form a cap-shaped structure. In the section shown in Fig-3, both maxillary and
mandibular early cap stages are shown, each enamel organ appearing relatively poorly
histodifferentiated. However, 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 external and internal enamel epithelia. In the late cap stage of tooth development
(Fig-4), by about the 12th week, the central cells of the enlarging enamel organ have become
separated (although maintaining contact by desmosomes).
Figure –[3]: Early cap stage of tooth development (arrows).
A = Meckel’s cartilage; B = developing tongue.
A
B

CAP STAGE
The cells of the external enamel epithelium remain cuboidal, whereas those of the internal
enamel epithelium become more columnar. The latter show an increase in RNA content and
hydrolytic and oxidative enzyme activity, while the adjacent mesenchymal cells continue to
proliferate and surround the enamel organ. The part of the mesenchyme lying beneath the
internal enamel epithelium is termed the dental papilla, while that surrounding the tooth germ
forms the dental follicle (stellate reticulum). A model describing the arrangement of deciduous
tooth germs at 13th weeks on the dental lamina of the lower jaw is shown in Fig-4.
Fig- 4 - Late cap stage of tooth development.
A = stellate reticulum; B = external enamel epithelium;
C = internal enamel epithelium; D = dental papilla;
E = dental follicle

EARLY BELL STAGE
By the 14th week, further morpho-differentiation and histo-differentiation of the tooth germ lead to
the early bell stage (Fig-5). The configuration of the internal enamel epithelium broadly maps out
the occlusal pattern of the crown of the tooth. This folding is related to differential mitosis along the
internal enamel epithelium. The future cusps and incisal margins are sites of precocious cell
maturation associated with cessation of mitosis, while areas corresponding to the fissures and
margins of the tooth remain mitotically active. Thus, cusp height is related more to continued
downward growth at the margin and fissures than to upward extension of the cusps. During the
bell stage, any bone resorption defects that restrict the space for development of the tooth germ
may be associated with the increased folding pattern of the internal enamel epithelium, leading
to changes in tooth shape. Consequently, spatial impediment, and the changing mechanical
forces that ensue, may be a co-factor in dental morphogenesis.
Figure - 5 - Early bell stage of tooth development.
A = inner investing layer of dental follicle;
B = outer layer of dental follicle
A
B

EARLY BELL STAGE
It is during the bell stage of development that the dental lamina breaks down and the enamel
organ loses connection with the oral epithelium. At the same time, the dental lamina between
tooth germs also degenerates. Remnants of the dental lamina may remain in the adult mucosa
Fig–6: as clumps of resting cells (epithelial pearls (of Serres)) that may contain keratin and can be
involved in the etiology of cysts.
Interposed between the enamel organ and the wall of the developing bony crypt is the
mesenchymal tissue of the dental follicle, which is generally considered to have three layers Fig-7.
The inner investing layer is a vascular, fibro-cellular condensation, three to four cells thick,
immediately surrounding the tooth germ; the nuclei of the cells tend to be elongated
circumferentially. The outer layer of the dental follicle is represented by a vascular mesenchymal
layer that lines the developing alveolus.
A
Fig-6- Epithelial pearls (of Serres) (arrows). A = Enamel space

EARLY BELL STAGE

EARLY BELL STAGE
Between the two layers is loose connective tissue with no marked concentration of blood vessels.
There is evidence that the cells of the inner layer of the dental follicle may be derived from the
neural crest. A high degree of histo-differentiation is achieved in the early bell stage (Figure - 7).
The enamel organ shows four distinct layers: external enamel epithelium, stellate reticulum, stratum
inter-medium and internal enamel epithelium. The cervical loop at the margins of the enlarging
bell-shaped enamel organ is a site of mitotic activity. Here, the central cells of the stellate
reticulum/stratum inter-medium may be the site of a stem cell niche providing cells that pass to
the internal enamel epithelium and later form ameloblast.
This may be under the control of Notch protein in the epithelium and growth factors, such as BMP4
and FGF10, in the adjacent dental mesenchyme.
A
B
C
D
E
F
G
Fig-7- A high-power view of the early bell.
A = external enamel epithelium;
B = cervical loop; C = stellate reticulum;
D = enamel cord; E = stratum inter-medium;
F = internal enamel epithelium; G = dental papilla

Late BELL STAGE
The late bell stage (appositional stage) of tooth development (Fig-8) is associated with the formation
of the dental hard tissues, commencing at about the 18th week. Dentine formation always precedes
enamel formation. In the section shown in (Fig-8), down growths of the external enamel epithelium
appear from the lingual sides of the enamel organs. In deciduous teeth, these lingual down-growths
give rise to the tooth germs of the permanent successors and first appear alongside the incisors at
about 5 months in utero. In enamel organs of permanent teeth, however, these down growths
eventually disappear. Behind the deciduous second molar, the dental lamina grows backwards to
bud off successively the permanent molar teeth.
A
Fig-8 Late bell stage (appositional stage) of tooth development. Dentine
matrix stained blue; enamel matrix stained red. A = permanent tooth.

Late BELL STAGE
The first permanent molar appears at about 4 months in utero, the tooth bud for the second
permanent molar appears about 6 months after birth, while that for the third permanent molar
appears at about 4–5 years after birth (Fig-9) provides a high-power view of a region of a tooth germ
at the late bell stage to show enamel and dentine formation commencing at the tips of future cusps
(or incisal edges). Under the inductive influence of developing ameloblasts (pre-ameloblasts), the
adjacent mesenchymal cells of the dental papilla become columnar and differentiate into
odontoblasts. The odontoblasts then become involved in the formation of predentine and dentine.
The presence of dentine then induces the ameloblasts to secrete enamel.
A C
D
B
E
Fig-9- High-power view of a region of a tooth germ at the late bell
stage to show enamel and dentine formation. A = odontoblasts; B =
ameloblasts; C = stratum intermedium; D = stellate reticulum;
E = external enamel epithelium;
dentine matrix stained green; enamel matrix stained red.

TRANSITORY STRUCTURES
During the early stages of tooth development, three
transitory structures may be seen: the enamel knot,
enamel cord and enamel niche.
ENAMEL KNOT
The enamel knot is a localized mass of cells in the centre
of the internal enamel epithelium. Characteristically, the
enamel knot forms a bulge into the dental papilla, at the
centre of the enamel organ. It was once thought that
the enamel knot played a role in the formation of crown
pattern by outlining the central fissure. However, the
enamel knot soon disappears and seems to contribute
cells to the enamel cord (see below). Although transitory,
recent studies of the enamel knot suggest it may
represent an important signalling centre during tooth
development. Unlike adjacent cells, those within the
enamel knot are non-proliferative and produce
molecules associated with signalling in other sites.
The enamel knot (A)
A

ENAMEL CORD
The enamel cord is a strand of cells seen at the early bell
stage of development, that extends from the stratum
intermedium into the stellate reticulum. When present,
the enamel cord overlies the incisal margin of a tooth or
the apex of the first cusp to develop (primary cusp).
When it completely divides the stellate reticulum into two
parts, reaching the external enamel epithelium, it is
termed the enamel septum. Where the enamel cord
meets the external enamel epithelium, a small
invagination termed the enamel navel may be seen. The
cells of the enamel cord are distinguished from their
surrounding stellate reticulum cells by their elongated
nuclei. It has been suggested that the enamel cord may
be involved in the process by which the cap stage is
transformed into the bell stage (acting as a mechanical
tie) or that it is a focus for the origin of stellate reticulum
cells.
A = enamel cord
B = enamel navel
A
B

ENAMEL NICHE
The enamel niche is seen where the tooth germ appears
to have a double attachment to the dental lamina (the
lateral and medial enamel strands). These strands
enclose the enamel niche, which appears as a funnel-
shaped depression containing connective tissue. The
functional significance of the enamel niche is unknown.
The enamel cord and the double attachment of the
tooth germ around the enamel niche were once
regarded as evidence supporting the view that the
complex crown form of mammalian teeth evolved from
fusion of a number of individual, simpler elements.
However, this view is not now accepted.
A= The enamel niche
B = lateral enamel strand
C = medial enamel strand
A
B
C

Structures atAppositional Stage
Dental follicle
Outer Dental Epith.
Stellate reticulum
Inner Dental Epith.
Pulp
Odontoblasts
Stratum intermedium

Development of face

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