The early development of tooth from six week of prenatal life. Description of different stages- bud,cap and bell stage and amelogenesis, dentinogenesis. Description of root development.

1. Early Development of
tooth
Dr. Ridwana Kawsar
BDS (CMC), BCS (Health)
MS (Conservative Dentistry &Endodontics - BSMMU)
Lecturer, Dept. of Conservative Dentistry & Endodontics
Shaheed Suhrawardy Medical College( ShSMC)
Sher-E-Bangla Nagar, Dhaka

2. Odontogenesis of the primary dentition begins between the sixth
and seventh week of prenatal development, during the embryonic
period.
The primitive oral cavity, or stomodeum, is lined by stratified
squamous epithelium called the oral ectoderm or primitive oral
epithelium.

3. • The oral ectoderm contacts the endoderm of the foregut to form
the buccopharyngeal membrane.
• 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.

4. • Primary epithelial band is a band of epithelium that has invaded the
underlying ectomesenchyme along each of the horseshoe-shaped
future dental arches.
• Most of the connective tissue cells underlying the oral ectoderm are of
neural crest or ectomesenchyme in origin.
• These cells are thought to instruct or induce the overlying ectoderm to
start tooth development, which begins in the anterior portion of what will
be the future maxilla and mandible and proceeds posteriorly

5. • At about 7th week the primary epithelial band divides into an
inner (lingual) process called Dental lamina and an outer
(buccal) process called Vestibular lamina.
• The dental laminae 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.

6. Initiation stage of odontogenesis of the primary teeth on cross section within
the developing mandibular arch. The stomodeum is now lined by oral
epithelium, with the deeper ectomesenchyme derived from neural crest cells.
A similar situation is occurring in the maxillary arch.

7. Development of the dental lamina from the oral epithelium lining the
mandibular arch where primary teeth will later form during the initiation stage,
which is surrounded by ectomesenchyme. A similar situation is also occurring
in the maxillary arch.

9. From this point, tooth development proceeds in three stages:
the bud, cap, and bell.
These terms are descriptive of the morphology of the developing
tooth germ.

10. Bud Stage
 The bud stage is represented by the first epithelial incursion into the
ectomesenchyme of the jaw.
 The epithelial cells show little if any change in shape or function.
 The supporting ectomesenchymal cells are packed closely beneath
and around the epithelial bud.
 As the epithelial bud continues to proliferate into the ectomesenchyme,
cellular density increases immediately adjacent to the epithelial
outgrowth.
 This process is classically referred to as a condensation of the
ectomesenchyme.

12. At the end of the proliferation process involving the dental lamina of
the primary dentition, both the future maxillary arch and the future
mandibular arch will each have 10 buds.
However, only proliferation of these two tissue types occurs during
this stage; no structural change occurs in the cells of either the
dental lamina or ectomesenchyme.

13. Thus the development of tooth germs is initiated, and the cells
continue to proliferate faster than adjacent cells.
In the bud stage, the enamel organ consists of peripherally
located low columnar cells and centrally located polygonal cells.
Both the dental papilla and the dental sac become more well
defined as the enamel organ grows into the cap and bell shapes

14. Cap stage :
 As the tooth bud continues to proliferate, it does not expand uniformly
into a larger sphere. Instead, unequal growth in different parts of the
tooth bud leads to the cap stage, which is characterized by a shallow
invagination on the deep surface of the bud.
 As it develops, it takes on a shape that resembles a cap, with an
outer convex surface facing the oral cavity and an inner concavity.
 Because the enamel organ sits over the dental papilla like a cap, this
stage of tooth development is known as the cap stage.

15. Outer and inner enamel epithelium
The peripheral cells of the cap stage are cuboidal, cover the convexity
of the ‘cap,’ and are called the outer enamel (dental) epithelium. The
cells in the concavity of the ‘cap’ become tall, columnar cells and
represent the inner enamel (dental) epithelium.

16. Stellate reticulum
Polygonal cells located in the center of the epithelial enamel organ,
between the outer and inner enamel epithelia, begin to separate due
to water being drawn into the enamel organ from the surrounding
dental papilla as a result of osmotic force exerted by
glycosaminoglycans contained in the ground substance.
As a result the polygonal cells become star shaped but maintain
contact with each other by their cytoplasmic process. As these star-
shaped cells form a cellular network, they are called the stellate
reticulum.
This gives the stellate reticulum a cushion like consistency and acts
as a shock absorber that may support and protect the delicate
enamel-forming cells.

18. The cells in the center of the enamel organ are densely packed and form the
enamel knot. This knot projects in part toward the underlying dental papilla, so that
the center of the epithelial invagination shows a slightly knob-like enlargement
that is bordered by the labial and lingual enamel grooves. At the same time a
vertical extension of the enamel knot, called the enamel cord occurs. When the
enamel cord extends to meet the outer enamel epithelium it is termed as enamel
septum, for it would divide the stellate reticulum into two parts. The outer enamel
epithelium at the point of meeting shows a small depression and this is termed
enamel navel as it resembles the umbilicus. These are temporary structures
(transitory structures) that disappear before enamel formation begins.

20. Dental papilla
Under the organizing influence of the proliferating epithelium of the
enamel organ, the ectomesenchyme (neural crest cells) that is partially
enclosed by the invaginated portion of the inner enamel epithelium
proliferates. The ectomesenchyme continues to increase its cellular
density. The cells are large and round or polyhedral with a pale cytoplasm
and large nuclei It condenses to form the dental papilla, which is the
formative organ of the dentin and the primordium of the pulp

21. Dental sac (dental follicle)
Concomitant with the development of the enamel organ and the dental
papilla, there is a marginal condensation in the ectomesenchyme
surrounding the enamel organ and dental papilla. Gradually, in this
zone, a denser and more fibrous layer develops, which is the primitive
dental sac , it is the precursor of the cementum, the periodontal
ligament (PDL), and the alveolar bone

23. Bell Stage
•As the invagination of the epithelium deepens and its margins continue to grow,
the enamel organ assumes a bell shape. In the bell stage, crown shape is
determined. The folding of enamel organ to cause different crown shapes is shown
to be due to differential rates of mitosis and differences in cell differentiation time.
•Four different types of epithelial cells can be distinguished bell stage of the enamel
organ. The cells form the inner enamel epithelium, the stratum intermedium, the
stellate reticulum, and the outer enamel epithelium.
•The junction between inner and outer enamel epithelium is called cervical loop and
it is an area of intense mitotic activity.

24. Inner enamel epithelium
 The inner enamel epithelium consists of a single layer of cells that differentiate
prior to amelogenesis into tall columnar cells called ameloblasts .
 The cells of the inner enamel epithelium exert an organizing influence on the
underlying mesenchymal cells in the dental papilla, which later differentiate
into odontoblasts.

25. Stratum intermedium
 A few layers of squamous cells form the stratum intermedium between the
inner enamel epithelium and the stellate reticulum.
 The cells of stratum intermedium work synergistically with cells of inner
enamel epithelium as a single functional unit and form enamel.
 It is absent in the part of the tooth germ that outlines the root portions of
the tooth which does not form enamel.

26. Stellate reticulum
 The stellate reticulum expands further, mainly by an increase in the amount
of intercellular fluid. The cells are star shaped, with long processes that
anastomose with those of adjacent cells.
 Before enamel formation begins, the stellate reticulum collapses, reducing
the distance between the centrally situated ameloblasts and the nutrient
capillaries near the outer enamel epithelium. Its cells then are hardly
distinguishable from those of the stratum intermedium. This change begins
at the height of the cusp or the incisal edge and progresses cervically.

27. Outer enamel epithelium
 The cells of the outer enamel epithelium flatten to a low cuboidal form.
 At the end of the bell stage, preparatory to and during the formation of enamel,
the formerly smooth surface of the outer enamel epithelium is laid in folds.
Between the folds the adjacent mesenchyme of the dental sac forms papillae that
contain capillary loops and thus provide a rich nutritional supply for the intense
metabolic activity of the avascular enamel organ. This would adequately
compensate the loss of nutritional supply from dental papilla owing to the
formation of mineralized dentin.

28. 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.
 First, they assume a cuboidal form; later they assume a columnar form and
acquire the specific potential to produce dentin. The dental papilla ultimately gives
rise to dental pulp, once the dentin formation begins at the cuspal tip of the bell
stage tooth germ.

29. Dental lamina Dental sac
 The dental lamina is seen to extend
lingually and is termed successional
dental lamina as it gives rise to
enamel organs of permanent
successors of deciduous teeth
(permanent incisors, canines and
premolars).
 The enamel organs of deciduous
teeth in the bell stage show
successional lamina and their
permanent successor teeth in the
bud stage.
 Before formation of dental tissues
begins, the dental sac shows a
circular arrangement of its fibers
and resembles a capsular structure.
 With the development of the root the
fibers of the dental sac differentiate
into the periodontal fibers that
become embedded in the
developing cementum and alveolar
bone.

32. PREAMELOBLAST FORMATION
 The events in the production of enamel and coronal dentin include the formation
of preameloblasts, odontoblasts and dentin matrix, ameloblast and enamel
matrix as well as the dentinoenamel junction from the basement membrane.
 After the formation of the IEE in the bell-shaped enamel organ, these innermost
cells grow even more columnar as they elongate and differentiate into
preameloblasts, lining up alongside the basement membrane.
 During this differentiation process, the nucleus in each cell moves away from the
center of the cell to the position farthest away from the basement membrane
that separates the enamel organ from the dental papilla. This movement of the
nuclei in each IEE cell occurs during cellular repolarization. In addition, the
majority of the each cell’s organelles become situated in the cell body distal to
the nucleus.

33.  In future development, the preameloblasts will induce dental papilla cells to
differentiate into dentin-forming cells (odontoblasts), and then will themselves
differentiate into cells that secrete enamel (ameloblasts).
 Thus, these initial changes to the cells occur within a presecretory stage with
distinct phases where IEE is formed during the bell stage of tooth development
(morphogenic phase) and then become preameloblasts (differentiation phase),
which much later become ameloblasts (secretory stage).

34. ODONTOBLAST
 After the IEE differentiates into preameloblasts, the outer cells of the
dental papilla are induced to differentiate into odontoblasts by the
newly stepped up preameloblasts.
 During this time, these cells also undergo repolarization, which results
in their nuclei moving from the center of the cell to a position in the cell
farthest from the separating basement membrane. These repolarized
cells also line up adjacent to the basement membrane in a mirror-
image orientation on the opposite side from the already orderly
preameloblasts.

35. Advanced bell stage
 This stage is characterized by the commencement of mineralization
and root formation.
 During the advanced bell stage, the boundary between inner enamel
epithelium and odontoblasts outlines the future dentinoenamel
junction.

36.  The formation of dentin occurs first as a layer along the future
dentinoenamel junction in the region of future cusps and proceeds pulpally
and apically.
 After the first layer of dentin is formed, the ameloblast which has already
differentiated from inner enamel epithelial cells lay down enamel over the
dentin in the future incisal and cuspal areas.

38.  The enamel formation then proceeds coronally and cervically, in all regions
from the dentinoenamel junction (DEJ) towards the surface.
 In addition, the cervical portion of the enamel organ gives rise to the
epithelial root sheath of Hertwig. The Hertwig’s epithelial root sheath (HERS)
outlines the future root and is thus responsible for the shape, length, size, and
number of roots.

39. DENTIN MATRIX FORMATION
 After the differentiation and repolarization, the odontoblasts begin
dentinogenesis, the appositional growth of dentin matrix, or predentin,
laying it down on their side of the now disintegrating basement membrane.
 Thus, the odontoblasts start their synthetic and secretory activity for the
most part some time before enamel matrix production begins.
 After the differentiation of odontoblasts from the outer cells of the dental
papilla and their formation of predentin, the basement membrane between
the preameloblasts and the odontoblasts disintegrates.

40.  This disintegration of the basement membrane allows the preameloblasts to
contact the newly formed predentin, which induces the preameloblasts to
differentiate into ameloblasts.
 After differentiation, the ameloblasts begin amelogenesis, the appositional
growth of enamel matrix, laying it down on their side of the now disintegrating
basement membrane.
 The enamel matrix is directly secreted from Tomes (tomes) process, an
angled distal part of each ameloblast that faces the fully disintegrated
basement membrane, formed from the group movement of the ameloblasts
away from the basement membrane.

41.  With the newly formed enamel matrix in contact with the predentin,
mineralization of the disintegrated basement membrane now occurs, forming
the dentinoenamel junction (DEJ), the inner junction between the dentin
and enamel tissue.

42.  Continued appositional growth of both types of dental matrix becomes regular
and rhythmic, as the cellular bodies of both the odontoblasts and ameloblasts
initially retreat away from the DEJ, forming their perspective tissue types.
 However, the odontoblasts (unlike the ameloblasts) will leave attached cellular
extensions in the length of the predentin, the odontoblastic process, as they
move away from the DEJ.
 Each odontoblastic process is contained in a mineralized cylinder, the dentinal
tubule. The maturation through mineralization of each type of matrix occurs
later and is a different process for both enamel and dentin. However, the cell
bodies of odontoblasts will remain within adjoining pulp attached by the
odontoblastic processes

46. Hertwig’s Epithelial Root Sheath and Root Formation
 On completion of the crown, the cervical loop, formed by the union of inner and
outer enamel epithelia, proliferates to form the Hertwig’s epithelial root sheath,
which determines the size and shape of the root of the tooth.
 The enamel organ plays an important part in root development by forming
HERS, which molds the shape of the roots and initiates radicular dentin
formation.
 The development of the roots begins after enamel and dentin formation has
reached the future cementoenamel junction.

47.  Hertwig’s root sheath consists of the outer and inner enamel epithelia only,
and therefore it does not include the stratum intermedium and stellate
reticulum.
 The cells of the inner layer remain short and normally do not produce enamel.

48.  When these cells have induced the differentiation of radicular dental papilla
cells into odontoblasts and the first layer of dentin has been laid down, the
epithelial root sheath loses its structural continuity and its close relation to
the surface of the root.

49.  If cells of the epithelial root sheath remain adherent to the dentin surface, they
may differentiate into fully functioning ameloblasts and produce enamel. Such
droplets of enamel, called enamel pearls, are sometimes found in the area of
furcation of the roots of permanent molars.

50.  Prior to the beginning of root formation, the root sheath forms the epithelial
diaphragm.
 The tip of the Hertwig’s epithelial root sheath proliferates horizontally between
the dentinal papilla and the dental follicle; this process partially encloses the
dental papilla and delineates the apical foramen or foramina. (The outer and
inner enamel epithelia bend at the future cementoenamel junction into a
horizontal plane, narrowing the wide cervical opening of the tooth germ).
 This proliferation is called the epithelial diaphragm.

51.  There is a pronounced difference in the development of HERS in teeth with
one root and in those with two or more roots.
 In single-rooted teeth, the epithelial diaphragm has a single opening which
guides the formation of the root, root canal, and apical foramen.
 In double-rooted teeth, the diaphragm evaginates in two predetermined
places that come together and form two openings. In three-rooted teeth,
the evagination occurs in three predetermined places to form three
openings.

52.  The differentiation of odontoblasts and the formation of dentin follow the
lengthening of the root sheath.
 At the same time the connective tissue of the dental sac surrounding the root
sheath proliferates and invades the continuous double epithelial layer dividing it
into a network of epithelial strands.
 The epithelium is moved away from the surface of the dentin so that connective
tissue cells come into contact with the outer surface of the dentin and differentiate
into cementoblasts that deposit a layer of cementum onto the surface of the dentin.

54. CEMENTOBLASTS
 As dentin is formed, the basement membrane of the inner enamel epithelium
disintegrates and the epithelial cells lose their continuity.
 The disintegration of the basement membrane and the loss of continuity of the
epithelial cells allow the mesenchymal cells from the dental follicle to penetrate
the newly deposited dentin. These mesenchymal cells differentiate into
cementoblasts.

55.  The collagen fibers followed by the ground substance elaborated by the
cementoblasts are deposited between the epithelial cells.
 The cluster of cells left behind from the epithelial root sheath migrates
toward the dental follicle and the future PDL. This cluster of epithelial cells
comprises the cell rests of Malassez.
 When some matrix production has taken place, mineralization of the
cementum starts by the spread and deposition of the hydroxyapatite crystals
from the dentin into the collagen fibers and the matrix.
 As dentinogenesis progresses in incremental phases, the apical foramen or
foramina are formed by an apposition of dentin and cementum that reduces
the size of the opening of the epithelial diaphragm.

56. Diagrams showing three stages in root development. (A) Section through tooth
germ. Note epithelial diaphragm and proliferation zone of pulp. (B) Higher
magnification of cervical region of A. (C) ‘Imaginary’ stage showing elongation of
Hertwig’s epithelial sheath coronal to diaphragm. Differentiation of odontoblasts in
elongated pulp. (D) In area of proliferation, dentin has been formed. Root sheath
is broken up into epithelial rest and is separated from dentinal surface by
connective tissue. Differen- tiation of cementoblasts.