1. Tooth Development (Odontogenesis)

2. Dentition Primary dentition – develops during prenatal period
20 teeth
Permanent dentition – develops as the jaw grows and matures
32 teeth
period in between – during the preteen years
mixed dentition period

3. THERE ARE MULTIPLE STAGES IN TOOTH DEVELOPMENT – but three major ones actually
initiation stage – 6th to 7th week
bud stage – 8th week
cap stage – 9th to 10th weeks
bell stage – 11th to 12th weeks
apposition stage – vaires per tooth
maturation stage – varies per tooth

4. Tooth formation first signs of formation – day 11
thickening of the epithelium where tooth formation will occur on the 1st branchial arch
signals – earliest mesenchymal markers for tooth formation are the Lim-homeobox genes (Lhx-6 and Lhx-7) – expressed as early as day 9 in the neural crest cells of the tooth region
positions of the teeth are controlled by signals from the oral epithelium – role of FGF-8 and Pax-9 – determine the position of the tooth germs
more than 90 genes have been identified in the oral epithelium, dental epithelium and dental mesenchyme!! so exact signaling mechanisms remain unclear

5. Tooth formation: Initial stages
-involves the physiologic process of induction
-induction of ectodermal tissues by the developing mesenchyme
-mechanisms remain unknown
-at the 6th week the stomatodeum is lined with ectoderm – outer portion is the
oral epithelium
-this gives rise to the primary epithelial bands
-also is a developing mesenchyme which contains neural crest cells that have migrated
to the area
-a basement membrane separates the developing oral epithelium and mesenchyme
Primary epithelial bands: Horseshoe-shaped bands that appear approximately around the 37th day of development, one for each jaw.
-there are two subdivisions: vestibular lamina and dental lamina
-the dental lamina develops a series of epithelial outgrowths - grow deep into the mesenchyme
-develops in the future spot for the dental arches
-will form the midline for these arches
-arches then form posteriorly from this point
-the ingrowths represent the future sites
for each deciduous tooth
-the vestibular lamina cells rapidly enlarge and then degenerate – forms a cleft that becomes the vestibule of the oral cavity
The initiation of tooth formation starts around the 37th day of gestation.

6. Bud Stage Tooth bud Oral epithelium Mesenchyme
marked by the incursion of epithelium into the mesenchyme
period of extensive proliferation and growth of the dental lamina
forms into buds or oral masses that penetrate into the mesenchyme
each tooth bud is surrounded by the mesenchyme
buds + mesenchyme develop into the tooth germ and the associated tissues of the tooth
this developing tooth forms from both the ectoderm and mesenchyme and from neural crest cells that have migrated into the mesenchyme
Tooth bud
Oral epithelium
Mesenchyme

7. Cap Stage
characterized by continuation of the ingrowth of the oral epithelium into the mesenchyme.
tooth bud of the dental lamina proliferates unequally in different parts of the bud
forms a cap shaped tissue attached to the remaining dental lamina
looks like a cap sitting on a ball of condensing mesenchyme
occurs for the primary dentition (during the fetal period)
this stage marks the beginning of histodifferentiation (differentiation of tissues)
the tooth germ also begins to take on form – start of morphodifferentiation
a depression forms in the deepest part of each tooth bud and forms the cap or enamel organ (or dental organ) – produces the future enamel (ectodermal origin)
below this cap is a condensing mass of mesenchyme – dental papilla – produces the future dentin and pulp tissue (mesenchymal origin)
the basement membrane separating the dental organ and the dental papilla becomes the future site for the dentinoenamel junction (DEJ)
remaining mesenchyme surrounds the dental/enamel organ and condenses to form the dental sac or the dental follicle

8. Cap stage
-together the enamel organ + dental papilla + dental follicle is considered the developing tooth germ or tooth primordium
-these primordium will be housed in the developing dental arches and will develop into the primary dentition

9. Bell Stage
Continuation of histodifferentiation and morphodifferentiation
cap shape then assumes a more bell-like shape
differentiation produces four types of cells within the enamel/dental organ
1. inner enamel epithelium
2. outer enamel epithelium
3. stellate reticulum
4. stratum intermedium
during the bell stage the dental lamina is separated from the dental organ
the dental papilla undergoes differentiation and produces two types of cells
1. outer cells of the DP – forms the dentin-secreting cells (odontoblasts)
2. central cells of the DP – forms the primordium of the pulp
dental sac increases its collagen content and differentiates at a later stage than the EO and DP

10. Differentiation of the Enamel/Dental organ
outer enamel epithelium (OEE) – cuboidal shape
protective barrier during enamel production
may also be called the outer dental epithelium
very little cytoplasm
cells are separated from the dental follicle by a basement membrane
inner enamel epithelium (IEE)
short, columnar cells
differentiates into the enamel secreting cells = ameloblasts
separated from the dental papilla below it by a basement membrane also
cells accumulate large amounts of glycogen
may also be called the inner dental epithelium
the IEE and OEE are continuous
region where they connect – curved rim of the EO = cervical loop
stellate reticulum
star-shaped cells in many layers
center of the enamel organ
forms a network = reticulum
supports production of enamel
stratum intermedium
inner layer of compressed flat to cuboidal cells
very high levels of the enzyme alkaline phosphatase
OEE
IEE
cervical loop

11. Bell Stage -the cells in the center of the enamel organ begin to
synthesize and secrete
GAGs
-this pulls water into the EO
-increasing amount of fluid
in the EO forces the central
cells apart
-however, they remain
connected via cellular
processes which makes them
star shaped = stellate ret.
B = inner dental epithelium (inner enamel epithelium)

12. Bell stage – early crown formation
the dental papilla is separated from the enamel organ by a basement membrane
immediately below this BM is a region called the acellular zone
this is where the first enamel proteins will be laid down
the dental lamina begins to break up into discrete islands of epithelial cells (epithelial pearls)– separates the oral epithelium from the developing tooth
these pearls may form cysts and delay eruption or they may develop into supernumerary teeth
the IEE completes its folding and you can begin to identify the shape of the future crown pattern

13. Tooth development so far

14. Cap and Bell stages & Permanent teeth
during the cap stage the development of the permanent dentition begins – anterior teeth
the primordia for these teeth appears as an extension off the developing dental lamina
penetrates into the mesenchyme lingual to the primary primordium
its site of origin is called the succesional dental lamina
these permanent teeth are called succedaneous teeth (anterior teeth and the premolars)
teeth that form with the primary tooth buds (primary predecessors)
permanent molars are non-succedaneous - they are formed by posterior proliferation of the dental lamina.

15. Appositional stage secretion of enamel, dentin and cementum
these tissues are initially secreted as a matrix that is partially calcified – serves as a framework for later calcification
time period varies
multiple inductions occur between the ectodermal tissues of the enamel organ and the mesenchymal tissues of the dental papillae and dental sac
these inductions are crucial for the production of enamel, dentin and cementum
these interactions are mediated by the basement membrane found in between these ectodermal and mesenchymal tissues

16. Maturation stage
characterized by the completion of calcification

17. Ameloblasts and Odontoblasts
the cells of the IEE assume a more columnar shape or they elongate
differentiate into pre-ameloblasts
this differentiation is characterized by the repolarization of these PAs – movement of the nucleus away from the basement membrane
this repolarization is critical to the differentiation of the PAs
continued differentiation and maturation results in the formation of ameloblasts
the pre-ABs induce the cells of the dental papilla to differentiate also
odontoblasts
differentiation by the mesenchyme of the dental papilla
occurs after differentiation of pre-ABs begins
results because the pre-ABs induce differentiation of the mesenchymal cells also
also undergo repolarization – mirror image of the pre-ABs (see Figure 6-12 and 6-13)
after differentiation – the ODs then start dentinogenesis
begin to deposit predentin on the side of their basement membrane – forms a layer immediately below the BM and above the cells (figure 6-13)
therefore dentin formation begins before enamel synthesis
explains why dentin is thicker than enamel

18. At 1 the epithelium is separated from the dental papilla by an acellular zone.
At 2 the cells of the inner dental epithelium have elongated, and the acellular zone begins to be eliminated as odontoblasts differentiate from ectomesenchymal cells in the tooth pulp.
At 3 the odontoblasts retreat toward the center of the pulp, leaving behind formed dentin.
At 4 the cells of the inner dental epithelium, now ameloblasts, begin to migrate outward and leave behind formed enamel.
before dentin forms – cells of the EO receive blood supply from vessels of the dental lamina
as dentin forms, it cuts of this papillary source of blood/nutrients
this causes a drastic reduction in the amount of nutrients that reach the EO
but the ABs require extensive nutrients to form enamel – stellate reticulum collapses and invagination of the OEE – this brings in blood supply from peripheral vessels found outside the tooth

19. Dentinoenamel junction
after OD differentiation and the initiation of dentinogenesis – the BM between the pre-ABs and ODs disintegrates
this allows direct contact between the pre-ABs and ODs – results in the completion of pre-AB differentiation to mature ABs
ABs then begin amelogenesis – apposition of enamel matrix
replaces the disintegrating BM
each ameloblast forms a tapered portion that faces the disintegrating BM - called a tome or Tome’s process
upon contact of the enamel matrix and dentin – the disintegrating BM begins to mineralize – forms the dentinoenamel junction or DEJ
the ODs form cellular process as they retreat toward the dental papilla (figure 6-14) that penetrate the forming predentin = dentinal tubules
mineralization of the developing dentin and enamel is distinct for each type of tissue
the cell bodies of the ODs remain in the pulp tissue
the cell bodies of the ABs participate in tooth eruption and will disappear shortly after

20. Timetable for tooth development
Entire primary dentition initiated between 6 and 8 weeks of embryonic development.
Successional permanent teeth initiated between 20th week in utero and 10th month after birth
permanent molars between 20th week in utero (first molar) and 5th year of life (third molar)

21. ROOT FORMATION
takes place as the crown is completely shaped and the tooth begins to erupt
therefore the tooth forms from the “top down” – i.e. crown to root
root formation is through the formation of a cervical loop
the CL is the most cervical portion of the enamel/dental organ – two layers consisting of IEE and OEE
the CL begins to grow down into the dental sac – elongates and moves away from the dental papilla
it forms a Hertwig's root sheath
rim of the sheath = epithelial diaphragm – encloses the developing primary alveolar foramen
also grows down to encompass all but the basal portion of the pulp
this sheath shapes the root and induces dentin formation in the root area by the ODs of the dental papilla
– ensures that it is continuous with the crown dentin
this sheath lacks the stellate reticulum and stratum intermedium
is capable of differentiating into ODs BUT NOT ABs
A, The root is beginning to form as an extension of the inner and outer dental epithelia in the cervical loop region (circles) which form a bilayered structure called Hertwig’s epithelial root sheath.
B. formation of dentin by odontoblasts above the root
sheath

22. Root Dentin The root of the tooth is composed by dentin and cementum
dentin forms when the outer cells of the dental papilla are induced to differentiation into ODs
similar to what occurs at the crown area
influenced by the IEE of Hertwig’s root sheath
the ODs then undergo dentinogenesis and secrete predentin
after dentin formation – the BM disintegrates along with the Hertwig’s sheath

23. Cementum and Pulp formation
cementogenesis in the root area also occurs upon degradation of the H. root sheath
the degradation allows contact of the dental sac cells with the dentin surface – induces the formation of cementoblast cells
the CBs cover the root dentin and undergo cementogenesis – laying down cementoid
the CBs do NOT leave cellular processes within the cementum but many CBs become entrapped in the forming cementum
these entrapped CBs are called cementocytes
only upon mineralization of the cementoid can it be called cementum
the region of contact between cementum and root dentin = dentinocemental junction or DCJ
while the cementum is forming - the central cells of the dental papilla form the pulp (Chapter 13)

24. Periodontal ligament
the surrounding tissues of the tooth also develop as the crown and root form
the mesenchyme of the dental sac condenses to form the periodontal ligament
forms adjacent to the new cementum
involves synthesis of collagen and bundling into fibers
ends of these fibers insert into the outer layer of cementum and surrounding alveolar bone
the cells of the disintegrating H. root sheath develop into discrete islands of epithelial cells
become epithelial rests of Malassez (figure 6-22)
cells become located in the mature periodontal ligament
no known function
they can be identified in the periodontal ligament and are responsible for the development of radicular cysts.

25. Multirooted teeth
anterior teeth, premolars and molars all begin as a single root – root trunk
root of the posterior teeth divides from the trunk into the correct number of root branches
differential growth of the H. root sheath results in the division of the root trunk into two or three roots

26. Primary tooth eruption
eruption takes place in chronological order
involves active eruption – vertical movement of the tooth
as opposed to passive eruption – receding of the gingiva
how it occurs is not understood
root growth, hormonal action, contractile collagen, vascular pressure??
several stages
after enamel apposition into the crown area of each primary and permanent tooth, the ABs place an acellular dental cuticle on the new enamel’s surface
as the tooth erupts it is still covered with a layer of amelobasts and the remaining layers of the enamel organ
these layers become compressed as the tooth moves – forming a reduced enamel epithelium (REE) or reduced dental epithelium
the REE fuses with the oral epithelium lining the oral cavity (figure 6-28)
the REE produces enzymes which disintegrates the central portion of this fused tissue – results in an epithelial tunnel through which the tooth erupts
this disintegration results in an inflammatory response – interpreted as the teething response
as the tooth erupts, the portion of the epithelium covering the crown pulls back and exposes the crown

27. Primary tooth eruption
-the epithelium remains attached to the tooth at the cervical portion
-i.e. as the tooth pierces the oral epithelium, the cells of the
reduced dental epithelium and the oral epithelium form the initial junctional epithelium (thin dotted line)
-this will eventually form the dentogingival junction

28. Permanent tooth eruption
as the succedaneous permanent teeth develop below the primary teeth, the primary tooth is exfoliated
eruption is lingual to the roots of the primary teeth
exception is the maxillary incisors which move to a more facial position as they erupt
eruption process is the same for the primary teeth – formation of an epithelium lined tunnel for eruption
the process is also similar for the nonsuccedaneous teeth except no primary tooth is shed