Odontogenesis

Question 1

What does mean diphyodont and heterodont mean

Answer 1

DIPHYODONT
2 sets of teeth that are seen in humans (initially primary and consequently become secondary) Both following the same developmental pattern.

HETERODONT
Permanent teeth in humans are of different shapes and sizes

Question 2

Primary (Deciduous) teeth

Answer 2

Develop @ 6-8 weeks in utero
20 teeth : 2 incisors, 1 canines, 2 molars in each quadrant

Question 3

Secondary (Permanent) teeth

Answer 3

Develop at 20 weeks in utero
32 teeth: 2 incisors, 1 canines, 2 premolars and 3 molars in each quadrant.

Question 4

Origin and composition of embryonic ectomesenchyme

Answer 4

First branchial arch epithelium from ectoderm develops the embryonic ectomesenchyme

Responsible for initiating tooth development by signalling the underlying ectomesenchymal cells from mesoderm

Question 5

Importance of normal embryological development of the oro-facial complex and teeth

Answer 5

Normal embryological development is important to understand the abnormalities as well as defects.
Failure of normal embryological development affects the growth of embryogenesis which is a complicated and delicately balanced process, therefore malfunctioning produces congenital defects.

Question 6

Role of neural crest cells in the initiation of odontogenesis

Answer 6

NCC separate from the neuroectoderm.
They have the capacity to migrate and differentiate extensively with the developing embryo.

During their induction, NCC undergo an epithelial-mesenchymal transformation in the head region they assist in the formation and differentiation of most of the connective tissue (derived from ectomesenchyme)

Proper migration of NCC is essential for the development of the face and teeth.
All the tissue of the teeth (except enamel and some cementum) and it’s supporting apparatus is derived from NCC.

Question 7

Induction and it’s role in tooth development

Answer 7

Reciprocal interaction: Where cell layers ‘talk’ to one another
1st induction
Epithelial layer sends a signal to the underlying ectomesenchymal cells that induces them to condense in an area of tooth development.
Once condensation of cells take place, a signal will be sent back to the overlying oral epithelium which results in epithelium.

Question 8

Epithelial-mesenchymal interaction is responsible for

Answer 8

Inducing tooth formation (initiation of development)
Forming the right tooth in the right place (Patterning of dentition)
Determination the tooth shape ( Morphodifferentiation)
Inducing histodifferentiation ( Cells differentiate to become equipped to secrete specific tooth structure

Question 9

Tooth initiation

Answer 9

Stomotodeum (1st branchial arch) lined by ectoderm. Epithelium is anchored to the underlying ectomesenchyme.
First arch epithelium + caudal or cranial neural crest in the anterior chamber of the eye = Teeth

First indication of odontogenesis
✓ 37 days of development
✓ Primary epithelial band: A horse shoe shaped thickening (Placodes) of epithelium in the presumptive upper and lower jaw
✓ Each band gives rise to
1. Dental lamina
2. Vestibular lamina

Smaller than normal placodes = results in missing/smaller teeth
Larger than normal placodes = results in supernumerary/ larger teeth

Question 10

Patterning of the dentition

Answer 10

*Development of specific tooth types in the correct positions
* Coordinated by 2 theories
1. Field model type
✓ Factors within the ectomesenchymal tissue in distinct graded and overlapping fields that determines the position of each tooth class or family
✓ Supported by the fact that each field express different combinations of patterning homeobox genes
✓ Explains why molors only form in the posterior regions and incisors only anteriorly

2. Clone model theory
   ✓Each tooth class or family is derived from a clone of ectomesenchymal cells that are programmed by epithelium to produce teeth of a specific pattern
   ✓ Permanent molars develop from one dental lamina and seem to be a clone of the specific one

Question 11

Morphogenesis

Answer 11

Cells of the inner enamel epithelial are involved in establishing the crown pattern of the tooth
At this time they are small and low columnar with centrally placed nuclei and undergo frequent mitosis
Then the cells undergo morphologic changes and then become ameloblasts (histodifferentiation)

Question 12

Primary dental lamina

Answer 12

1. Overlying primary epithelial band sends a signal to the underlying ectomesenchymal cells
2. Condense at the site of a future tooth
3. Condensed ectomesenchyme reciprocates by sending a signal back to the overlying epithelium that responds by proliferating and growing downwards
4. Into the underlying ectomesenchyme as the primary dental lamina
   Process for each of the primary teeth that are to develop but at different times for each tooth

Question 13

Secondary dental lamina

Answer 13

Tooth germs give rise to the secondary incisors, canines and premolars that will develop from an additional lamina (secondary/ successional dental lamina) that forms on the lingual and palatal aspect of the primary tooth germ dental lamina.

Secondary lamina gives rise to secondary/permanent teeth.

5 primary teeth in each quadrant will be replaced by the first 5 permanent teeth in each quadrant i.e primary teeth 81 will be replaced by the permanent tooth 41

The three permanent molars do not have primary predecessors and develop in a different manner

Once sufficient growth of jaws has occurred, a single dental lamina burrows into the ectomesenchyme posteriorly of the last primary molar tooth

Lamina displays a backwards extension and gives off epithelial outgrowths that will give rise to the 1st, 2nd and 3rd permanent molars from this ONE lamina

Process is the same for all secondary molars, both maxillary and mandibular

Question 14

Lamina responsible for development of secondary molars

Answer 14

Secondary dental lamina

Question 15

Derivatives of the dental laminae in the adult gingiva

Answer 15

Tooth 51, 52, 53, 54, 54 becomes tooth 11, 12, 13, 14,15 in an adult from secondary dental lamina

16,17,18 come from secondary dental lamina

Question 16

Bud Stage

Answer 16

After the formation of dental lamina, the epithelial cells of the apical portion proliferate forming a bud shaped structure

Surrounding ectomesenchymal cells adjacent to it continue to condense around the bud

Question 17

Cap Stage

Answer 17

• There is an uneven proliferation of epithelial cells is the bud which result in the cap shape morphology
• This change in morphology results in a ‘cap’ of epithelial cells that sit on a ‘ball’ of condensed ectomesenchymal cells
• The cap consisting of epithelial cells is referred to as the ENAMEL ORGAN
• The ball of condensed epithelial cells in the hollow of the cap is referred to as the DENTAL PAPILLA
• The condensed ectomesenchymal cells that surround the cap and limit the dental papilla is called the DENTAL FOLLICLE

DENTAL ORGAN (TOOTH GERM) = Enamel organ+ dental papilla+ dental follicle

The enamel organ cells begin to undergo some changes
• Cells at the outer aspect of the enamel organ : Low cuboidal shape
: Outer enamel epithelium
• Cells bordering the dental papilla assume : Short columnar shape
: Inner enamel epithelium

CERVICAL LOOP is the region where the inner and outer enamel epithelium’s meet

• Some of the central cells of the enamel organ secrete glycosaminoglycans that are hydrophilic in nature and pull water into the enamel organ
• This results in the epithelial cells of the central potion of the enamel organ been pushed apart from one another
• The epithelial cells maintain their desmosomal junctions to the adjacent epithelial cells and resemble stars

STELLATE RETICULUM is the morphology of the central portion of enamel organ

Question 18

Bell Stage

Answer 18

Differentiation begins at the future cusp tip regions and spreads down the slope of the enamel organ with the time

Cells at the periphery of the bell continue to proliferate until the crown attains its full size

✓ Differentiation of cells is important to determine the morphology of the crown
✓ The number of enamel knots sh determine the number of cups that the crown will have
✓ Folding of the inner epithelial enamel also influences the final stage of the tooth
✓ Once the hard tissues of the crown are being secreted the enamel organ still covers the enamel
✓ They is reorganization of the rest of the components of the enamel organ that surround the crown of the tooth

✓ Late in the bell Stage, fragmentation of the dental lamina takes place resulting in a separation of the developing tooth germ from the overlying oral epithelium
✓ RESTS Of SERRES : dental lamina fragments that are due to apoptosis if Any remants of epithelial cells remain behind.
: Located in the gingiva of the oral cavity

                                : Have potential of becoming cysts or tumors

Question 19

Tooth root formation

Answer 19

Hers initiates odontoblast differentiation (induction) from peripheral ectomesenchymal cells of dental pulp.

Root = dentine covered by cementum
Inner and outer enamel epithelium = Hertwig’s root sheath (HERS) from the cervical loop. First component to send signals for differentiation

Root formation begins after the bell Stage as the tooth begins to erupt

# Odontoblasts begin secreting dentine and HERS begins to fragment
# Stretching of root sheath leads to fragmentation-epithelia rests of Malassez
# Induction of cementoblasts for cementum formation
# Epithelial ectomesenchymal transformation - HERS cells differentiate into cementoblasts

Question 20

Origin, composition and structure of each component of the tooth germ

Answer 20

ENAMEL ORGAN
Origin: Mesenchymal cells
Composition: The cap consisting of epithelial cells
Structure: OEE, IEE, Stratum intermedium and stellate reticulum
DENTAL PAPILLA
Origin: Neural crest cells
Composition:The ball of condensed epithelial cells in the hollow of the cap
Structure: Pulp, stellate
DENTAL FOLLICLE
Origin:condensed ectomesenchymal cells that surround the cap and limit the dental papilla
Composition:
Structure: Connective tissue surrounding tooth germ

Question 21

Explain the function and fate of all components of tooth germ

Answer 21

Dental lamina: SERRES rests in gingiva
Enamel organ: Enamel and reduced enamel epithelium
HERS: Some cementum and melassez rest in PDL
Dental papilla: Pulp and dentine
Dental follicle: Alvelor bone, cementum, and periodontal ligament

Question 22

Vascular and nerve supply of the tooth germ during development

Answer 22

Vascular supply of dental organ
• Rich in blood supply
• Branching out of vessels around tooth germ in the follicle entering papilla during the cap Stage
• Vessels increase in number during histodifferentiation and reach maximum during the crown stage
• Enamel organ is avascular receiving it’s blood supply from dental follicle and papilla vessels

•

Nerve supply to dental organ
• Pioneer fibres form a rich nerve plexus in the dental follicle around the tooth germ
• As soon as dentinogenesis (formation of dentine) starts the nerve fibres penetrate the papilla
•No fibres enters enamel organ

Question 23

Tooth root formation with specific reference to the formation of multiple rooted teeth

Answer 23

✓ If 2 tongues are present it would result in 2 roots and so on with 3
✓ Tongues of epithelium grow from the root sheath epithelium towards each other
✓ Once the entire root is formed we only have the apical entrance (apical foramen) though which the vascular and neural structure enter the tooth

Question 24

Development of accessory root canals and the clinical implications

Answer 24

✓ HERS fragments prematurely
✓ HERS will not signal for odontoblast differentiation therefore no dentine formed
✓ No signals sent to cementum formation
✓ Failure of epithelial ectomesenchymal interaction
✓ No hard tissues structure forms
Other factors
• If a blood vessel or nerve is found in the path of HERS during root formation
• Failure of fusion of the epithelial tongues (cusps)

Clinical Implications
1. Allows infection to spread from the dental pulp to the adjacent periodontal ligament
2. If a root canal is performed it provides a area door bacteria to hide and result in difficulty in being able to thoroughly clean the infected root system resulting in failure of root canal procedure

Question 25

Tooth eruption begins

Answer 25

As the root of the tooth begins to develop and the root is only completely formed after the tooth has erupted

Question 26

Odontogenesis

Answer 26

Complex process whereby teeth form from embryonic cells, grow and erupt into the mouth

Stages
✓ Initiation
✓ Bud
✓ Cap
✓ Bell (crown formation)
✓ Start of tooth eruption and root formation