Development of Teeth: Crown Formation

Lesson - 1

Text and pictures in this presentation are taken

from Oral Histology text books: Ten Cates and

James Avery

ObjectivesAt the end of this chapter the student should be able to describe the origin of the formative cells of teeth, the stages of tooth formation and the process of mineralization of enamel and dentin.

Sub topicsTopic 1. Formative cells of dental tissuesTopic 2. Stages of tooth formationTopic 3. DentinogenesisTopic 4. Amelogenesis

Stages of tooth development

New terms - (1)

1 Formative cells of dental tissues

Neural crest cells arise from the neural folds that develop on the neural plate They are also termed as ectomesenchyme or neuroectoderm These cells form all of the connective tissues of the face including the dental structures During the 6th week of embryonic life the ectoderm covering the oral cavity is composed of epithelial layer, two to three cells thick In the future alveolar area the epithelium proliferates and forms dental lamina

Migration of neural crest cells

The dental lamina then proliferates to form rounded or ovoid structures that protrude in the mesenchyme, called tooth buds or tooth germs The maxillary and mandibular dental lamina eventually gives rise to 20 such tooth buds ( primary dentition) between the 6th and the 8th pre natal week Tooth buds of the permanent dentition develop lingual to the deciduous buds (except for permanent molars) beginning from 5months I.U life The lingual extension of the dental lamina that gives rise to the permanent tooth buds is called the successional lamina A second lamina also develops along with the dental lamina called the vestibular lamina which later forms the oral vestibule

New terms - (1)

Tooth bud

2 Stages of tooth formation

Most organ systems like digestive system, cardiovascular system, urinary system etc are functionally complete within 9 months (at birth)Tooth formation is a continuous process that also continues long after birth According to the shape of the epithelium different stages of tooth formation can be classified: lamina, bud, cap and bell stages

1. The dental lamina

This stage is characterized by thickening of the epithelium and there are no distinguishable tooth sites

2. Bud stage This stage is marked by rounded growth of

epithelial cells of the dental lamina It is the stage of initial proliferation of epithelial

cells and adjacent mesenchymal cells Proliferation of epithelial cells result in formation of

bud-shaped structure called enamel organ In this stage the mesenchymal cells surrounding

the bud form an ectomesenchymal condensation

3. Cap stage Gradually the enamel organ gains a concave

surface towards the mesenchyme, then, it is considered to be in cap stage

In this stage the dental mesenchyme that partially surrounds the enamel organ is called the dental papilla or embryonic dental pulp

Cells that lie outside the enamel organ ( and those adjacent to the papilla) divide and grow around the enamel organ to form the dental follicle or dental sac

These three structures constitute the tooth germ and give rise to the tooth and its supporting structures

The epithelial component of enamel organ forms the enamel, dental papilla forms the dentine and pulp, the dental follicle forms the cementum, periodontal ligament and alveolar bone

Cap stage

4. Bell stage Bell stage is also called the stage of differentiation

because of the following;

a. the shape of the future tooth crown is outlined – morphodifferentiation

b. differentiation of various cells of enamel organ and dental papilla – cytodifferntiation

c. differentiation of different tissues – histodifferentiation

Enamel organ in bell stage consists of four different type of cells

1. Outer enamel epithelium

2. Inner enamel epithelium

3. Stratum intermedium

4. Stellate reticulum

Bell stage

Bell stage

1. Outer enamel epithelium - the cells that cover the convex surface of the enamel organ

• These cells function to bring nutrition and oxygen to ameloblasts and other enamel organ cells

2. Inner enamel epithelium - the cells that line the concavity of the bell-shaped enamel organ.

• This is the layer that is closest to the papilla.

• The inner enamel epithelium cells elongate and differentiate into ameloblasts. Ameloblasts are enamel-forming cells

3. Stratum intermedium – the layer adjacent to the inner enamel epithelium

• It is formed from a layer of spindle shaped cells

• They function with the ameloblasts in the mineralization of enamel

4. Stellate reticulum (star-shaped) – those cells that fill the remainder of the enamel organ

Cervical loop The area of the enamel organ where the inner and

outer enamel epithelial cells join is called the cervical loop

It is an area of active cell proliferation and lies in a region that will become the cervix of the tooth

After the crown formation the cells of cervical loop give rise to the epithelial root sheath and epithelial diaphragm

Cervical loop

Odontoblasts During the bell stage the cells in the periphery of

the dental papilla differentiate into odontoblasts Odontoblasts form the dentine and the process of

dentine formation is called dentinogenesis They are mesenchymal matrix-producing cells During dentinogenesis, the dental papilla becomes

surrounded by dentine and it is then termed the dental pulp

During this stage the dental lamina begin to degenerate and disappears. This leaves the tooth bud independent of the oral epithelium

3 Dentinogenesis

During bell stage odontoblast differentiation begins with cells near the basal lamina (layer of cells separating the enamel organ and the dental papilla) which, transform into preodontoblastsFollowing multiplication, the preodontoblasts elongate and become young differentiating odontoblastsOdontoblasts further elongate resulting in the formation of apical/ odontoblastic processesOdontoblasts then secrete matrix protein at the apical end of the cell and along its processThe secreted matrix is collagenous and not mineralized hence it is called predentin

Morphological changes in odontoblasts during dentinogenesis

As the matrix is being secreted the odontoblasts move away from the basal lamina towards the centre of the future pulpAs the odontoblasts retreat the ends of the processes maintain their positions while there is lengthening of the process at DEJThe DEJ will lie at the junction between the inner enamel epithelium and the basal laminaThe matrix that forms around the elongated cell process eventually mineralizes and the odontoblastic will lie within a dentine tubuleDentinogenesis takes place in two phases: first the formation of organic collagen matrix and second the deposition of hydroxyapatite (calcium phosphate) crystals

Predentine and dentine

The average crystals attains a size of 100 nm in length and 3 nm in widthAs each day passes predentin is formed along the pulpal boundary, the adjacent predentin that was formed during the previous day mineralizes and becomes dentineDuring the period of crown development approximately 4µm of dentine is laid down in every 24 hoursIncremental deposition and mineralization of dentine begins at the tips of the pulp horns at the DEJDentinogenesis continues until the entire crown is complete and long after the tooth begins to erupt

Odontoblasts in dentinal tubules

Odontoblasts in dentinal tubules

Formation of enamel and dentine in increments

4 Amelogenesis

The inner enamel epithelium cells differentiate into preameloblasts which later differentiate into ameloblasts to start amelogenesisPrior to secretion of enamel matrix the preameloblasts begin the process of elimination of the basal lamina which lies between them and the preodontoblastsThe ameloblasts will only become functional after the first layer of dentine is formedThe first area of the crown to be completely formed is the cusp tip and the last is the cervical regionCrowns of teeth increase in size by incremental deposition of enamel matrix

Considering the role of ameloblasts, amelogenesis can be divided into three main functional stages; presecretory, secretory and the maturation stage

Presecretory stage

During this stage the ameloblasts change polarity, develop an extensive protein synthetic apparatus, and prepare to secrete the organic matrix of enamel

Recent research has shown that secretion of enamel proteins starts even before the basal lamina is lost

Secretory stage

During this stage the ameloblasts secrete the matrix proteinsThey develop cytoplasmic extension on the apical end called Tomes processThe acquisition of Tomes process signals the beginning of the secretory stageProtein in the ameloblasts are packed in secretory granulesThe contents of secretory granules are released against the newly formed mantle dentine which, immediately becomes partially mineralized to form the initial layer of enamel. This layer does not contain enamel rods

As the first increment of enamel is formed, ameloblasts move away from the dentine surfaceWhen enamel formation begins Tomes process comprises only a proximal portion, after the initial layer is formed it develops a distal portion as an outgrowth of the proximal processTherefore secretion of enamel proteins is established from two sites, the proximal portion and the distal portion of the Tomes processSecretion from the first site (proximal portion) results in the formation of enamel partitions that surround a pit in which resides the distal portionThese partitions form the interrod enamelSecretion from the second site fills the pit which, after mineralization forms the enamel rod

Enamel Rods and Interrod enamel

Enamel rod and Interrod enamel

Enamel formed from both sites is of identical composition but differ only in orientation of crystalsThe distal Tomes process retreats leaving a narrow space that is filled with organic material forming the rod sheathEventually the ameloblasts become smaller like they were while secreting the initial enamel layer . Because rods form in relation to the distal portion of the Tomes process the final few increments of enamel do not contain any rodsThus enamel is composed of a rod-containing layer sandwiched between thin rodless initial and final layers

Initial enamel and final enamel

Pits that were filled by Tomes process

Crystal orientation of enamel

Immunocytochemical preparation showing rodsheath

Maturation stage

When secretion of the full thickness of enamel is complete, ameloblasts enter the maturation stageDuring this stage the ameloblasts undergo considerable morphological changes. They shorten and transform into cuboidal shaped cells. The adjacent stratum intermedium and stellate reticulum reorganize to form a papillary layerFinally when the enamel is fully mature the ameloblasts and the papillary layer regress and form the reduced enamel epithelium which, thereafter performs protective functionDuring the maturation stage physiochemical changes take place in the width and thickness of the pre-existing crystals seeded during the formative stage of amelogenesis

Smaller secretory stage crystals and larger maturation stage crystals

Next the ameloblasts remove water and organic material from the enamel thus forming mature enamelAmeloblasts in this stage undergo ‘modulation’ , alternating between ruffle-ended apical surfaces to smooth ended apical surfacesEventually they undergo programmed cell death - apoptosis