Morphological stages

1) Initiation of dental lamina:

At this stage neither the upper nor lower jaws show separate lip or gum regions. The separation of lips and cheeks from the gums is associated with the initial development of the teeth.

The first indication of the processes which will result in tooth formation consists of condensation of ectomesenchymal tissue immediately beneath the surface epithelium. This condensation first appears close to the midline and spreads backwards along each jaw quadrant.

Interaction between the ectomesenchymal tissue and the oral epithelium, induces epithelial proliferation to produce odontogenic epithelial islands . Potential odontogenic tissue can be identified as early as the 35^th day of development.

It seems that there are four maxillary odontogenic epithelial islands, two developing from the frontonasal process and give rise to the maxillary anterior teeth, and two from the maxillary processes and give rise to the maxillary posterior teeth. The mandibular dentition develops from only the two (right and left) initial odontogenic epithelial islands from the mandibular arch.

Fusion of the four maxillary odontogenic epithelial islands and the two mandibular islands form upper and lower horseshoe-shaped epithelial bands which correspond in position to the future dental arches. This band of epithelium- is called the Primary Epithelial Band which gives rise to two subdivisions, the vestibular lamina and the dental lamina.

The dental and vestibular laminae .

They are the subdivisions of the Primary Epithelial Band at about the 37^th day of development. – The vestibular lamina .

It is located labial and buccal to the dental lamina. The vestibule forms as a result of the proliferation of the vestibular

lamina into the ectomesenchyme. Its cells rapidly enlarge and then degenerate at the center to form a cleft which becomes the vestibule between the cheek and the tooth-bearing area. In the mid plane of each lip, the degeneration is less extensive leaving a midline fold known as the labial frenum.

Primary epithelial band. Primary epithelial band giving off

vestibular lamina (A) and dental lamina (B).

- The Dental laminae .

The main (mother, or general) dental lamina, is the epitheiiai ingrowths into the ectomesenchyme at sites corresponding to the positions of the future deciduous teeth.

The lateral dental lamina, appears during the second month in utero and deals with the initiation of the deciduous dentition (It connects between the main dental lamina and the epithelial bud of the deciduous tooth).

The successionai lamina, begins at about the fifth month intrauterine (M.I.U.) to give the dental organs of the permanent successor, lingual to the deciduous germ.

The main dental lamina extends distally to give the dental organ of the first permanent molar at the fourth M.I.U, while that of the second molar is at the first year, and that of the third molar is at four years.

2) The Bud stage

The ectomesenchymal tissue adjacent to the dental lamina appeared to clump in regions which correspond with those of the future deciduous germs. Within the dental lamina, localized proliferative activity leads to formation of series of epithelial outgrowths into the ectomesenchymal clumps. This epithelial outgrowth is called Dental organ (Enamel organ). According to the shape of this dental organ the stage of tooth development derives its name.

The tooth bud is therefore formed of the dental organ and adjacent condensation of the ectomesenchymal tissue.

Developing dental lamina.

3) The Cap stage

The ectomesenchymal cells continue to proliferate increasing the size of the ball of cells adjacent to the epithelial part of the tooth germs. This ball of condensed ectomesenchymal cells is called Dental papilla. The epithelium continues to proliferate in an attempt to surround the growing ball of ectomesenchymal cells. The dental organ spreads out to form a "cap" of cells on top of the ectomesenchymal tissue.

The condensed ectomesenchyme limiting the dental papilla and encapsulating the dental organ called the Dental follicle.

The dental organ, the dental papilla and dental follicle together constitute the tooth germ.

The epithelial dental organ gives -> Enamel.
The dental papilla gives -> Dentin,

Pulp.

The dental follicle forms -> Cementum,

Periodontal ligament Alveolar bone.

As the epithelial cap (dental organ) grows, the cells in its center begin to secrete acidic mucopolysaccharides into the intercellular spaces separating them. This substance is intensely hydrophilic and water is pulled into the intercellular spaces. This process results in the development of the Stellate reticulum.

The histological features of the cap stage

1- Dental organ consists of:

a- Outer dental epithelium b- Inner dental epithelium C- Stellate reticulum

2- Dental papilla

3- Dental sac or follicle

1 – Dental organ

a- Outer dental epithelium:

The peripheral cells line the convexity of the cap are cuboidal and called outer dental epithelium which serves to control the exchange of substances between the dental organ and its environment.

b- Inner dental epithelium:

The cells on the concavity of the cap are cuboidal and then become columnar in shape represent the inner dental epithelium.

The area of the enamel organ where the inner and outer dental epithelia join one another is called the Cervical loop. It is an area of active cell proliferation and will give rise to the epithelial diaphragm and epithelial root sheath of Hertwig.

c- Stellate reticulum:

Consists of several layers of star shaped cells between the outer and inner dental epithelia, their intercellular spaces are filled with mucopolysaccharides. It serves several functions, which include:

- Mechanical: It absorbs the pressure generated by the expanding

surrounding mesoderm to protect the developing tooth (act as Cushion).

- Nutritive: It acts as store- house for the nutritive components mainly minerals needed for the enamel forming cells (The ameloblasts).

- It keeps room for the developing enamel

A localized area of cell proliferation forms a relatively dense mass of cells near the center of the enamel organ close to the inner dental epithelium. This area is known as Enamel knot. Closely associated with the knot is a cellular condensation traversing the stellate reticulum vertically to unite with the outer dental epithelium called Enamel cord. These structures disappear with enamel formation and their function is not known. They may act as reservoir for the developing cells of the dental organ.

2- Dental Papilla

The ectomesenchymal tissue which is partly enclosed by the invaginated portion of the inner dental epithelium proliferates and condenses forming the dental papilla. The dental papilla shows active budding of capillaries and mitotic figures. The dental papilla provides also nutrition to the growing tooth germ.

A delicate basement membrane separates between the inner dental epithelium and the dental papilla. The peripheral part of the dental papilla appears free from cells but contains their cytoplasmic processes and fine argyrophilic fibers. This layer appears narrow, light and is called cell free zone or acellular zone.

3- The Dental sac or follicle

The dental organ and papilla are surrounded by the ectomesenchymal cells of the tooth sac.

Within the sac, the developing tooth is able to undergo slight positional adjustments caused by growth forces generated during crown development.

The follicle provides sufficient reinforcement to prevent distortion of the outer dental epithelium.

During the cap stage, the dental organ of the deciduous tooth germ is connected with the main dental lamina by the lateral dental lamina.

The room between the main dental lamina, the lateral dental lamina and the deciduous dental organ is called the Enamel niche.

4) The Bell Stage

The bell stage constitutes two phases.

i- The early bell stage, before any mineralization starts.

ii- The late bell stage, starts once the first layer of dentin is
layed down.

The histological feature of the Early Bell stage

As the dental papilla continues to increase to size, and the dental organ cells continue to encircle it, the invagination of the epithelium deepens and its margins continue to grow, the organ assumes a bell shape.

This bell shape is resulted due to differential growth between the inner and outer dental epithelia, in which the outer dental epithelium rate of proliferation is more.

During this stage, the inner dental epithelium folds taking the shape of the future crown pattern of the tooth.

Early bell. Stellate reticulum (SR), outer dental epithelium (ODE),

inner dental epithelium (IDE), cervical loop (CL), dental papilla (DP), secondary tooth bud (STB), and developing bone (B).

Experimental works suggest that the ectomesenchyme of the tooth bud determines the shape into which a tooth develops.

During this stage the dental organ shows:

• More histodifferentiation and morphodifferentiation than the cap stage.

• A new layer of cells is differentiated between the inner dental epithelium and the stellate reticulum. This is the Stratum intermedium which is formed of 2-3 layers of flat cells and is characterized by high activity of the alkaline phosphatase enzyme.

•

The stellate reticulum expands furthermore by increasing the intercellular fluid.

• The inner dental epithelial cells elongate and assume columnar shape and are characterized by a high glycogen content. Most of their elongation is toward the dental papilla resulting in the disappearance of the cell free zone.

• As the inner dental epithelial cells get in contact with peripheral cells of the dental papilla, they exert an organizing influence on the undifferentiated mesenchymal cells to differentiate into odontoblasts. This process is known as

induction.

The Dental Papilla shows:

• Increase in size and the capillary loops increase in number.

• The cells of the dental papilla appear as undifferentiated mesenchymal cells with few scattered collagen fibrils occupy the extracellular spaces. The inner dental epithelium induces the peripheral cells adjacent to it to differentiate into odontoblasts.

« The odontoblasts secrete the soft matrix of dentin, the pre-dentin.

At the end of the early bell stage, thickening of the basement membrane separating the odontoblasts and the inner dental epithelium occurs.

The Dental sac shows:

• More collagen fibrils occupy the extracellular spaces between the follicular fibroblasts which are generally oriented in a pattern around the dental organ and papilla.

• The inner layer is relatively more cellular and lies close to the outer dental epithelium and the papilla, while the outer layer is less cellular lies against the walls of the bony crypt.

The Histological feature of the advanced ( late ) Bell stage

The advanced bell stage is marked by the beginning of dentin formation and mineralization.

During this stage, the boundary between the inner dental epithelium and odontoblasts outlines the future dentino-enamel junction.

Later the cervical loop gives rise to the epithelial root sheath of Hertwig, which is important for root formation.

Dental organ

As the mineralization of dentin starts, the main source of nutritional supply to the dental organ from the dental papilla is cut off and the dental organ reacts to compensate and satisfy the needs for the nutritive materials from the connective tissue of the dental sac surrounding it.

a- Outer dental epithelium

Its cells flattened to a low cuboidal or flattened form and show folds. The adjacent mesenchyme of the dental sac forms papillae that contain capillary loops and enter between these folds. Thus they provide a rich nutritional supply for the intense metabolic activity of the avascular dental organ, particularly the ameloblasts.

The cells of the outer dental epithelium become specialized for the active transport of material. They develop villi, cytoplasmic vesicles and large number of mitochondria. The capillaries in contact with them show areas with very thin walls and fenestrations (a structural modification commonly found in areas of active transport).

b- Inner dental epithelium

Under the influence of the first formed dentin layer, the inner dental epithelial cells assume tall columnar shape and undergo differentiation to become active secretory cells called ameloblasts which produce enamel matrix that eventually mineralize to enamel.

It has been stressed that the enamel formation cannot begin until some dentin has been formed, likewise, it has been stated that odontoblasts differentiate under an organizing influence stemming from the cells of the inner dental epithelium. This interdependency between the two tissues is called reciprocal induction.

c- Stellate reticulum

When the first layer of dentin is laid down, the stellate reticulum layer is noticeably reduced in thickness due to loss of intercellular fluid. This makes the active developmental site of the tooth closer to the nutritional supply of the surrounding connective tissue capillary loops and provides space for the developing enamel.

d- Stratum intermedium

Its cells show strong reaction for alkaline phosphatase enzyme which is important for mineralization of enamel. Also the increased glycogen and mucopolysaccharides indicate a high metabolic activity.

These cells should be considered together with the ameloblasts as the functional unit for the production of enamel.

Dental papilla

It is designated dental pulp by the beginning of dentin formation. As dentin increases in thickness, the pulp space becomes narrower.

Dental follicle

The mesenchyme of the dental follicle shows increased condensation and becomes more cellular.

At the advanced bell stage, the dental lamina joining the tooth germ breaks up by invasion of the mesenchymal tissue of the dental sac, and by this the connection of the growing tooth germ with the oral epithelium becomes severed . Remnants of the dental lamina may persist in the gingiva and the jaw and are called epithelial rests of Serre’s (Serre’s pearls). These epithelial cells normally degenerate, or may form small cysts over the developing tooth and delay eruption.

a- Outer dental epithelium

• It regulates the transport and exchange of materials between the dental sac and dental organ.

• At the end of enamel formation, it becomes included as a part of the reduced dental epithelium which also contains the inner dental epithelium, the stellate reticulum and the stratum intermedium.

• Together with the inner dental epithelium it forms the Hertwig’s epithelial root sheath.

b- Inner dental epithelium

• The differentiation of the odontoblasts occurs under the organizing influence of the inner dental epithelium (Induction).

• It regulates the transport of materials from the dental papilla to the dental organ before dental hard tissue formation.

• It determines the morphology of the dentoenamel junction.

• Its cells become differentiated into ameloblasts which are responsible for enamel formation and maturation.

• Its cells (the ameloblasts) secrete an organic layer over the fully matured enamel called the primary enamel cuticle (Nasmyth membrane). It also becomes included as a part of the reduced dental epithelium.

• Together the inner and outer dental epithelia form the Hertwig’s epithelial root sheath which is responsible for root formation.

• Part of reduced dental epithelium.

c-Stellate reticulum

Acts as a buffer against physical forces protecting the sensitive ameloblasts. Store house for the nutritive materials.

Keeps room for the developing enamel.

Part of reduced dental epithelium.

d- Stratum intermedium

• Control diffusion of materials to the ameloblasts.

• Provides the enamel organ with enzymes needed for formation and mineralization of enamel especially alkaline phosphatases. Part of redujqedjientaLejithelium. origin

Functions of the dental papilla

Its analague (ectomesenchymal cells) initiate the primary sign of tooth development. It is the primordium of the dental pulp. It is the formative organ of dentin. The odontoblasts are differentiated from the peripheral ectomesenchymal cell layer of the dental papilla under the influence of the inner dental epithelium.

It determines the identity of the tooth. It provides nutrition to the growing tooth germ (it shows ive mitotic division and capillary budding).

Functions of the dental sac

- Its analague (the ectomesenchymal cells) initiate the primary sign of tooth development.

- It provides nourishment to the growing tooth germ especially at the late stage of tooth development.

Its outer layer constitutes condensed collagen fibers which gives protection and reinforcement to the tooth germ printing its distortion..