Organogenesis

  Organogenesis

I) Introduction

Teeth are natural components of the craniofacial organization of vertebrates and more particularly of mammals.

Each tooth constitutes an exceptional model of development, cyto-differentiation and spatial organization. Teeth begin to form in the embryo at the sixth week of its development; the cells of the oral epithelium divide to form the tooth buds, ultimately leading to the formation of the dental organs.

II) Origin of the dental system

The embryology of the dental system is inseparable from cephalic embryogenesis because teeth arise from neurilation. Teeth develop from ectoderm and mesoderm, enamel arises from the ectoderm of the oral cavity, and all other tissues arise from the associated mesenchyme.

1. the ectoderm of the stomodeum: it is formed of deep cubic and superficial fusiform cell layers; this epithelium is separated from the underlying ectomesenchyme by the basement membrane. This ectomesenchyme is responsible for inducing the formation of the enamel organ.
2. Ectomesenchyme: derived from neural crest cells (NCC) is responsible for the rapid volumetric increase of the facial buds; in these, the branches of the trigeminal nerve surrounded by Schwann cells are already present .

III) Formation of odontogenic mesenchyme

It is around the 5th-6th week of intrauterine life that we observe, from the ectomesenchyme of the neural crests surrounding the trigeminal, cell migrations which will, among other things, regroup in the presumptive region of the future dental arches, due to numerous mitoses, the presumptive zone of odontogenesis will present a higher cell density than the neighboring zones. This condensation is called odontogenic mesenchyme. During the initial stages of development of the dental germ, the odontogenic mesenchyme will have an inductive role, in fact, in opposition to the other regions of the oral epithelium, only the epithelium which surmounts the odontogenic mesenchyme is competent to respond to this induction. The formation of the odontogenic mesenchyme begins in the incisor region and continues progressively towards the future molar region, with a slight precocity in the mandible compared to the maxilla.  

IV) formation of the primitive blade:

The increase in mitoses at the level of the germinal base of the epithelium under the induction of the odontogenic mesenchyme results in an increase in the number of superficial bases, a slight epithelial protuberance is observed on the lingual surface of the maxillary and mandibular nasal buds, this stage is called the salient wall, it is a transitional stage, in fact the germinal base of the epithelium quickly invaginates into the mesenchyme, we then speak of the primitive lamina or plunging wall, from the external slope of the wall an epithelial expansion called the vestibular lamina and another internal one is the dental lamina itself.

1. Evolution of the vestibular blade:

Gradually the vestibular blade will become hollowed out as a result of the cytolysis of the central cells, leading to the formation of a groove or vestibule which separates the labio-jugal regions from the maxillary territories.

2. Evolution of the dental blade:

Facing the mesenchymal papillae from the odontogenic mesenchyme, the dental lamina itself is in a palatal position in the maxilla and in a lingual position in the mandible relative to the vestibular lamina.

V) Formation of dental placodes or dental germs:

The primary dental lamina will give rise to 10 lacteal dental germs per arch, then apoptosis (programmed death) of the latter and appearance of the secondary dental lamina which is at the origin of the permanent teeth.

VI) evolution of the dental germ (dental morphogenesis)

   A) Bud stage  :

The dental lamina hypertrophies in relation to the mesenchymal papilla following mitotic activity. This is the stage of the dental bud.

   B) Young cupule stage:

The germ evolves rapidly on the morphological level.                                                     

The epithelial cap gradually takes the form of a cup as it increases in surface area, and we see a grouping of cells in the center of the epithelium: this is the primary enamel knot (PEK) which is a transient formation, so from the outside we describe:

-an external adamantine epithelium (EAE).

-an internal adamantine epithelium (IAE)

The latter is separated from the mesenchymal cells by a basement membrane; and between the two are filling cells.

-In the middle the primary enamel node (NEP) which is responsible for the axes of development of the dental germ.

C) The stage of the aged cupule:

-disappearance of the NEP

-The filler cells will dissociate into a star shape and will be united by the desmosomes, these cells become the Stellate Reticulum (ER)

-the EAI cells elongate.

So we have three 3 epithelial cell layers:

EAE—RE—EAI;

*in the ecto-mesenchymal (EM) part: the ecto-mesenchymal (EM) part takes the name of mesenchymal papilla, the vascularization is more organized and it has the beginning of innervation.

*the peripheral part: the follicular sac is organized into cellular layers.

D) The bell stage:

1-The epithelial part  :

-A fourth cellular layer called Stratum Intermedium is inserted between the ER and the EAI;

-Secondary enamel nodules (NES) appear in the areas of future cusps. Their number will depend on the number of cusps (as many cusps as secondary nodes). They will control the differentiation phenomena responsible for dental morphogenesis.

-The EAI cells will further lengthen to become future ameloblasts at the origin of enamel.

-EAI and EAE on the periphery of the bell are juxtaposed to give the HERTWIG sheath which will sink into the ectomesenchyme; this structure will be at the origin of root formation.

-so at this stage we find four layers: EAE-RE-SI-EAI and which will be at the origin of the formation of the enamel organ

1. External Adamantine Epithelium (EAE): formed of cuboidal or flattened cells, regular with a regular arrangement, mitoses are numerous in the vicinity of the reflection zone, at the base of the enamel organ, they gradually decrease as one approaches the top of the latter. At this level the cells practically do not multiply.

The capillaries coming from the dental sac ensure the latter’s nutritional supply and metabolism.

2. The stellate reticulum (SR): it occupies the center of the enamel organ, the cytoplasm of these cells is compressed by the presence of a gel that occupies the intercellular spaces, the latter are considerably widened, while the cytoplasmic ends are stretched by the desmosomes that connect the cells together.
3. The stratum intermedium (SI): is located between the ER and the EAI, formed of 3 to 5 contiguous cell layers linked together by numerous desmosomes, the cells have a very flattened shape, the cytoplasm presents numerous enzymatic activities.
4. Internal adamantine epithelium (IAE): its cells are slightly flattened compared to those of the external epithelium which they resemble.

These cells represent the future pre-ameloblasts.

      2-The mesenchymal papilla  : the ectomesenchymal cells which face the EAI cells will differentiate into odontoblasts at the origin of the coronal dentin.

3-The follicular sac  : originating from the periodontal mesenchyme, which surrounds the enamel organ and the mesenchymal papilla, it will have the following role:

– to protect the dental germ during the later stages of its development; – to provide the enamel organ with the nutritional elements necessary during amelogenesis through its vascularization.

 – it provides the cellular and fibrillar elements inducing the formation of cementum, the desmodont and the alveolar bone .  

VII) Formation of the crown

A) amelogenesis

Enamel formation occurs in three stages: precursor stage, secretory stage and maturation stage.

1. Precursor stage: this is the stage of differentiation of ameloblasts which is the consequence of interactions between epithelial cells and peripheral fibroblasts of the mesenchymal papilla.
2. Secretion stage: it corresponds to the secretion of precursors of the organic matrix of the enamel then its mineralization by the deposition of hydroxyapatite crystals.
3. Maturation stage: the enamel will form in successive layers, from the enamel-dentine junction to the surface of the tooth, until the eruption of the tooth into the oral cavity, the crown will remain covered by a cap of epithelial cells whose metabolism is zero, it is the reduced adamantine organ.
4. Dentinogenesis: 

It is done in several successive stages:

1. The differentiation of odontoblasts which occurs from the peripheral fibroblasts of the mesenchymal papilla, conditioned by the overlying presence of pre-ameloblasts.
2. Synthesis and secretion of dentin matrix precursors by odontoblasts and it is composed of proteins and proteoglycans.
3. Maturation of dentin.
4. Mineralization of dentin.

VIII) Root formation

A) Formation of the HERTWIG epithelial sheath:

As soon as amelogenesis is complete, mitotic activity intensifies at the reflection zone; the two epithelial layers (EAI and EAE) attached to each other lengthen by bending towards the central axis of the germ. This cell proliferation leads to the formation of a bi-stratified epithelial sheath that stretches in the apical direction: this is the HERTWIG epithelial sheath; this sheath is interposed between the dental papilla and the internal layer (or “investing layer”) of the dental follicle that encapsulates the dental germ.

This sheath has a circular opening at its apical end which constitutes the primary foramen and through which vascular and nervous elements will penetrate into the future dental pulp.

The orientation of this sheath will condition the shape and number of dental roots for each germ.

B) Formation of root dentin and pulp  :

Root dentin is formed as the Hertwig sheath grows apically and progressively induces the differentiation of odontoblasts.

The dentin is applied in a centripetal direction and is organized around the cytoplasmic extensions of the odontoblasts and the non-mineralized part of the mesenchymal papilla (coronal and radicular) will give rise to the dental pulp.

Apex closure occurs approximately 1 year after tooth eruption.

C) Formation of cementum  :

Cementogenesis occurs at the periphery of the root dentin, begins in the cervical area and continues apically to the apex of the tooth, as the Hertwig sheath disintegrates. Cementoblasts from undifferentiated mesenchymal cells will develop an organic matrix including collagen fibers, then mineralize the latter along the root surface.

D) Formation of alveolar bone  :

On the external face of the follicular sac, we observe the beginning of cellular differentiation processes leading to the formation of osteoblasts responsible for the synthesis and secretion of the organic matrix of the bone and then its mineralization.

At the end of its construction, the alveolar bone will join with the maxillary bone bases.

E) Formation of the desmodont

During root formation, the fibers of the follicular sac will reorient themselves, the ends of the fibers will be incorporated into the cementum on the one hand, and into the alveolar bone on the other hand, whose ligamentous insertions are put in place as the processes of cementogenesis and osteogenesis progress.

The last part to form is the apical portion . 

XI) CONCLUSION

Embryology is, in fact, the basis that allows us to understand the entire anatomical organization of a region.

Organogenesis

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Organogenesis