development of dentition and occlusion.
2. DEVELOPMENT OF DENTITION & OCCLUSION
Final year part 1
Dept of orthodontics
The development of dentition is an important part of craniofacial growth as the formation, eruption, exfoliation and exchange of teeth take place during this period. This is an assimilation of facts, predictions, studies, in both static and dynamic situations; the factors influencing them and their clinical implications.
4. PRE-NATAL DEVELOPMENTOF TEETH
The embryonic oral cavity is lined by stratified squamous epithelium known as the oral ectoderm
Around the 6th week of intra uterine life, the infero-lateral border of maxillary arch and supero- lateral border of mandibular arch shows localised proliferation of oral ectoderm resulting in the formation of a horse-shoe shaped band of tissue called dental lamina
Dental lamina plays a important role in the development of dentition
5. Early Dental Lamina
Tongue Dental laminaVestibular lamina
The deciduous teeth are formed by the direct proliferation of the dental lamina
The permanent molars develops from the distal proliferation of the dental lamina
And permanent teeth which replace the deciduous teeth develop from the lingual extension of the dental lamina
6. In certain areas the dental lamina proliferate and forms knob like structurethat grows into underlying mesenchyma.
Each knob represents a future deciduous tooth and is called enamel organ
Enamel organ passes through a number of stages ultimately forming the teeth
Based on shape of the enamel organ develops can be divided into,
7. BUD STAGE
Differentiation of dental lamina leads to formation of round, ovoid swelling at 10 different points corresponding to future position of deciduous teeth. These are the primordia of enamel organ.
Enamel organ consists of peripherally located low columnar cells and centrally located polygonal cells.
Dental papilla : It is the area of ectomesenchymal condensation subjacent to enamel organ. Cells of dental papilla will form tooth pulp & dentine.
Dental sac: It is area of ectomesenchymal condensation surrounding the tooth bud & dental papilla. Cells of dental sac will form cementum & periodontal ligament.
8. Bud Stage
9. CAP STAGE :
Characterized by a shallow invagination of deep surface of abud.
Outer & inner enamel epithelium.
Cuboidal cells cover the convexity of the cap.
Columnar cells cover the concavity of the cap.
Polygonal cells begin to separate as more intercellular fluid is produced and forms cellular network called stellate reticulum.
Cells in center of the enamel organ are densely packed. This knot projects towards underlying dental papilla.Vertical extension of enamel knot forms enamel cord. Both the structures disappear before enamel formation begins.
Dental papilla & dental sac :
Dental papilla is for mature organ of dentine & shows active budding of capillaries.
Dental sac are important for formations of cementum & periodontal ligament.
10. Cap Stage of Tooth Development
Dental (enamel) organ
Copyright 2007, Thomas G. Hollinger, Gainesville, Fl
11. 12. BELL STAGE :
Stage of Histodifferentiation and Morph differentiation.
During histodifferentiation some cells of dental organ diffentiates into specific form and shape. This is seen inearly Bell stage.
During morph differentiation Dental organ assumes characteristic shape of the tooth. This is seen in late Bell stage.
The invagination of the epithelium deepens and its margins continue to grow and enamel organ assumes a bell shape.
During histodifferentiation cells acquire their functional assignment. Odontoblasts are differentiated from mesenchymal cells with formation of dentin the cells of inner dental epithelium transform into ameloblasts and enamel matrix is lead down opposite the dentin. Presence of dentin is absolutely essential for laying down of enamel.
Differentiation of epithelial cells are essential for differentiation of epithelial Odontoblasts and initiation of dentin formation. Future dentino enamel junction is outlined and the form of crown is established.
Tooth germ shows the following structures
13. Dental organ :
a) Outer dental epithelium: A single rows of cuboidal cells is thrown into folds and contain blood vessels at late bell stage.
b)Stellate reticulum:There is increase in intercellular fluid and layer expands. The cells assume star shape with long processes that anastomose with adjacent cells.
c)Stratum intermedium:Several layers of squamsus cells appear between stellate reticulum and inner dental epithelium and are called stratum intermedium. This layer is essential for enamel formation. It helps in calcification of enamel and is a reserve source for new ameloblasts.
d)Inner dental epithelium: This consists of single layer of cells that differentiates into tall columnar cells, the ameloblasts. They have a hexagonal shape on cross section and are 4u in diameter and 40u in height. These cells influence the underlying mesenchymal cells, which differentiates into Odontoblasts.
14. II. Dental Papilla :
It is the mesenchyme enclosed portion of the Dental organ. The peripheral cells under the influence of inner dental epithelium assume an cuboidal shape first & columnar later and are called Odon oblast, which produce dentin. The basement membrane separating the epithelial dental organ and dental papilla is called membranaperformativa which forms future dentino enamel junction.
III. Dental sac:
Before formation of dental tissues begins, the dental sac shows a circular arrangement of its fibres and resembles a capsular structure. With the development of the root the fibers of dental sac differentiates into the periodontal fibres that become embedded in the developing cementum and alveolar bone.
15. 16. Apposition
The tooth germ forms calcified tissues of the tooth, the enamel, the dentin and the cementum. There is a layer like deposition of an extra cellular matrix resulting in additive growth. There is regular and rhythmic deposition, which is incapable of further growth.
17. FORMATION OF ROOT
Root start forming after dentin formation has reached future cementoenamel junction. Both dental organ and dental papilla play part in formation of root.
Hertwig's epithelial root sheath :
*The outer and inner dental epithelium meets one another at future cervical area and is called cervical loop.
*This cervical loop forms epithelial sheath of Hertwig, which moulds the shape of the root and initiates dentin formation.
- The root sheath consists of only outer and inner dental epithelium.
19. The inner layer of cells remains short and do not produceenamel. These cells induce the differentiation of cell of dental papilla into Odontoblasts, which lay a layer of dentin. At the same time the continuity of Hertwig's sheath is destroyed due to infiltration of connective tissue and the root sheath breaks up into small strands of epithelium called epithelial rests of Molassez. 20. While the coronal part of the sheath degenerates, the apical part continues to grow in length and aid in lengthening of root. 21. The cells of dental sac differentiate into cementoblasts, which lay cementum over the outer surface of the dentin in root portion. At the same time precollagenous fibers appear between cementoblasts and become continuous with outer surface of dentin. They become collagenous and are transformed into cementoid tissue, which calcifies to form cementum.