Crown formation

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Text of Crown formation

  • 1.Development of Teeth: Crown FormationLesson - 1

2. Text and pictures in this presentation are takenfrom Oral Histology text books: Ten Cates and James Avery 3. ObjectivesAt the end of this chapter the student should be ableto describe the origin of the formative cells of teeth,the stages of tooth formation and the process ofmineralization of enamel and dentin.Sub topicsTopic 1. Formative cells of dental tissuesTopic 2. Stages of tooth formationTopic 3. DentinogenesisTopic 4. Amelogenesis 4. Stages of tooth development 5. 1 Formative cells of dental tissuesNeural crest cells arise from the neural folds that developon the neural plate They are also termed as ectomesenchyme orneuroectoderm These cells form all of the connective tissues of the faceincluding the dental structures During the 6th week of embryonic life the ectodermcovering the oral cavity is composed of epithelial layer, twoto three cells thick In the future alveolar area the epithelium proliferatesand forms dental lamina New terms - (1) 6. Migration of neural crest cells 7. The dental lamina then proliferates to form rounded orovoid structures that protrude in the mesenchyme, calledtooth buds or tooth germs The maxillary and mandibular dental lamina eventuallygives 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 tothe deciduous buds (except for permanent molars)beginning from 5months I.U life The lingual extension of the dental lamina that gives riseto the permanent tooth buds is called the successionallamina A second lamina also develops along with the dentallamina called the vestibular lamina which later forms theoral vestibule New terms - (1) 8. Tooth bud 9. 2 Stages of tooth formationMost organ systems like digestive system, cardiovascularsystem, urinary system etc are functionally completewithin 9 months (at birth)Tooth formation is a continuous process that alsocontinues long after birth According to the shape of the epithelium different stagesof tooth formation can be classified: lamina, bud, cap andbell stages 10. 1. The dental laminaThis stage is characterized by thickening of the epithelium and there are no distinguishable tooth sites2. 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 11. 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 12. Cap stage 13. 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 ofdifferent tissues histodifferentiation Enamel organ in bell stage consists of four differenttype of cells 1. Outer enamel epithelium 2. Inner enamel epithelium 3. Stratum intermedium 4. Stellate reticulum 14. Bell stage 15. Bell stage 16. 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 cells2. 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 anddifferentiate into ameloblasts. Ameloblasts are enamel-forming cells3. Stratum intermedium the layer adjacent to the inner enamel epithelium It is formed from a layer of spindle shaped cells They function with theameloblasts in themineralization of enamel 17. 4. Stellate reticulum (star-shaped) those cells that fill the remainder of the enamel organCervical 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 18. Cervical loop 19. Odontoblasts During the bell stage the cells in the periphery of thedental papilla differentiate into odontoblasts Odontoblasts form the dentine and the process ofdentine formation is called dentinogenesis They are mesenchymal matrix-producing cells During dentinogenesis, the dental papilla becomessurrounded by dentine and it is then termed the dentalpulpDuring this stage the dental lamina begin to degenerateand disappears. This leaves the tooth bud independentof the oral epithelium 20. 3 DentinogenesisDuring bell stage odontoblast differentiation begins withcells near the basal lamina (layer of cells separating theenamel organ and the dental papilla) which, transform intopreodontoblastsFollowing multiplication, the preodontoblasts elongateand become young differentiating odontoblastsOdontoblasts further elongate resulting in the formationof apical/ odontoblastic processesOdontoblasts then secrete matrix protein at the apicalend of the cell and along its processThe secreted matrix is collagenous and not mineralizedhence it is called predentin 21. Morphological changes in odontoblasts during dentinogenesis 22. As the matrix is being secreted the odontoblasts moveaway from the basal lamina towards the centre of thefuture pulpAs the odontoblasts retreat the ends of the processesmaintain their positions while there is lengthening of theprocess at DEJThe DEJ will lie at the junction between the innerenamel epithelium and the basal laminaThe matrix that forms around the elongated cell processeventually mineralizes and the odontoblastic will lie withina dentine tubuleDentinogenesis takes place in two phases: first theformation of organic collagen matrix and second thedeposition of hydroxyapatite (calcium phosphate) crystals 23. Predentine and dentine 24. The average crystals attains a size of 100 nm in lengthand 3 nm in widthAs each day passes predentin is formed along the pulpalboundary, the adjacent predentin that was formed duringthe previous day mineralizes and becomes dentineDuring the period of crown development approximately4m of dentine is laid down in every 24 hoursIncremental deposition and mineralization of dentinebegins at the tips of the pulp horns at the DEJDentinogenesis continues until the entire crown iscomplete and long after the tooth begins to erupt 25. Odontoblasts in dentinal tubules 26. Odontoblasts in dentinal tubules 27. Formation of enamel and dentine in increments 28. 4 AmelogenesisThe inner enamel epithelium cells differentiate intopreameloblasts which later differentiate into ameloblaststo start amelogenesisPrior to secretion of enamel matrix the preameloblastsbegin the process of elimination of the basal lamina whichlies between them and the preodontoblastsThe ameloblasts will only become functional after thefirst layer of dentine is formedThe first area of the crown to be completely formed is thecusp tip and the last is the cervical regionCrowns of teeth increase in size by incrementaldeposition of enamel matrix 29. Considering the role of ameloblasts, amelogenesis can bedivided into three main functional stages; presecretory,secretory and the maturation stagePresecretory stageDuring this stage the ameloblasts change polarity, developan extensive protein synthetic apparatus, and prepare tosecrete the organic matrix of enamelRecent research has shown that secretion of enamelproteins starts even before the basal lamina is lost 30. Secretory stageDuring this stage the ameloblasts secrete the matrixproteinsThey develop cytoplasmic extension on the apical endcalled Tomes processThe acquisition of Tomes process signals the beginning ofthe secretory stageProtein in the ameloblasts are packed in secretorygranulesThe contents of secretory granules are released againstthe newly formed mantle dentine which, immediatelybecomes partially mineralized to form the initial layer ofenamel. This layer does not contain enamel rods 31. As the first increment of enamel is formed, ameloblastsmove away from the dentine surfaceWhen enamel formation begins Tomes process comprisesonly a proximal portion, after the initial layer is formed itdevelops a distal portion as an outgrowth of the proximalprocessTherefore secretion of enamel proteins is established fromtwo sites, the proximal portion and the distal portion of theTomes processSecretion from the first site (proximal portion) results inthe formation of enamel partitions that surround a pit inwhich resides the distal portionThese partitions form the interrod enamelSecretion from the second site fills the pit which, aftermineralization forms the enamel rod 32. Enamel Rods and Interrod enamel 33. Enamel rod and Interrod enamel 34. Enamel formed from both sites is of identical compositionbut differ only in orientation of crystalsThe distal Tomes process retreats leaving a narrow spacethat is filled with organic material forming the rod sheathEventually the ameloblasts become smaller like theywere while secreting the initial enamel layer . Because rodsform in relation to the