Crown formation

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Crown formation

  1. 1. Development of Teeth: Crown Formation Lesson - 1
  2. 2. Text and pictures in this presentation are takenfrom Oral Histology text books: Ten Cates and James Avery
  3. 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. 4. Stages of tooth development
  5. 5. 1 Formative cells of dental tissuesNeural 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. 6. Migration of neural crest cells
  7. 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. 8. Tooth bud
  9. 9. 2 Stages of tooth formationMost 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. 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. 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. 12. Cap stage
  13. 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 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
  14. 14. Bell stage
  15. 15. Bell stage
  16. 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 and differentiate 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 the ameloblasts in the mineralization of enamel
  17. 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. 18. Cervical loop
  19. 19. 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 pulpDuring this stage the dental lamina begin to degenerate and disappears. This leaves the tooth bud independent of the oral epithelium
  20. 20. 3 DentinogenesisDuring bell stage odontoblast differentiation begins withcells near the basal lamina (layer of cells separating theenamel organ and the dental papilla) which, transform intopreodontoblastsFollowing multiplication, the preodontoblasts elongateand become young differentiating odontoblastsOdontoblasts further elongate resulting in the formationof apical/ odontoblastic processesOdontoblasts then secrete matrix protein at the apicalend of the cell and along its processThe secreted matrix is collagenous and not mineralizedhence it is called predentin
  21. 21. Morphological changes in odontoblasts during dentinogenesis
  22. 22. As the matrix is being secreted the odontoblasts moveaway from the basal lamina towards the centre of thefuture pulpAs the odontoblasts retreat the ends of the processesmaintain their positions while there is lengthening of theprocess at DEJThe DEJ will lie at the junction between the innerenamel epithelium and the basal laminaThe matrix that forms around the elongated cell processeventually mineralizes and the odontoblastic will lie withina dentine tubuleDentinogenesis takes place in two phases: first theformation of organic collagen matrix and second thedeposition of hydroxyapatite (calcium phosphate) crystals
  23. 23. Predentine and dentine
  24. 24. The average crystals attains a size of 100 nm in lengthand 3 nm in widthAs each day passes predentin is formed along the pulpalboundary, the adjacent predentin that was formed duringthe previous day mineralizes and becomes dentineDuring the period of crown development approximately4µm of dentine is laid down in every 24 hoursIncremental deposition and mineralization of dentinebegins at the tips of the pulp horns at the DEJDentinogenesis continues until the entire crown iscomplete and long after the tooth begins to erupt
  25. 25. Odontoblasts in dentinal tubules
  26. 26. Odontoblasts in dentinal tubules
  27. 27. Formation of enamel and dentine in increments
  28. 28. 4 AmelogenesisThe inner enamel epithelium cells differentiate intopreameloblasts which later differentiate into ameloblaststo start amelogenesisPrior to secretion of enamel matrix the preameloblastsbegin the process of elimination of the basal lamina whichlies between them and the preodontoblastsThe ameloblasts will only become functional after thefirst layer of dentine is formedThe first area of the crown to be completely formed is thecusp tip and the last is the cervical regionCrowns of teeth increase in size by incrementaldeposition of enamel matrix
  29. 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. 30. Secretory stageDuring this stage the ameloblasts secrete the matrixproteinsThey develop cytoplasmic extension on the apical endcalled Tomes processThe acquisition of Tomes process signals the beginning ofthe secretory stageProtein in the ameloblasts are packed in secretorygranulesThe 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. 31. As the first increment of enamel is formed, ameloblastsmove away from the dentine surfaceWhen enamel formation begins Tomes process comprisesonly a proximal portion, after the initial layer is formed itdevelops a distal portion as an outgrowth of the proximalprocessTherefore secretion of enamel proteins is established fromtwo sites, the proximal portion and the distal portion of theTomes processSecretion from the first site (proximal portion) results inthe formation of enamel partitions that surround a pit inwhich resides the distal portionThese partitions form the interrod enamelSecretion from the second site fills the pit which, aftermineralization forms the enamel rod
  32. 32. Enamel Rods and Interrod enamel
  33. 33. Enamel rod and Interrod enamel
  34. 34. Enamel formed from both sites is of identical compositionbut differ only in orientation of crystalsThe distal Tomes process retreats leaving a narrow spacethat is filled with organic material forming the rod sheathEventually the ameloblasts become smaller like theywere while secreting the initial enamel layer . Because rodsform in relation to the distal portion of the Tomes processthe final few increments of enamel do not contain any rodsThus enamel is composed of a rod-containing layersandwiched between thin rodless initial and final layers Initial enamel and final enamel
  35. 35. Pits that were filled by Tomes process
  36. 36. Crystal orientation of enamel
  37. 37. Immunocytochemical preparation showing rodsheath
  38. 38. Maturation stageWhen secretion of the full thickness of enamel iscomplete, ameloblasts enter the maturation stageDuring this stage the ameloblasts undergo considerablemorphological changes. They shorten and transform intocuboidal shaped cells. The adjacent stratum intermediumand stellate reticulum reorganize to form a papillary layerFinally when the enamel is fully mature the ameloblastsand the papillary layer regress and form the reducedenamel epithelium which, thereafter performs protectivefunctionDuring the maturation stage physiochemical changestake place in the width and thickness of the pre-existingcrystals seeded during the formative stage of amelogenesis
  39. 39. Smaller secretory stage crystals and larger maturation stage crystals
  40. 40. Next the ameloblasts remove water and organic materialfrom the enamel thus forming mature enamelAmeloblasts in this stage undergo ‘modulation’ ,alternating between ruffle-ended apical surfaces to smoothended apical surfacesEventually they undergo programmed cell death -apoptosis

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