Odontogenesis (1)

odontogenesis

1. ODONTOGENESIS PRIYANKA. SHETTY

2. CONTENTS INTRODUCTION NEURAL CREST PRIMARY EPITHELIAL BAND DENTAL LAMINA VESTIBULAR LAMINA EPITHELIAL-MESENCHYMAL INTERACTIONS GENES EXPRESSED DURING TOOTH DEVELOPMENT

3. STAGES IN DEV OF TOOTH ROOT FORMATION FORMATION OF SUPPORTING TISSUES CLINICAL IMPLICATIONS CONCLUSION RECENT ADVANCES

4. In human,20 primary and 32 permanent teeth develop from the interaction of oral epithelial cells and underlying mesenchymal cells. Each developing tooth grows as an anatomically distinct unit, but the basic developmental process is similar for all teeth. INTRODUCTION:

5. neural crest Group of cells-separated from neuroectoderm -capacity to migrate& differentiate extensively within embryo. Structures-spinal sensory ganglia, sympathetic neurons, Schwann cells, pigment cells& meninges. Avian embryo-crest of neural folds-name Mammalian embryo-lateral aspect of neural plate .

6. H&N-imp role. Differentiate-most of the CT. CT elsewhere-mesoderm: ‘ mesenchyme ’. Head- ‘ ectomesenchyme ’-origin from neuroectoderm . Proper migration-essential- dvp of face& teeth. All the tissues of teeth & supporting app ( exc enamel)- dvp directly from NC. Depletion prevents proper dvp.

7. The primitive oral cavity or stomadeum is lined by stratified squamous epithelium called the ORAL ECTODERM. The oral ectoderm contacts the endoderm of the foregut to form the buccopharyngeal membrane. At about 27 th day of gestation this membrane ruptures and the primitive oral cavity establishes a connection with the foregut. Most of the connective tissue cells underlying the oral ectoderm are neural crest or ectomesenchymal in orgin . These cells are thought to instruct or induce the overlying ectoderm to start tooth development,which begins in anterior portion of what will be future maxilla and mandible and proceeds posteriorly .

8. PRIMARY EPITHELIAL BAND After 37 days of development, a continuous band of thickened epithelium forms Roughly horse shoeshaped Each band of epithelium called primary epithelial band gives rise to DENTAL LAMINA which forms first VESTIBULAR LAMINA A key feature of initiation of tooth development is formation of localized thickenings or placodes within primary epithelial bands.

9. DENTAL LAMINA : o o 6 weeks old basal cells of oral ectoderm proliferates This leads to the formation of DENTAL LAMINA which is a band of epithelium that has invaded the underlying ectomesenchyme along each of horse shoe shaped future dental arches. Dental lamina primodium for the ectodermal portion of deciduous teeth

10. A-DENTAL LAMINA B-VESTIBULAR LAMINA Permanent molars arise from distal extension of dental lamina. Development of first permanent molar is initiated at 4 th month in utero . 2 nd molar at first year after birth 3 rd molar at 4 th or 5 th years

11. The successors of decidous teeth lingual extension of the free end of dental lamina successional lamina and develops from 5 th month in utero to tenth month of age

12. FATE OF DENTAL LAMINA Total activity of dental lamina period of 5 yrs degenerate . D L may be still active in 3rd molar region after it has disappeared elsewhere. Remnants persist as epithelial pearls or islands within the jaw as well as in gingiva (Epithelial rests of serre / Cell rests of Serre ) Proliferate odontogenic tumours or cysts

13. VESTIBULAR LAMINA Buccal ext of Pri epi band VESTIBULAR LAMINA OR LIP FURROW BAND. Proliferation of vestibular lamina into the ectomesenchyme soon after formation of dental lamina VESTIBULE V shaped Separates the cheek and lip from tooth bearing areas

14. EPITHELIAL MESENCHYMAL INTERACTION

15. GENES EXPRESSED DURING TOOTH DEVELOPMENT Barx Bar H1 homologue in vertebrates(TF) Bmp Bone morphogenic proteins(SP) Dlx Distaless homologue in vertebrae(TF) Fgf Fibroblast growth factor(SP) Gli Glioma -associated oncogene homologue. Hgf Hepatic growth factor(SP) Lef Lymphoid enhances binding factor(TF) Lhx Lim- homeobox domain gene(TF) Msx Msh -like genes in vertebrae(TF)

16. Otlx Otx -related homeobox gene(TF). Pax Paired box homeotic gene(TF). Pitx Transcription factor named for its expression in the pituitary gland Ptc Patched cell-surface cell-surface receptor for sonic hedgehog( Shh ). Shh Sonic hedgehog(SP). Slit Homologue to dorsophila slit protein(SP). Smo Smoothed PTC coreceptor for Shh . Wnt Wingless homologue in vertebrates.(SP) TF –transcription factor SP- secretory proteins

17. INITIATION OF TOOTH DEVELOPMENT TOOTH PATTERNING REGIONALISATION OF DENTAL ECTODERM

18. Initiation of tooth development: Odontogenesis is first initiated by factors resident in the 1 st arch epithelium influencing ectomesenchyme but with time this potential is assumed by ectomesenchyme

19. lhx-6 & lhx-7 and lim-homeobox domain gene (transcription factor): Earliest mesnchymal markers which initiate tooth development. Induced by fgf-8 present in epithelium of 1 st brachial arch. 2. pax-9 Tooth initiation & Tooth germ location Induced by fgf-8 & down regulated by BMP2 and BMP4(seen where fgf-8 not seen). 3. Gli gene 4. Lef-1 1 st expressed in dental epithelial thickenings and during bud formation shifts to being expressed in condensing mesenchyme . 5. Shh-have a role in stimulating epithelial cell proliferation. 6. Dlx-1 ,Dlx-2

20. Regionalisation of dental ectoderm: Shh is expressed where dental ectoderm is formed. Wnt is expressed throughout in oral cavity except for presumptive dental ectoderm where shh is expressed

21. TOOTH TYPE PATTERNING: Determination of specific tooth types at their correct positions in the jaws. In heterodonts , 3 families of shapes: Incisiform Caniniform Molariform HYPOTHESIS Odontogenic homeobox code model of dental patterning(Field theory) Clone theory

22. odontogenic homeobox code model (field model) of dental patterning : The factors related to tooth shape reside within the ectomenchyme in distinct graded and overlapping fields for each tooth family

23. Clone theory Each tooth is derived from a clone of ectomesenchymal cells programmed by epithelium to produce teeth of given pattern. A The molar clone ectomesenchyme has induced the dental lamina to begin tooth development. The clone and dental lamina progress posteriorly B When a clone reaches the critical size, a tooth bud is initiated at its center. C The next tooth bud is not initiated until the progress zone of the clone escapes the influence of a zone of inhibition surrounding the tooth bud

24. Developmental stages Continuous process. Different morphologic ‘stages’-descriptive purposes. Different sizes& shapes; similar stages of dvp . Named after shape of EO: Bud Cap Bell

25. Stages of Tooth Development Morphologic Stages Dental lamina Bud stage Cap stage Bell stage- early Bell stage- advanced Formation of enamel and dentin matrix Physiologic process Initiation Proliferation Morphodifferentiation Apposition Histodifferentiation

26. Tooth development A-BUD STAGE B-CAP STAGE C-BELL STAGE D,E-DENTINOGENESIS AND AMELOGENESIS F-CROWN FORMATION G-ROOT FORMATION AND ERUPTION H-FUNCTION

27. TOOTH GERM Includes all the formative tissues for the entire tooth and its supporting structures. Three main components:- Enamel organ - ectodermal component that gives rise to enamel. Dental papilla - ectomesenchymal component that gives rise to dentin and pulp. Dental follicle or dental sac - ectomesenchymal component giving rise to cementum , periodontal ligament, and part of the alveolar socket

28. BUD STAGE Enamel organ differentiate into round or ovoid swelling called tooth bud . Enamel organ at this stage consists of: 1.Peripherally located low columnar cells. 2.Centrally located polygonal cells

29. Epithelium of dental lamina is separated from underlying mesenchyme by a basement membrane. Ectomesenchymal condensation occurs in relation to enamel organ. Ectomesenchymal condensation just below enamel organ is known as dental papilla . It forms future dentin & pulp. Ectomesenchymal condensation that surrounds tooth bud & dental papilla is known as dental sac . It forms future cementum & periodontal ligament

30. Molecular Basis: Msx gene expression induces ectomesnchymal condensation . BMP4 Induces ectomesenchymal condensation . Induces Msx . Induces BMP2 & Shh which helps in transition from Bud to Cap stage.

31. CAP STAGE Unequal growths in different parts of the tooth bud leads to cap stage Shallow invagination on the deep surface of the bud. Resembles a cap sitting on a ball of condensed ectomesenchyme .

32. Outer enamel epith Stellate reticulum Dental sac Dental papilla Inner enamel epith Dental lamina

33. Outer and inner enamel epithelium The peripheral cells of the cap stage: outer enamel epithelium. The cells in the concavity of cap becomes tall,columnar : inner enamel epithelium. The outer enamel epithelium is separated from the dental sac Inner enamel epithelium from dental papilla by a delicate basement membrane Hemidesmosomes

34. stellate reticulum Bw IEE& OEE- polygonal cells- synthesize& secrete GAGs into extracellular component bw epi.cells. GAGs-hydrophilic-osmotic pressure-drag water into EO. Increasing amount of fluid-increases-volume of extracellular compartment. Central cells-forced apart.

35. Retain connection with each other through cytoplasmic extensions& desmosomal attachments: star shaped. Gives central area cushion-like consistency. Shock absorber. Protects delicate enamel-forming cells.

36. DENTAL PAPILLA Under proliferating epithelium of enamel organ ectomesenchyme proliferates condenses to form the dental papilla formative organ of dentin and the primodium of the pulp. The dental papilla shows active budding of capillaries and mitotic figures

37. DENTAL SAC Marginal condensation in the ectomesenchyme surrounding the enamel organ and dental papilla. Zone a dense and fibrous layer develops : primitive dental sac. Cells of dental sac : cementum and periodontal ligament.

38. TRANSITORY STRUCTURES ENAMEL KNOT

39. ENAMEL KNOT They are clusters of non dividng epithelial cells visible in sections of molar cap stage tooth germ. Vertical enamel knot : enamel cord Enamel cord extends to meet OEE : Enamel septum Each tooth germ has single primary enamel knot at the cap stage and as these disappears Secondary enamel knots appear at the tips of future cusps in molars.

40. Enamel cord & Enamel Navel

41. A small depression at the point of meeting- ‘enamel navel’. Transient structures; disappear before enamel formation. Function of enamel knot& cord may be- reservoir of dividing cells for the growing EO. Fgf-4 & Slit-1: best markers; both primary& sec. knots. Exact physical role-not yet established. Current view- E.knot -organization centre- orchestrates cuspal morphogenesis.

42. ENAMEL NICHE Apparent structure in histologic sections created because the dental lamina is a sheet : contains a concavity filled with connective tissue. Impression that the tooth germ has double attachment to oral epithelium by 2 separate strands .

43. BELL STAGE As the invagination of epithelium deepens and its margins continue to grow, the enamel organ assumes a bell stage EARLY BELL STAGE Inner enamel epithelium Outer enamel epithelium Stratum Intermedium Stellate reticulum Cervical loop or zone of reflexion Dental Papilla Dental Sac

44. inner enamel epithelium Single layer of cells-differentiate prior to amelogenesis into tall columnar cells- ‘ ameloblasts ’. 4-5 µm in diameter; 40µm high. Contains nucleus away from basement membrane. Nucleus/ cytoplasmic ratio is high. high glycogen content . Cytoplasm contains free ribosomes ,a few RER ,some mitochondria & few scattered tonofilaments . Attached to one other by junctional complexes laterally& to SI by desmosome s .

45. stratum intermedium A few layers of squamous cells b/w IEE& OEE. Closely packed- desmosomes & gap jn . Desmosomal jn bw SR& IEE. Cells-well developed cytoplasmic organelles, acid mucopolysaccharides & glycogen deposits: high mitotic activity. High activity of alk.ph. Responsible for enamel formation

46. stellate reticulum Expands further-increased extracellular fluid. Desmosomal jns - bw -SR,SI& OEE. Before enamel formation- SR collapses. Reduces distance bw ameloblasts& nutrient capillaries near OEE. Then, cells-hardly distinguishable from SI. This change begins-height of cusp, progresses cervically.

47. Outer enamel epithelium Flatten to a low cuboidal form, centrally placed nuclei At the end of bell stage-laid in folds. Bw folds-DS forms papillae-contain capillary loops- nutritional supply to avascular enamel organ Compensate for loss of nutritional supply from DP - caused due to form of mineralized dentin. .

48. dental papilla Enclosed in the invaginated portion of EO. Before IEE bigins to produce enamel, peripheral cells of DP- odontoblasts - organizing influence of EO. First they assume cuboidal form, then columnar; specific potential for dentin production. BM-separating EO& DP- ‘ membrana preformativa ’

49. Dental Sac: Consists of undifferentiated mesenchymal cells & circularly arranged collagen fibrils around enamel organ & dental papilla. Collagen fibrils are more in dental sac than dental papilla. Ramifying nerves & vessels are also seen. Dvp of root DS differentiate into perio fibres that gets embedded in dvp cementum n alv bone Cervical loop or zone of reflexion : Its region where the inner and outer enamel epithelia meet at the rim of the enamel organ This is the point where cells continue to divide until tooth attains its full size & which after crown formation gives rise to epithelial component of root formation.

50. Dental sac Outer enamel epi Ameloblasts Stellate reticulum Stratum intermedium Odontoblasts Dental papilla Cervial loop

51. advanced bell stage Characterized by commencement of mineralization & root formation. Boundary bw IEE& odontoblasts - future DEJ. Formation of dentin occurs first as a layer along future DEJ. Begins at cusp tips, proceeds cervically& apically .

52. After 1st layer of dentin- ameloblasts - enamel- cusp area. Proceeds coronally& cer Cervical portion of enamel gives vically , from DEJ towards surface. rise to HERS. Outlines future root; responsible for shape, length, size& no. of roots.

53. break of DL& crown pattern determination 2 imp events occur in the bell stage : 1 . Dental lamina joining tooth germ to OE fragments, eventually- separates developing tooth from OE. Fragmentation-> discrete clusters of epithelial cells- normally degenerate. Some may persist- epithelial pearls (formerly- glands of Serres )

54. 2. IEE completes its folding, making it possible to recognize the shape of future crown pattern. Folding occurs not from growth pressures within DP. Results from intrinsic growth caused by differential rates of mitotic divisions within cells of IEE.

55. Stellate reticulum Enamel Dentin OEE odontoblasts

56. AMELOGENESIS The cells responsible for amelogenesis are ameloblasts. LIFE CYCLE OF AMELOBLASTS Presecretory stage Morphogenic stage Organizing stage Secretory stage Formative stage Post- secretory stage Maturative stage Protective stage Desmolytic stage

57. MORPHOGENIC STAGE The function of ameloblasts :determination of shape of the tooth. The IEE interacts with the underlying connective tissue and through differential growth helps to establish DEJ and thereby determine shape of the tooth. The ameloblasts at this stage are low columnar, centrally placed nucleus. Cytoplasmic organelles are not abundant,the centrioles and golgi apparatus are at apical part of cytoplasm and mitochondria is evenly distributed through out the cytoplasm.

58. ORGANIZING STAGE/DIFFERENTIATION STAGE The ameloblasts exert organizing influence on dental papilla cells and help in their differentiation to odontoblasts . Ameloblasts increase in length about 40 microns and has abundant cytoplasmic organelles for protein synthesis Reverse polarity is seen.

59. It’s a preparation to secretion because organelles are moved to secretory end of cell which is at the basal region. The cells also develop intercellular junctions at proximal and distal ends called proximal and distal terminal bars. During the terminal phase : dentin formation begins and basal lamina supporting the ameloblasts layer disintergrates . Ameloblasts attain secretory function only after a layer of dentin is formed. This interdependence b/w ameloblasts and odontoblasts is called RECIPROCAL INDUCTION

60. FORMATIVE/SECRETORY STAGE In this stage ameloblasts performs functin of secretion of enamel matrix and partial mineralisation. Here they synthesise enamel proteins In initial stages ameloblasts have flat basal region. As the enamel matrix is deposited, the ameloblasts move away from dentin side but a part of distal end of ameloblasts gets trapped in its own matrix and this conical trapped portion is called tomes process. After Tomes process is formed ,the secretion of enamel takes place from 2 different sites and is responsible for the rod structure of enamel.

61. MATURATIVE STAGE Ameloblasts : helps in the mineralization and maturation of enamel. Introduce the inorganic material necessary for maturation and also removes proteins and water to provide space for minerals. Show morphologic alterations. Ameloblasts are ruffle ended when they are reabsorbing proteins and water. Shows slight reduction in height and decrease in volume and organelles content.

62. The basal plasma membrane of ruffle ended ameloblasts shows a brush border with many foldings while that of smooth ended ameloblasts is smooth. The cytoplasm of ameloblasts also has vacoules which contains material resembling enamel matrix indicating absorptive function of these cells. Excess synthetic organelles are removed and the remaining organelles are shifted to the distal end of the cells.

63. PROTECTIVE STAGE After the enamel formation is completed the basal plasma membrane of ameloblasts looses the brush border and become smooth. They secrete protein : to the surface of newly formed enamel. Dev hemidesmosomal attachments to these basal lamina structure which help in holding these firmly to the tooth surface.

64. Columnar ameloblasts shorten to cuboidal and along with the other collasped layers of enamel organ forms 2-3 layered stratified epithelium which is termed as REE. This reduced enamel epithelium covers the newly formed enamel and protects : till tooth erupts into the oral cavity . Helps establishing the dentino gingival junction .

65. DESMOLYTIC STAGE In this stage the REE secretes collagenase enzyme which destroys the connective tissue between oral mucosa & erupting tooth. This facilitates eruption process. During this stage the REE proliferates and fuses with the oral epithelium to form a solid plug of epithelial cells. The central cells of this degenerate to form a canal through which the tooth erupts. Amelogenesis , the process of formation of enamel involves 2 steps : Matrix deposition & Mineralization.

66. ENAMEL MATRIX DEPOSITION: Enamel formation begins :tips and the incisal edges progresses outwards and cervical. As matrix deposition progresses ameloblasts move outward away from the matrix. In the early stages of amelogenesis the enamel matrix consists of 20-30% of proteins and this protein gradually decreases Ameloblasts synthesise and secrete enamel matrix which is composed of enamel proteins. Amelogenin and non amelogenin.

67. Enamel matrix also contains sulphated glyco conjugates. After a little thickness of enamel matrix is deposited ameloblasts develop a conical process at the base which is called Tome’s process. Enamel secretion takes place through two sites in Tome’s process 1. Interod growth region 2. Rod growth region Tome’s process is responsible for the rod structure of enamel

68. MINERALIZATION OF ENAMEL MATRIX In Immediate partial mineralization . 25-35% of total mineral content is deposited in matrix. It begins at DEJ where the tuftelin , ameloblastin/enamel proteins are deposited on layer of dentin. The initial enamel crystallizes to form a ribbon of minerals perpendicular to DEJ and the process continues till the entire thickness of enamel matrix secreted. After nucleation , the ameloblasts deposit matrix rich in amelogenin ..

69. Maturation Massive reabsorption of matrix protein and water takes place by ameloblasts and there is rapid growth of crystallites. Initially amelogenin proteins are absorbed onto specific crystal faces , thus controlling their growth. The amelogenin proteins are removed from mineralizing front by proteolytic cleavage mediated by proteinase enzyme and is reabsorbed by endocytic action of ameloblasts to bring about crystal growth. The maturation process starts at DEJ and progresses to the surface similarly it proceeds from cusp/ incisal tip to cervical region.

70. DENTINOGENESIS Dentin is formed by cells called odontoblasts that differentiates from ectomesenchymal cells of dental papille following an organizing influence that emanates from inner enamel epithelium. ODONTOBLAST DIFFERENTIATION The dental papilla cells : small and undifferentiated , a central nucleus and a few other organelles. Separated from the inner enamel epithelium by an acellular zone that contains some fine collagen fibrils

71. The ectomesenchymal cells adjoining acellular zone rapidly enlarge and elongate preodontoblasts first and then odontoblasts , as their cytoplasm increases in volume to contain increasing amount of protein synthesisng organelles. The acellular zone between the dental papilla and IEE gradually is eliminated as odontoblasts differentiate and increase in size and occupy this zone. These cells are highly polarised with their nuclei positioned away from the IEE.

72. FORMATION OF MANTLE DENTIN Formation of organic matrix. Odontoblasts differentiate in the preexisting ground substance of dental papilla and 1 st dentin collagen synthesised by them is deposited in this ground substance. Von korffs fibres appear. They are type 3 associated collagen fibres. As odontoblasts increase in size, they produce smaller type 1 collagen fibrils that orient themselves parallel to the future DEJ. A layer of mantle predentin appears.

73. The plasma mem of odontoblasts adjacent to IEE extends stubby process into extracellular matrix which penetrate and interpose itself b/w cells of IEE to form enamel spindle. As odontoblasts forms these process, it also buds off a number of small mem bound vesicles known as matrix vesicles The odontoblasts then develops a cell process odontoblast process, which is left behind in the forming dentin matrix ,as the odontoblast moves away towards the pulp.

74. The mineral phase first appears within the matrix vesicles as single crystals seeded by phospholipids present in the vesicle membrane. The crystals grow and rupture spread as a cluster of crystallites that fuse with adjacent clusters to form a continuous layer of mineralized matrix . The deposition of mineral layer lags behind the formation of the organic matrix so that a layer called predentin is found between the odontoblasts and mineralization front.

75. Following mineral seeding , non collagenous matrix proteins produced by odontoblasts come into play to regulate mineral deposition In this way coronal mantle dentin is formed in a layer approximately 15-20 micrometer thick onto which primary dentin is added.

76. FORMATION OF ROOT DENTIN The epithelial cells of Hertwig’s root sheath initial differentiation of odontoblasts that form root dentin. SECONDARY AND TERTIARY DENTINOGENESIS Secondary dentin is deposited after root formation is completed and is formed by the same odontoblasts that formed primary dentin. Tertiary dentin is deposited at specific site in response to injury by damaged odontoblasts /replacement cells from pulp.

77. ROOT FORMATION Begins after enamel& dentin formation has reached future CEJ. Enamel organ-imp role-HERS- molds shape of root& initiates radicular dentin formation. Cells of IEE& OEE- proliferate as double layered sheath from cervical loop. Devoid of SI& SR.

78. HERS-extends around pulp, bw pulp& DF. Rim of this HERS- ‘epithelial diaphragm’- encloses primary apical foramen. Cells of IEE- remain short, do not form enamel. But, induce differentiation of odontoblasts at periphery of pulp, facing HERS. Odontoblasts- eventually- dentin.

79. Degeneration of HERS and formation of cell rests of malassez Differentiation of cementoblasts from dental sac Cementum formation Orientation of periodontal ligament

80. In case of multirooted teeth, there is differential growth of epithelial diaphragm in the form of tongue like extensions which grow towards each other & fuse causing division of trunk into two or three roots.

81. fate of HERS After fragmentation: cells drift away; positioned in PDL. Conventional sections-discrete clusters or islands of epi.cells: ‘cell rests of Malassez’.

82. Formation of supporting tissues The supporting tissues of the tooth form from the dental follicle. As root sheath fragments ectomesenchymal cells of the dental follicle penetrate b/w the epithelial fenestrations and become apposed to the newly formed dentin of the root. Cells differentiate into cementoblasts . The cells of periodontal ligament and fibre bundles also differentiate from dental follicle .

83. histophysiology A no. of physiologic growth processes. Overlap. Many are continuous through morphologic stages. Each process-predominate in one stage than other. Ex.: histodifferentiation characterizes bell stage.

84. Dental lamina Initiation Bud stage Cap stage Bell stage (early) Bell stage (advanced) Morphodifferentiation Formation of enamel & dentin matrix Proliferation Histodifferentiation Apposition PHYSIOLOGIC PROCESS

85. initiation Dental lamina & tooth buds- potential for tooth formation. Specific cells within DL-potential to form EO of certain teeth by responding to factors. Different teeth-initiated at different times. Requires ectomesenchymal-epithelial interactions.

86. Clinical implication : A lack of initiation results in absence of either single tooth or multiple teeth. Most frequently the permanent upper lateral incisor ,third molar, and lower second premolars. Abnormal initiation may result in development of single or multiple supernumerary teeth

87. Genes: Lhx-6 & 7 Fgf-8 Pax-9 Sonic hedgehog Activin -A Barx Bmp Dlx Gli Hgf Lef Lhx Msx Oflx Ptc Pitx Slit Smo wn t

88. proliferation Enhanced proliferative activity bud, cap & bell. Proliferative growth causes regular changes in size & proportion of growing tooth germ. Tooth germ already has potential to become more highly developed.

89. Genes: Bmp2,4 Dlx 1-3 EGR 1 FGFs Lef1 Msx 1& 2 Notch 1 -3 Pax9 RAR- α , β , γ RXR- α , β , γ Syndecan Tenascin TGF- βs

90. histodifferentiation Succeeds proliferative phase. Formative cells-definite morphologic as well as functional changes. Acquire their functional assignment. Cells become restricted in their functions. Differentiate; give up capability of multiplying.

91. This phase-reaches highest development in bell stage, just preceding formation of enamel& dentin. Organizing influence of IEE on mesenchyme- evident in bell stage. Causes differentiation of adj DP cells-odontoblasts. With formation of dentin: IEE -> ameloblasts; form enamel opposite to dentin.

92. Enamel does not form in the absence of dentin. Dentin formation precedes & essential for enamel formation. Differentiation of epithelial cells precedes & is essential for differentiation of odontoblasts .

93. Clinical implications: In vitamin A deficiency ameloblasts fail to differentiate ,as a result of which adjacent mesenchyme fails to differentiate & a atypical dentin known as osteodentin is formed Defective histodifferentiation of ameloblasts lead to Amelogenesis Imperfecta Defective histodifferentiation of odontoblasts results in Dentiinogenesis imperfecta .

94. morphodifferentiation Morphologic pattern or basic form & relative size-established-differential growth. Therefore, morphodifferentiation-impossible without proliferation. Advanced bell stage - imp stage of morphodiff , in the crown, outlining future DEJ. DEJ& DCJ- different & characteristic for each type of tooth. Act as a blueprint pattern. Enamel, dentin & cementum -in conformity with this pattern. Therefore- responsible for form& size. Ex.: size& form of cuspal portion of 1st permanent M- established at birth, before formation of hard tissues begin.

95. Genes: Bmp4 Collagens Dlxl-3 Lef 1 Msxl Msx2 Notch1 -3 Pax9 RAR RXR Tuftelin Clinical implications: Endocrinal disturbances can affect tooth shape only during this stage. In hypopitutarism small clinical crown is often mistaken for a small anatomic crown Disturbances in morphodifferentiation Supernumerary cusp or tooth Twinning Suppression of parts may occur Peg or malformed tooth

96. apposition Deposition of matrix of hard dental structures. Appositional growth- layer-like deposition of extracellular matrix. This type of growth is therefore additive. Fulfillment of plans outlined at histo& morphodiff . Regular& rhythmic deposition of extra cellular matrix. Periods of activity& rest- alternatively at definite intervals.

97. Clinical implications : Genetic & environmental factors may disturb the normal synthesis & secretion of organic matrix of enamel leading to condition called enamel hypoplasia . If organic matter is defective, then enamel or dentin is said to be hypocalcified or hypomineralised .

98. Conclusion Teeth are serially homologous structures, which allow the localization and quantification of the effects of specific gene mutations. It is possible to determine the phase of odontogenesis affected by these conditions. These features make tooth development an important system, to understand the intricate molecular mechanisms that regulate development.

99. RECENT ADVANCES: Ectodysplasin , a recently identified signal molecule in the tumor necrosis factor (TNF) family, and its receptor Edar mediate signalling between ectodermal compartments in tooth germs. The genes regulated by the different signals include transcription factors and signal receptors that regulate the competence of the cells to respond to the next signals, as well as new signals that act reciprocally and thereby continue the communication between cells and tissues (reviewed by  Jernvall and Thesleff , 2000 ;  Thesleff and Mikkola , 2002 ). The transcription factor c- Myb is involved in the control of cell proliferation, survival and differentiatiON The expression of c- Myb in both species was strong in the odontoblasts and ameloblasts at the stage of dentin and enamel production suggesting a possible novel role of c- Myb during tooth mineralization Eva Matalova , et al,. Expression and characterization of c- Myb in prenatal odontogenesis Develop. Growth Differ. (2011) 53, 793–803

100. Msx1  and  Pax9  interact synergistically throughout lower incisor development and affect multiple signaling pathways that influence incisor size and symmetry and that a combined reduction of  PAX9  and  MSX1  gene dosage in humans may increase the risk for orofacial clefting and oligodontia . Mitsushiro Nakatomi ,  Xiu -Ping Wang , Darren Key ,  Jennifer J. Lund ,Annick Turbe-Doan,et al, Genetic interactions between  Pax9  and  Msx1  regulate lip development and several stages of tooth morphogenesis Developmental Biology Volume 340, Issue 2 , 15 April 2010, Pages 438–449 Zn affects biological events especially in tooth development, with recent progress uncovering the roles of the Zn transporter Zip13 in mammalian health and diseases Toshiyuki Fukada ,  Yoshinobu Asada ,  Kenji Mishima ,  Shinji Shimoda ,  Ichiro Saito Slc39a13/Zip13: A Crucial Zinc Transporter Involved in Tooth Development and Inherited Disorders Journal of Oral Biosciences Volume 57, Issue 1, Pages 1-44 (February 2015)

101. References B.K.B.Berkovitz , G.R.Holland , B.J.Moxham Oral Anatomy, Histology, And Embryology 4 th Edition Oral Development And Histology James Avery Third Edition. Tencate’s Oral Histology Antonio Nanci 8 th Edition Orban Oral Histology And Embryology 13 th Edition

102. Javier Caton and S Tucker Current knowledge of tooth development: patterning and mineralization of the murine dentition J Anat. 2009 Apr; 214(4): 502–515 Jukka Jernvall and Irma Thesleff Tooth shape formation and tooth renewal: evolving with the same signals Development 139, 3487-3497 (2012) Chatterjee .Sa, Boaz Kb Molecular biology of odontogenesis.J Orofac Sci , 3(1)2011 Irma Thesleff Epithelial mesenchymal signalling regulating tooth morphogenesis Journal of cell science 116, 1647-1648