Hertwig’s epithelial root sheath and root formation: - PowerPoint Presentation

1. Hertwig’s epithelial root sheath and root formation:

2. The cervical portion of the enamel organ gives rise to the epithelial root sheath of Hertwig. The Hertwig’s epithelial root sheath (HERS) outlines the future root and is thus responsible for the shape, length, size, and number of roots.

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4. The development of the roots begins after enamel and dentin formation has reached the future cementoenamel junction. The enamel organ plays an important part in root development by forming HERS, which molds the shape of the roots and initiates radicular dentin formation. Hertwig’s root sheath consists of the outer and inner enamel epithelia only, and therefore it does not include the stratum intermedium and stellate reticulum. The cells of the inner layer remain short and normally do not produce enamel.

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6. When these cells have induced the differentiation of radicular dental papilla cells into odontoblasts and the first layer of dentin has been laid down, the epithelial root sheath loses its structural continuity and its close relation to the surface of the root. Its remnants persist as an epithelial network of strands or clumps near the external surface of the root. These epithelial remnants are found in the periodontal ligament of erupted teeth and are called rests of Malassez.

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9. There is a pronounced difference in the development of HERS in teeth with one root and in those with two or more roots. Prior to the beginning of root formation, the root sheath forms the epithelial diaphragm .The outer and inner enamel epithelia bend at the future cementoenamel junction into a horizontal plane, narrowing the wide cervical opening of the tooth germ. The plane of the diaphragm remains relatively fixed during the development and growth of the root.

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11. The proliferation of the cells of the epithelial diaphragm is accompanied by proliferation of the cells of the connective tissue of the pulp, which occurs in the area adjacent to the diaphragm. The free end of the diaphragm does not grow into the connective tissue, but the epithelium proliferates coronal to the epithelial diaphragm.

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13. The differentiation of odontoblasts and the formation of dentin follow the lengthening of the root sheath. At the same time the connective tissue of the dental sac surrounding the root sheath proliferates and invades the continuous double epithelial layer dividing it into a network of epithelial strands

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15. The epithelium is moved away from the surface of the dentin so that connective tissue cells come into contact with the outer surface of the dentin and differentiate into cementoblasts that deposit a layer of cementum on the surface of the dentin. The rapid sequence of proliferation and destruction of Hertwig’s root sheath explains the fact that it cannot be seen as a continuous layer on the surface of the developing root.

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17. In the last stages of root development, the proliferation of the epithelium in the diaphragm lags behind that of the pulpal connective tissue. The wide apical foramen is reduced first to the width of the diaphragmatic opening itself and later is further narrowed by apposition of dentin and cementum to the apex of the root. Differential growth of the epithelial diaphragm in multi- rooted teeth causes the division of the root trunk into two or three roots.

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20. During the general growth of the enamel organ the expansion of its cervical opening occurs in such a way that long tongue like extensions of the horizontal diaphragm develop. Two such extensions are found in the germs of lower molars and three in the germs of upper molars.

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22. Before division of the root trunk occurs, the free ends of these horizontal epithelial flaps grow toward each other and fuse. The single cervical opening of the coronal enamel organ is then divided into two or three openings. On the pulpal surface of the dividing epithelial bridges, dentin formation starts and on the periphery of each opening, root development follows in the same way as described for single-rooted teeth.

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28. If the continuity of HERS is broken or is not established prior to dentin formation, a defect in the dentinal wall of the pulp ensues. Such defects are found in the pulpal floor corresponding to the furcation or on any point of the root itself if the fusion of the horizontal extensions of the diaphragm remains incomplete. This accounts for the development of accessory root canals opening on the periodontal surface of the root.

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30. HISTOPHYSIOLOGYA number of physiologic growth processes participate in the progressive development of the teeth .Except for their initiation, which is a momentary event, these processes overlap considerably, and many are continuous throughout the various morphologic stages of odontogenesis.

31. Nevertheless, each physiologic process tends to predominate in one stage more than in another. For example, the process of histodifferentiation characterizes the bell stage, in which the cells of the inner enamel epithelium differentiate into functional ameloblasts. However, proliferation still progresses at the deeper portion of the enamel organ.

32. 1-Initiation:The dental laminae and associated tooth buds represent those parts of the oral epithelium that have the potential for tooth formation. Specific cells within the horseshoe-shaped dental laminae have the potential to form the enamel organ of certain teeth by responding to those factors that initiate or induce tooth development.

33. Different teeth are initiated at definite times. Initiation induction requires ectomesenchymal–epithelial interaction. The mechanism of such interaction is not clearly understood. However, it has been demonstrated that dental papilla mesenchyme can induce or instruct tooth epithelium and even non tooth epithelium to form enamel.

34. 2-ProliferationEnhanced proliferative activity begins at the points of initiation and results successively in the bud, cap, and bell stages of the odontogenic organ. Proliferative growth causes regular changes in the size and proportions of the growing tooth germ. Even during the stage of proliferation, the tooth germ already has the potential to become more highly developed

35. 3-Histodifferentiation:Histodifferentiation succeeds the proliferative stage. The formative cells of the tooth germs developing during the proliferative stage undergo definite morphologic as well as functional changes and acquire their functional assignment (the appositional growth potential).

36. The cells become restricted in their functions. They differentiate and give up their capacity to multiply as they assume their new function; this law governs all differentiating cells. This phase reaches its highest development in the bell stage of the enamel organ, just preceding the beginning of formation and apposition of dentin and enamel.

37. The organizing influence of the inner enamel epithelium on the mesenchyme is evident in the bell stage and causes the differentiation of the adjacent cells of the dental papilla into odontoblasts. With the formation of dentin, the cells of the inner enamel epithelium differentiate into ameloblasts and enamel matrix is formed opposite the dentin. Enamel does not form in the absence of dentin, as demonstrated by the failure of transplanted ameloblasts to form enamel when dentin is not present.

38. Dentin formation therefore precedes and is essential to enamel formation. The differentiation of the epithelial cells precedes and is essential to the differentiation of the odontoblasts and the initiation of dentin formation. The importance of the basement membrane of this interface has been recognized.

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40. Morphodifferentiation:The morphologic pattern, or basic form and relative size of the future tooth, is established by morphodifferentiation, that is, by differential growth. Morphodifferentiation therefore is impossible without proliferation. The advanced bell stage marks not only active histodifferentiation but also an important stage of morphodifferentiation in the crown, outlining the future dentinoenamel junction.

41. The dentinoenamel and dentinocemental junctions, which are different and characteristic for each type of tooth, act as a blueprint pattern. In conformity with this pattern the ameloblasts, odontoblasts, and cementoblasts deposit enamel, dentin, and cementum, respectively, and thus give the completed tooth its characteristic form and size. For example, the size and form of the cuspal portion of the crown of the first permanent molar are established at birth long before the formation of hard tissues begin.

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45. Apposition:Apposition is the deposition of the matrix of the hard dental structures. Appositional growth of enamel and dentin is a layer like deposition of an extracellular matrix. This type of growth is therefore additive. It is the fulfillment of the plans outlined at the stages of histodifferentiation and morphodifferentiation. Appositional growth is characterized by regular and rhythmic deposition of the extracellular matrix, which is of itself incapable of further growth. Periods of activity and rest alternate at definite intervals during tooth formation.

46. CLINICAL CONSIDERATIONSA lack of initiation results in the absence of either a single tooth or multiple teeth (partial anodontia), most frequently the permanent upper lateral incisors, third molars, and lower second premolars. There also may be a complete lack of teeth (anodontia). On the other hand, abnormal initiation may result in the development of single or multiple supernumerary teeth

47. Complete and partial anadontia

48. Teeth may develop in abnormal locations, for example, in the ovary (dermoid tumors or cysts) or in the hypophysis. In such instances the tooth undergoes stages of development similar to those in the jaws. In vitamin A deficiency the ameloblasts fail to differentiate properly. Consequently, their organizing influence on the adjacent mesenchymal cells is disturbed, and atypical dentin, known as osteodentin, is formed.

49. Endocrine disturbances affect the size or form of the crown of teeth if such effects occur during morphodifferentiation, that is, in utero or in the first year of life. Size and shape of the root, however, may be altered by disturbances in later periods.

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51. Clinical examinations show that the retarded eruption that occurs in persons with hypopituitarism and hypothyroidism results in a small clinical crown that is often mistaken for a small anatomic crown. Abnormal curvatures in the root, termed dilacerations may be due to trauma sustained during development of the root.

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53. Disturbances in morphodifferentiation may affect the form and size of the tooth without impairing the function of the ameloblasts or odontoblasts. New parts may be differentiated like formation of supernumerary cusps or roots. Twinning, i.e. two similar teeth may be produced as a result of splitting of one tooth germ. Fusion of teeth produced from two tooth germ joined together before mineralization may occur.

54. FUSION:

55. A suppression of parts may occur like loss of cusps or roots. An abnormality in shape may result in a peg or malformed tooth with enamel and dentin that may be normal in structure. Peg shaped teeth (screw-driver shaped) with the permanent upper central incisor showing a notched incisal edge may be seen in individuals born with congenital syphilis

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57. This condition is known as Hutchinson’s incisor. Genetic and environmental factors may disturb the normal synthesis and secretion of the organic matrix of enamel leading to a condition called enamel hypoplasia. If the organic matrix is normal but its mineralization is defective, then the enamel or dentin is said to be hypocalcified or hypomineralized. Both hypoplasia and hypocalcification can occur as a result of an insult to the cells responsible for the apposition stage of tooth development

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