The diiferentiation of odontoblasts and the formation of dentin immediately succeed the lengthening of the root sheath.
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At the same time the connective tissue of the dental sac surrounding the sheath proliferates and breaks up the continuous double epithelial layer Fig. Coniiec-tive tissue cells differentiate into eementohlasts and deposit a layer of cementum onto the sur.
Differentiation of odontoblasts in the elongated pulp. In the cervical part of the root dentin has been formed. The root sheath is broken up into epithelial rests and is separated from the dentinal surface by connective tissue. Differentiation of cenzentoblasts.
Surface vigew of the epithelial djaphragm. Diagrammatic mesiodistal sections of a lower molar. Beginning of dentin formation at the bifurcation. Formation of the two roots in progress. Details as in Fig. Increasingly more of the diaphragm is bent i11to the long axis of the root, the wide apical foramen being thus reduced first to the width of the diaphragmatic opening itself and, later, further narrowed by apposition of dentin and cementum at the apex of the root.
The peculiar development of the diaphragm in multi-rooted teeth causes the division of the root stock into two or three roots. Two such extensions are found in the germs of lower molars, three in the germs of upper molars. Before the formation of the root begins. The single cervical opening of the coronal enamel organ is then divided into two or three openings. On the pulpal surface of the dividing bridges dentin formation starts Fig. If cells of the epithelial root sheath remain adherent to the dentin surface they may differentiate into fully functioning ameloblasts and produce enamel.
Such droplets of enamel, called enamel pearls, are sometimes found in the area of bifurcation of the roots of permanent molars. This accounts for the development of accessory root canals opening on the periodontal surface of the root see chapter on Pulp. A number of physiologic growth processes participate in the progressive development of the teeth Table I. Except for initiation which is a momentary event, these processes overlap considerably and many are continuous over several histologic stages. Nevertheless, each tends to predominate in one stage more than in another.
The dental lamina and tooth buds represent that part of the oral epithelium which has potencies for tooth formation. Initiation is set off by unknown chemical factors, as the growth potential of the ovum is set off by the fertilizing spermatozoon. Teeth may develop in abnormal locations such as the ovary dermoid tumors or cysts or the hypophysis.
In such instances the tooth undergoes similar stages of development as in the jaws. A lack of initiation results in the absence of teeth. This may occur in isolated areas, most frequently in the permanent upper lateral incisors, third molars and lower second hicuspids; or there 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. Marked proliferative activity ensues at the points of initiation, and results successively in the bud, cap and bell stages of the odontogenic organ.
Proliferative growth is the result of cellular division and is.
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It is marked by changes in the size and proportions of the growing tooth germ Figs. During the stage of proliferation the tooth germ has the potentiality to progress to more advanced development. This is illustrated by the fact that explants of these early stages continue to develop in tissue culture through the subsequent stages of histodiffercntiation and appositional growth.
A disturbance or experimental interference has entirely diiferent effects, according to the time of occurrence and the stage of development which it attects. If aberrations occur during the stage of proliferative growth, new parts may be differentiated supernumerary cusps or roots ; twinning may result; or a complete suppression of parts may occur loss of cusps. The cells become restricted in their potencies; they give up their capacity to multiply as they assume their new function a law which governs all differentiating cells.
This phase reaches its highest development in the bell stage of the enamel organ, just preceding the beginning of apposition of dentin and enamel Fig. With the formation of dentin. Enamel does not form in the absence of dentin as demonstrated by transplanted ameloblasts failing to form enamel when no dentin is present. Dentin formation therefore precedes and is essential to enamel formation.
The differentiation. A tumor is, therefore, characterized by unorganized proliferation and incomplete dilferentiation of the cells.
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The degree of nondifferentiation of the cells is an index to the rate of proliferation and, therefore, to the malignancy of the tumor. The result is formation of dentin groimd substance, with an absence or disarrangement of the dentinal tubules, resembling irregular secondary dentin. The shape of the tooth and the quality of the enamel are normal. This dentin is known as osteodentin since it resembles bone. The morphologic pattern or basic form and relative size of the future tooth is established by morphodifferentiation. The advanced bell stage marks not only active histodifferentiation but also an important stage of morphodifferentiation of the crown.
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Onto this area the ameloblasts. For example. Size and shape of the root. Clinical examination shows that the re tarded eruption which occurs in hypopituitary and hypothyroid cases results in a small clinical crown which is often mistaken for a small anatomical crown see section on Epithelial Attachment. Disturbances in morphodifferentiation may affect the form and size of the tooth without impairing the function of the ameloblasts or odontoblasts. The result is a peg or malformed tooth e.
Apposition is the deposition of the matrix of the hard dental structures; it will be described in separate chapters on the formation of. This chapter deals with certain aspects of apposition, in order to complete the discussion of the physiologic processes concerned in the growth of teeth.
Appositional growth of enamel and dentin is a layer-like deposition of an extracellular matrix.
This type of growth is, therefore, additive. These laws, for the most part, have been elucidated in growth studies of various other organs and organisms. The process of appositional growth may be compared with the construction of a house. The blueprints the dentino-enamel and dentino-cemental junctions outline the form and size of the structure and are different for each class of tooth. However, the workers cells , the materials used nutritive elements , the materials elaborated enamel and dentin , and the methods of construction the pattern of cellular activity are common to all classes and forms of teeth.
It begins at a given site, at the dentinal cusps, termed the growth center, and at a. The result is an incremental pattern which is a summation of gnomonic curves superposed on the morphogenetic pattern dentino-enamel junction. Please log in to add your comment.
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Search for books, journals or webpages All Pages Books Journals. Editors: Antonio Nanci. Hardcover ISBN: Imprint: Elsevier. Published Date: 13th October Page Count: Free Shipping Free global shipping No minimum order. Detailed histological and structural images help you differentiate the structures. Medical approach to histology helps you understand underlying biological events, and oral tissue repair and regeneration.