Book - Oral Histology and Embryology (1944) 2
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Orban B. Oral Histology and Embryology (1944) The C.V. Mosby Company, St. Louis.
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Chapter II Development and Growth of Teeth
1. Introduction
This chapter deals with the development of the tooth beginning with its initiation from the oral epithelium, up to the formation of enamel and dentin (Fig. 13) (Table I).
The life history of the tooth consists of the following stages:
1. Growth: (:1) Initiation; (1)) Proliferation; (c) Histodiiferentiation; (d) Morphodiiferentiation; and (e) Apposition
2. Calcification. 3. Eruption. 4. Attrition.
These stages, except for initiation, are not sharply demarcated, but overlap considerably and many of them are concurrent for some time (Fig. 13). Thus, one microscopic section shows the predominance of one stage and indicates characteristics of the preceding as well as succeeding stages.
The dilferent stages in the growth of the teeth will first be considered in terms of their morphologic and histologic appearance, and will then be discussed in terms of the physiologic processes which they represent. An understanding of the histologic structure is greatly facilitated by an appreciation of its physiologic aspects. The histologic stages in tooth development are well defined and, while more knowledge will be added to this field, it is probable that further advance will be largely in the direction of histophysiology. The histologic description of the subject matter in this chapter will, therefore, be followed by physiologic interpretation (Table I).
2. Developmental Stages
The tooth germ develops from ectoderm and mesoderm. The ectoderm of the oral cavity forms the epithelial enamel organ which molds the shape of the entire tooth and gives rise to the enamel. The mesoderm inside the enamel organ. the dental papilla. differentiates into the dental pulp and elaborates the dentin. The mesoderm surrounding the enamel organ, the dental sac. forms the eenientum covering the root, and the periodontal memlzrane.
First draft submitted by Isaac Schour in collaboration with Maury Hassle.
Fig. 13.—DIagran1matlc illustration of the life cycle of the tooth. Stage 0 shnws active from Schuur and Ma.-;sler.") nxorplladlrrerentlatian as well as hlstodifferentiatlon. (Modified 31
1, Fig.14.--Diagrammatic reconstruction uf the dental lamina and enamel organs of the niamlible. (.\Io1lified from I\'urberg.~)
A. 22 mm. embryo, bud stage (8th week). B. 43 mm. embryo, cap stage (10th week).
B. 163 mm. embryo. bell stage (about 4 months old). The primordia of permanent teeth are seen as thickenings of the dental lamina. on the lingual side of each tooth germ. Distal extension of the dental lamina with the primordium of the first molar.
A. Dental Lamina and Bud Stage
The first sign of human tooth development is seen during the sixth week of embr_vonic life (11 mm. e1nbr_vo). At this stage the oral epithelium consists of a basal layer of high cells and a surface layer of flattened cells. The rich glycogen content of their cytoplasm, which does not stain in routine preparations, gives them an empty appearance. The epithelium is separated from the connective tissue by a basement membrane. Certain cells in the basal layer of the oral epithelium begin to proliferate at a more rapid rate than do the adjacent cells. An epithelial thickening arises in the region of the future dental arch and extends along the entire free margin of the jaws (Figs. 1-1 and 15). It is the primordium of the ectodermal portion of the teeth known as the dental lamina. Mitotic figures are seen not only in the epithelium but also in the subjacent mesoderm (Fig. 15).
Fig. 15. Initie.tion of tooth development Human embryo 13.5 mm. long. 5th week. (0rba.n').
A. Sagittal section through upper and lower jaws. B. High magnification of thickened oral epithelium.
About the time of differentiation of the dental lamina there arise from it and in each jaw. round or oval swellings at ten different points corresponding to the future position of the deciduous teeth, the primordia of their enamel organs, the tooth buds ( Fig. 16 l. Here the development of tlic tooth germs is initiated and the cells proliferate faster than the adjacent cells. The dental lamina is shallow. and microscopic sections often show the tooth buds close to the oral epithelium.
Fig. 16. Bud stage of tooth development (proliferation stage). 16 mm. long, 6th week (0rban5).
A. Wax reconstruction of the germs of the central and lateral lower incisors. B. Sagittal section through upper and lower jaws. 0'. High magnification or the tooth germ or the lower incisor in bud stage.
B. Cap Stage
As the tooth bud continues to proliferate it does not expand uniformly into a larger sphere. Unequal growth in the different parts of the bud leads to formation of the cap stage which is characterized by a shallow invagination on the deep surface of the bud (Figs. 14, B and 17).
Fig. 17.—Cap stage of tooth development. Human embryo 31.5 mm. long. 9th week, torhanfi)
A. Wax reconstruction of the enamel organ of the lower lateral incisor. B. Labxoiingual section through the same tooth.
The following histologic changes seen in the cap stage are preparatory to those in the subsequent bell stage:
The peripheral cells of the cap stage appear in two portions, the outer enamel epithelium at the convexity consisting of a single row of short cells, and the inner enamel epithelium at the concavity consisting of a layer of tall cells (Figs. 17 and 18).
The cells in the central core of the enamel organ situated between the outer and inner enamel epithelia begin to separate by an increase of the intercellular fluid, and arrange themselves in a network called the stellate reticulum or enamel pulp (Figs. 20 and 21). The cells assume a branched reticular form, resembling mesenchyme. In this reticular network the spaces are filled with a mucoid fluid rich in albumin. giving the enamel pulp a cushion-like consistency which, later, protects the delicate enamel-forming cells.
Fig. 18.—Cap stage of tooth development Human embryo -11.5 mm. long, 10th week. (Orban.-")
A. Wax reconstruction of the enamel organ of the lower central incisor. B. Labiolingual section through the same tooth.
At first, there is no change into a stellate arrangement of the cells in the center of the tooth germ which contains the enamel knot (Fig. 17). The latter projects in part toward the underlying dental papilla, so that the center of the epithelial invagination shows a slightly budlike enlargement which is bordered by the labial and lingual enamel grooves (Fig. 17). At the same time there arises in the increasingly high enamel organ a vertical extension of the enamel knot, called the enamel cord (Fig. 20). Both are temporary structures which disappear before enamel formation begins. Another temporary appearance is an indentation in the outer enamel epithelium, next to the enamel cord, called the enamel navel.
Under the organizing influence of the proliferating epithelium of the enamel organ the mesench_vme, partially enclosed by the invaginated portion of the inner enamel epithelium, proliferates; it condenses to form the dental papilla which is the formative organ of the dentin and primordium of the pulp (Figs. 17 and 18). The changes in the dental papilla occur concomitantly with the development of the enamel organ. While the enamel organ exerts a dominating influence over the adjacent connective tissue, the condensation of the latter should not be considered as a passive reaction to the crowding by proliferating epithelium. The dental papilla shows active budding of capillaries and mitotic figures, and its peripheral cells adjacent to the inner enamel epithelium enlarge and, later, differentiate into the odontoblasts.
Concomitant with the development of the enamel organ and the dental papilla, there is a marginal condensation in the mesenchyme surrounding the outside of the enamel organ and dental papilla. At first this mesenchymal border is distinguished by a lesser number of cells. Soon, however, a denser and more fibrous layer develops which constitutes the primitive dental sac.
The enamel organ, the dental papilla and dental sac constitute the formative tissues for a11 entire tooth and its periodontal membrane, hence collectively form a tooth germ.
C. Bell Stage
As the invagination, developed during the cap stage, deepens and its margins continue to grow, the enamel organ assumes the bell stage of its development (Figs. 14, C, 19, and 20). The following histologic modifications of the cap stage are significant.
The inner enamel epithelium consists of a single layer of cells which differentiate prior to amelogenesis into tall columnar ameloblasts (Figs. 20 and 21). They are 4 to 5 microns in diameter and about 40 microns high. In cross-section they assume a hexagonal shape, similar to that seen later in transverse sections of the enamel rods.
There is a change in the polarity of the ameloblasts which is proved by the fact that their nuclei are no longer next to the dental papilla but are situated near the stratum intermedium {see chapter on Enamel Development).
The ameloblasts exert an organizing influence upon the underlying mesenchymal cells which differentiate into odontoblasts.
Several layers of low squamous cells, called stratum intermedium, appear between the inner enamel epithelium and stellate reticulum I: Fig. 21). This layer seems to be essential to enamel formation. It is absent in that part of the tooth germ which is not amelogenic and which outlines the root portions of the tooth.
The enamel pulp nstellate Ieticulunr expands further. mainly by increase of the intercellular fluid. The cells are star-shaped with long processes which anastomose with those of adjacent cells (Fig. 21).
The cells of the outer enamel epithelium flatten to a low cuboidal form. At the end of the bell stage, preparatory to and during the formation of enamel, the formerly smooth surface of the outer enamel epithelium is laid in folds. Between the folds the adjacent mescnchyme of the dental sac sends in papillae which contain capillary loops and thus provide a rich nutritional supply for the intense metabolic activity of the avascular enamel organ.
In all teeth excepting the permanent molars the dental lamina proliferates at its deep end to give rise to the enamel organ of the permanent successor. while it distintegrates in the region between the enamel organ and the oral epithelium. The enamel organ becomes gradually independent and separated from the dental lamina at about the time when the first dentin is formed.
The dental papilla is largely enclosed in the invaginated portion of the enamel organ. Before the inner enamel epithelium begins to produce enamel, the peripheral cells of the suhjacent mesenehymal dental papilla ( or primitive pulp: undergo histoditferentiation into odontoblasts under the organizing influence of the epithelium. They assume a high columnar form and acquire a specific potentiality to take part in dentin formation.
The basement membrane separating the enamel organ and dental papilla, at the time just preceding dentin formation is called membrana preformatira. Between this and the incompletely difierentiated odontoblasts there is a clear layer.
In the root the histoditfercntiation of the odontoblasts from the dental papilla takes place under the influence of the inner layer of HertWig’s epithelial root sheath. As the primary dentin is laid down the dental papilla becomes the dental pulp.
Before apposition begins the dental sac shows a circular arrangement of its fibers and resembles a capsular structure. With the development of the root, the fibers of the dental sac differentiate into the periodontal fibers which become embedded in the cementum and alveolar bone.
During the advanced bell stage the boundary between inner enamel epithelium and odontoblasts outlines the future dentino-enamel junction (Figs. 20 and 22). In addition, the junction of the inner and outer enamel epithelia at the basal margin of the enamel organ, in the region of the future cemento-enamel junction, proliferates and gives rise to the epithelial root sheath of Hertwig.
Fig. 19. Cap stage of tooth development. Human embryo 00 mm. long, 11th week. (Orbanfi) A. Wax reconstruction of the enamel organ of the lower lateral incisor.
B. Labiolingual section through the same tooth.
The functional activity of the dental lamina and its chronology may be considered in three phases: The first is concerned with the initiation of the entire deciduous dentition which occurs during the second month in utero (Fig. 1-1, .1 and B). The second phase deals with the initiation of the successors of the deciduous teeth. It is preceded by the growth of the free end of the dental lamina lsuecessional lamina), lingually to the enamel organ of each deciduous tooth. and occurs from about the fifth month in utero for the central permanent inc-isors, to 10 months of age for the second bic-uspicl eFig'. 1-1. ('1 . The thingl phase is preceded by the extension of the dental lamina distally to the enamel organ of the second deciduous molar whit-I1 begins in the 140 mm. emlu-_\'o Fig. 1-1:. C‘ .. The permanent molars arise directly from the distal extension of the dental lamina. The time of initiation is about -1 months of fetal life «in 160 mm. embryo) for the first permanent molar, the first year for the second permanent molar, and the fourth to fifth year for the third permanent molar.
Fig. 21.—The four layers of the e1Jlthe“1fi.1l_gl18.!§l31 organ in high magnification. Area X 0 ug.
Flg. 22.—Advanced bell stage of tooth development. Human embryo 200 mm. long, age about 18 weeks. Lablolingual section through the flrst deciduous lower molar.
It is thus evident that the total activity of the dental lamina extends over a period of about 5 years, while any particular portion of it functions for a much briefer period, since only a relatively short time elapses after initiation before the dental lamina begins to disintegrate at that particular location. Thus, whereas the free and deeper end of the dental lamina gives rise to the bud of the permanent successor, its gingival portion breaks up. Similarly, the dental lamina may be still active in the third molar region although it has disappeared elsewhere except for occasional epithelial remnants.
Fig. .‘:3.—Sagitt.al section through the head of a human fetus 200 mm. long. age about 18 weeks. in the region of the central incisors.
During the cap stage the dental lamina maintains a broad connection with the enamel organ but, in the bell stage, it begins to break up by niesenehgmal invasion which first penetrates its central portion and divides it into the lateral lamina and the dental lamina proper. The mesenehyinal invasion is at first incomplete and does not perforate the dental lamina (Fig. 20). The dental lamina proper proliferates only at its deeper margin which becomes a free end situated lingually to the enamel organ a11d forms the bud (anlage) for the permanent successor ' Fig. ‘20 s. The rest of the structure becomes more fenestrated and finally mostly resorbed. The epithelial connection of the enamel organ with the oral epithelium is severed by the mesoderm. The tooth germ then becomes a free internal organ. Remnants of the dental lamina may persist as epithelial pearls.
Labially and buccally to the dental lamina, another epithelial thickening develops independently and somewhat later. It is the Vestibular lamina also termed the bucco-gingival lamina or lip-furrow band (Figs. 18 and 19). It subsequently hollows out and forms the oral vestibule between the alveolar portion of the jaws and the lips and cheeks (Figs. 22 and 23).
D. Hertwig’s Epithelial Root Sheath and Root Formation
The development of the roots begins after enamel and dentin formation has reached the future cemento-enamel junction. The epithelial enamel organ plays an important part in root development by forming the Hertwig’s epithelial root sheath which initiates formation and molds the shape of the roots. It consists only of the outer and inner enamel epithelia, without the stratum intermedium and stellate reticulum.” The cells of the inner layer remain short and, normally, do not produce enamel. When these cells have induced the differentiation of connective tissue cells into odontoblasts and the first layer of dentin has been laid down, the epithelial root sheath loses its continuity and its close relation to the surface of the tooth. Its remnants persist as epithelial rests of Malassez.
There is a marked difference in the development of Hertwig’s epithelial root sheath in teeth with one root and those with two or more roots. Prior to the beginning of root formation the root sheath forms the epithelial diaphragm in single-rooted teeth (Fig. 2-1:). The outer and inner enamel epithelia bend at the future cemento-enamel 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‘ (see chapter on Eruption). The proliferation of the cells of the epithelial diaphragm is accompanied by that 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 epithelial organ lengthens coronally to the epithelial diaphragm (Fin. 24, B). The diiferentiation of odontoblasts and the formation of dentin immediately succeed the lengthening of the root sheath. At the same time the connective tissue of the dental sac surrounding the sheath proliferates and breaks up the continuous double epithelial layer (Fig. 24,
(‘i into a network of epithelial strands ~Fig. 2-}. D . The epithelium is pushed away from the dental surI'a(~e so that ('01ill€L‘Tl\'E' tissue comes into contact with the outer surfac-e of the dentin. Coniiec-tive tissue cells differentiate into eementohlasts and deposit a layer of cementum onto the sur
Fig. 2-l.—~Three stages in root develoinnent (diagrams).
A. Section through a tooth germ showing the epithelial diaphragm and proliferation zone of pulp.
B. Higher magnification of the cervical region of A.
C’. "Imaginary" stage showing the elongation of Hertwig’s epithelial sheath between diaphragm and future cemento-enamel junction. Differentiation of odontoblasts in the elongated pulp.
D. 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. face of the dentin. The rapid seqiience of p1'oli1'e1-atioii and destruction of Hertwig ‘s root sheath explains the fact that it cannot be seen as a continuous layer on the s111'Iac-e of the developing root LFig. 2-1, D }. In the last stages of root development the proliferation of the epithelium in the diaphragm ‘ . '.—Three sta es in the development of a. tooth with two roots, and one with thrfelgrogtas. Surface vigew of the epithelial djaphragm. During growth o_t the tooth germ the simple diaphragm (4) expands eccentrlcally go that hon_zonta._1 epxthellal fla._ps are formed (.3). Later these flaps proliferate a.n_d umte (dotted lmes 1n 0') and divlde the single cervical opening into two or three opening .
Fig. 26 Two stages in the development of a. two-rooted tooth. Diagrammatic mesiodistal sections of a lower molar. A. Beginning of dentin formation at the bifurcation. 3. Formation of the two roots in progress. (Details as in Fig. 24.)
lags behind that of the pulpal connective tissue. 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.‘ During the general growth of the coronal epithelial enamel organ the expansion of its cervical opening occurs in such a way that long tongue-like extensions of the horizontal diaphragm develop (Fig. 25'}. 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 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 bridges dentin formation starts (Fig. 26, A), and on the periphery of each opening root development follows in the same Way as described for single-rooted teeth (Fig. 26, B).
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. If the continuity of Hertwig’s root sheath is broken or is not established prior to dentin formation, a defect in the dcntinal wall of the pulp ensues. Such defects are found in the pulpal floor corresponding to the bifurcation if the fusion of the horizontal extensions of the diaphragm remains incomplete, or on any point of the root itself. This accounts for the development of accessory root canals opening on the periodontal surface of the root (see chapter on Pulp).
3. Eistopeysiology and Clinical Considerations
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.
TABLE I
Sraens IS Toorn Gaowrn
JIOEPHOLOGIC‘ STAGES: rntsronoatc 1>3.oc1:ss£s: Dental Lamina %—- -—————> Initiation
Bud Stage . ‘
Cap Stage (early) ? ’ Proliferation
Gap Stage (advanced) '] I
Bell Stage (early) ‘ L—————-—Histodifiei-entiation Bell Stage (advanced) J‘ ) -——-—-—Morphodifierent‘iation Formation of Enamel and - Dentin Matrix } "‘PP°‘“‘°” Initiation
For example, the process of histodifiercntiation characterizes the bell stage in which the cells of the inner enamel epithelium differentiate into functional amelohlasts. However, proliferation still progresses at the deeper portion of the enamel organ where Hertwig’s epithelial root sheath is forming.
The dental lamina and tooth buds represent that part of the oral epithelium which has potencies for tooth formation. Specific cells contain the entire growth potential of certain teeth and respond to those factors which initiate tooth development. Ditferent teeth are initiated at definite times. 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. therefore, multiplicative in character. It is marked by changes in the size and proportions of the growing tooth germ (Figs. 15 and 19).
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. Aberrations in tooth development can, therefore, be classified according to the stage of development at which they occur. 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. roots or the entire tooth).
Histodilferentiation succeeds the prolit'erative stage The formative cells of the tooth germs developing during the proliferative stage undergo definite histologic as well as chemical changes and acquire their functional assignment (the appositional growth potential). 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. 20).
The organizing influence of the epithelial cells on the mesenchymc is evident in the bell stage. The ditt'erentiation of the inner layer of the enamel organ into ameloblasts has been shoxm to be an essential preliminary step to the difi'erentiation of the adjacent cells of the dental papilla into odontoblasts. With the formation of dentin. the ameloblasts are stimulated to appositional function and enamel matrix is formed opposite the 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. and prestnnably the chemical influence, of the epithelial cells precede and are essential to the difi'ercntiation of the odontoblasts and the initiation of dentin 1'o1'u1ation.
If differentiation does not occur, the nonspecific. unorganized growth energy expresses itself in the continued unhampered proliferation of cells. A tumor is, therefore, characterized by unorganized proliferation and incomplete dilferentiation of the cells. The degree of nondifferentiation of the cells is an index to the rate of proliferation and, therefore, to the malignancy of the tumor.
In dentinogenesis imperfecta {hereditary opalescent dentin) the odoutoblasts fail to difierentiate completely. 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.
In vitamin A deficiency the ameloblasts fail to ditferentiate properly. In consequence, their organizing influence upon the adjacent mesenchymal cells is distributed and atypical dentin formation results. 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. by outlining the future dentinoenamel junction (Figs. 20 and 22}.
The dentino-enamel and dentino-cemental junctions which are diflfercnt and characteristic for each type of tooth act as a blueprint pattern. Onto this area the ameloblasts. odontoblasts and cementoblasts deposit enamel. dentin matrix and cementum. 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, prior to appositional growth.
The frequent statement in the literature that endocrine disturbances affect the size or form of the crown of teeth is not tenable unless such effects occur during morphodil’ferentiation. 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. 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.g., Hutchinson ’s incisor) with enamel and dentin that may be normal in structure.’ The ultimate shape of the crown may be disturbed even in the presence of a normal bell stage if the enamel formation is insufficient, as in enamel hypoplasia.
Apposition is the deposition of the matrix of the hard dental structures; it will be described in separate chapters on the formation of
enamel, dentin and cementum. 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. It is the fulfillment of the plans outlined at the stages of histo- and morphoditferentiation. Appositional growth is characterized by regular and rhythmic deposition of the extracellular material, periods of activity and rest alternating at definite intervals, and by the fact that the deposited material is of itself incapable of further growth.
The matrix is deposited by the cells along the site outlined by the formative cells at the end of morphodilferentiation (the future dentinoenamel and dentino—cemental junctions), and according to a definite pattern of cellular activity which is common to all types and forms of teeth. The growth potential acquired by the formative cells at the histodifferentiation stage is, therefore, expressed according to definite and universal laws of growth. 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.
Appositional growth proceeds according to a definite pattern. It begins at a given site, at the dentinal cusps, termed the growth center, and at a. given time, and proceeds in definite directions at definite rates which follow gradients of time, locus and anteroposterior direction. The amount of growth is definitely set by the rate of work (averaging 4 microns per day in man) and the functional lifespan of the formative cells. The result is an incremental pattern which is a summation of gnomonic curves superposed on the morphogenetic pattern (dentino-enamel junction). DE\'ELOP.\IE.\'T AND GRO\\'TH OI‘ TEETH
References
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