Book - Oral Histology and Embryology (1944) 4
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Orban B. Oral Histology and Embryology (1944) The C.V. Mosby Company, St. Louis.
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Chapter IV - The Dentin
The dentin constitutes the bulk of the tooth. As a living tissue it consists of specialized cells, the odontoblasts, and an intercellular or ground substance. In its physicaland chemical qualities it closely resembles bone. The main morphological difference between bone and dentin is that some of the osteoblasts forming the bone are included in the intercellular substance as osteocytes, whereas the dentin contains only cytoplasmic processes of the odontoblasts.
1. Physical Properties
In teeth of young individuals the dentin is, usually, light yellowish in color. Unlike enamel which is very hard and brittle, dentin is slightly compressible and It is somewhat harder than b_one, but considerably softer than enamel. The smaller content of mineral salts in dentin renders it more radiolucent than enamel. Dentin is birefringent due to the positive birefringency of the collagenous fibrils of the ground substance and the negative birefringency of the mineral contents which form submicroscopic apatite crystals.
2. Chemical Composition
Dentin consists of 30, per cent organic matter and water, and 70 per cent inorganic material (see Table in chapter on Enamel). The organic substance is chiefly collagen, a substance which yields glue or gelatin when boiled in water. The inorganic component is mostly apatite, as in bone, cementum and enamel. Organic and inorganic substances can be separated by decalcification or incineration. In the process of deca1cification the organic constituents can be retained as a cartilage-like material which maintains the shape of the dentin structure. Incineration removes the organic constituents and the inorganic substances shrink but retain the shape of the organ and become very brittle and porous.
First draft submitted by Getrit iéevelander.
The dentin consists of a fibrillar calcified ground substance which contains cytoplasmic processes of the odontolilasts (odontoblastic processes or Tomes’ fibers) in small tubes known as dentinal tubules. The matrix consists of fine collagenous fibrils‘ of approximately 0.3 micron in diam
Fig. 72. Collagcnous fibrils in the dentirml ground substance (decalclflcd longitudinal section ; silver impregnation).
cter, set in a homogenous calcified cementing substance (Fig. 72). These are the collagenous fibrils of the dentinal ground substance which are densely packed together and which are arranged in a direction approximately at right angles to the dentinal tubules (Fig. 73). The external layers of the dentin contain a variable amount of coarse and irregularly ar ‘Fiber: "A filamentary or threadlike structure."
Fibril: "A small fiber or component filament of a. fiber." (Goulrl.) DENTIN 103 ranged fibrils which give this layer a diiferent appearance under the microscope; this is the so-called “mantle” dentin.” In each successively formed layer of the dentin the fibrils cross each other at an acute angle. It has been claimed that the arrangement of the fibrils in the ground substance is adapted to functional stresses.” The fibrils are bound together into small bundles by the homogenous cementing substance, to form the dentinal ground substance. The calcium salts are contained in the cementing substance, the fibrils being uncalcified.”
Fig. 73. Colls.genous fibrils in the dentinal ground substance‘ (decalcifled transverse section; Mallory-Azan). (Orbanfl)
The odontoblasts are arranged along the pulpal surface of the dentin (see chapter on Pulp). Each cell sends a long cytoplasmic process (Fig. 74) throughout the entire thickness of the dentin (odontoblastic processes) which is contained in a dentinal tubule. There is only a potential space between the fiber and Wall of the tubule. This space is frequently enlarged during histologic preparation, due to shrinkage. The course of the dentinal tubule is somewhat curved, resembling an S in shape (Fig. 75). Starting at right angles from the pulpal surface the first convexity of this doubly curved course is directed toward the apex of the tooth. In the root of the tooth, and in the area of incisal edges and cusps, the tubules are almost straight.
Fig. 74. 0dontob1a.stic processes (Tomes' fibers) lyin in dentinal tubules, extend from the odontoblasts into the entin.
The processes of the odontoblasts are thickest, and the dentinal tubules widest, near the cell body, tapering toward the outer surface. They divide dichotomously near the end into several terminal branches (Fig. 76). Along their course they send out thin secondary processes which seem to unite with similar processes from neighboring odontoblastic processes (Fig. 77). They may be compared to the anastomosing processes of osteocytes. Some terminal branches of the odontoblast processes extend into the enamel (see chapter on Enamel). Occasionally, an odontoblast process splits into two almost equally thick processes; the division can occur at any distance from the pulp (Fig. 78).
The odontoblast processes are cytoplasmic extensions of the cells, with a denser and slightly deeper staining outer layer. The dentinal tubules containing the odontoblast processes are relatively wide near the pulpal cavity (3 to 4 microns), becoming narrower at their outer end (1 micron) (Fig. 79). Near the pulpal surface of the dentin the number of tubules in 1 square millimeter varies, according to some investigators, from 30,000 to 7 5,000.“ The pulpal surface of the dentin is about one—third to onefifth of the outer surface of the dentin. Accordingly, the dentinal tubules are farther apart in the peripheral layers of the dentin, and more closely packed near the pulp (Figs. 79, A and B). The ratio of the number of tubules per unit area, on the pulpal and outer surfaces, is about 4 to 1. There are more tubules per unit area in the crown than in the root. Their fine lateral branches contain the secondary branches of Tomes’ fibers. A thin zone of the Wall of the tubules immediately bordering the lumen, appears dark in hematoxylin and eosin stain. The area can be compared to the so—called capsule of the lacunae in bone. It is said to be a layer of ground substance Incremental devoid of fibrils“ and is known as Neun1ann’s sheath (Fig. 79). Even studies with the electron 1nieroscope”““ have failed to decide the question as to the nature of Neuinann ‘s sheath.
Fig. 75.—Ground section of human incisor. Observe course of dentinal tubules.
The formation and calcification of dentin begin at the tip of the cusps and proceed inward by a rhythmic apposition of conic layers, one within the other. When the dentin of the crown has been laid down, the apical layers assume the shape of elongated, truncated cones (Fig. 80). The daily
Fig. 76.—Dentina.l tubules showing dichotomous branching close to the dentino-enamel junction.
rate of apposition of the dentin in the crown varies from 4 to 8 microns in thicl:11ess.29:“ The appositional growth is graded in such a way that the increments become thinner as root formation progresses.“ The rhythmic growth pattern of dentin is indicated in the completed structure by fine lines (Fig. 81). These seem to correspond to rest periods in cellular activity and are known as the imbrication or incremental lines of von Ebner
Fig. 77. Seconda.ry branches of dentlnal tubules anastomosiug with those of neighboring as well as distant tubules. (Be\'ulancler.)
Fig. ’l8.—Splitting of dentinal tubules into branches. (Bevela.nder.) Fig. 79.—We1I-fixed decalcifled section of dentin. A. Close to the pulp. B. Close to the outer surface.
(Magnification X2000. No shrinkage between dentinal fibers a d (1 ti 1 t b I the entire tubule is fllle by the fiber. Note size and number o1'nden:i':1ai1atub'{l1I¢:.lse?fi
A and B.
Fig. 80. Diag'rammatlc illustration of the inc ta! it‘ 1 tt ¢e°i'1“°“S central incisor) 5 m.i.u. = 5 montheilhelilitezzo.
and Owen. They run at right angles to the dentinal tubules. Sometimes, the incremental pattern is accentuated due to less complete calcification. These lines, readily demonstrated in ground sections, are known as “contour lines of Owen” (Fig. 82). In the deciduous teeth and first permanent molars, where the dentin is formed partly before and partly after birth
Fig. 81. Incrementa.l lines in the dentin. Imbricatlon or incremental lines of v. Ebner and Owen. Ground section.
Fig. 82. Accentuated incremental lines in the dentin: Contour lines of Owen. Postnatal dentin
(Fig. 44), the prenatal and postnatal dentin are separated by an accentuated contour line, the so-called neonatal line“ (Fig. 83). It corresponds to the incompletely calcified dentin formed in the first two weeks after birth; it is caused by metabolic disturbances at the time of adjustment of the newborn to the abrupt changes of environment and nutrition.
Fig. 83.—PostnataIly formed dentin is separated from the prenatally formed by an accentuated lncremental line: the neonatal llne. (Sehour and Poncher."-‘)
Calcificatiogf the dentinal ground substance occurs by deposition Fofrsmalflcalciuifij globules which, normally, fuse to form a seemingly liomogenousrstrbstance (pages 122, 123). If calcification remains incomplete, the uncalcified or hypoealcified ground substance bounded by the globules forms the interglobular dentin. The dentinal tubules pass uninterrupted through these uncalcified areas (Fig. 84). In ground sections of dried teeth the uncalcified ground substance is shrunk or lost, and interglobular areas are filled with air and, therefore, appear black in transmitted light (Fig. 85). Interglohular dentin is found chiefly in the crown, near the dentino-enamel junction and arranged according to the incremental pattern of the tooth. Minute areas of interglobular dentin are, presumably, also responsible for the contour lines of Owen.
A thin layer of dentin adjacent to the cementum shows, in ground sections, a granular appearance (Fig. 86) ; this is known as Tomes’ granular layer. It is a constant morphologic feature, which is limited-to the root. Numerous minute areas of interglobular dentin are believed to give this zone its granular appearance. The development of Tomes’ granular layer, a zone of inadequately calcified dentin, has been attributed to the presence of a highly vascularized tissue on the surface of the root before cementum
Fig. 84.—Inte1-globular dentin (decalcifled section). The dentinal tubules pass uninterrupted through the uncalcifled and hypocalcifled ‘areas.
is deposited. It is claimed that calcification is retarded in close proximity to highly vaseularized areas. The absence of a granular layer in the crown is explained by the fact that, during development, the outer area of dentin in the crown is separated from the vascularized connective tissue by the enamel organ.
The study of dentin by means of polarized light has added to the knowledge of its structure. By this method it was shown that the calcification of dentin is largely the result of calcium salt impregnation around the fibrils.“
The long axis of the crystals is parallel to fibrils” and, therefore, approximately at right angles to the direction of the dentinal tubules (Fig. 87).
Subsequent studies with polarized light“ have shown another arrangement of inorganic salts, unique in the dentin. The polarized light reveals
Fig. 85.—Interglobule.r dentin as seen in a. dry ground section. Interglobular areas are filled with air and appear black in transmitted light. (Bevelandex-.)
areas of semilunar calcification in the dentin“! 19' 2° which are due to an initial phase of calcification in spherite form, with the crystals of the calci fication units having their long axes radiating from a common center (Fig. 87). DENTIN 113
Studies of the dentin, by means of polarized light, are difficult because the organic fibers and the inorganic salts are both birefringent, the first optically positive and the second optically negative; the total effect is a combination of the two. To separate them it has been necessary to work with (1) dentin freed of the organic constituents, and (2) dentin freed from the inorganic parts. By such means it has been shown” that the collagenous fibrils are made up of submicroscopic units with their long axes parallel to the length of the fibrils. In calcified dentin these fibrils are coated with apatite crystals similar to those of the enamel, and ar ranged also With their long axes parallel to the direction of the collagenous fibrils.
Fig. 8s.—'.l.‘omes’ granular layer lies in theectperipheral zone of the root dentin. Ground 3 on.
4. Innervation
Despite the obvious clinical observation that dentin is highly sensitive to a diversity of stimuli, the anatomic basis for this sensitivity is still controversial. The literature contains many accounts on the presence of nerve fibers in the denti11al tubules but these findings have repeatedly been demonstrated as artefacts. The difficulties of histologic technique are the cause for the lack of definitive information.
The pulp contains numerous unmyelinated and myelinated nerve fibers. The former end on the pulpal blood vessels, while the latter can be followed into the subodontoblastic layer. Here they lose their myelin sheath and can be followed into the odontoblastic layer itself. Between the cell bodies of the odontoblast most of the fibers apparently end in contact with the odontoblastic perikaryon. Occasionally part of a nerve fiber seems to be embedded in the predentin curving back into the odontoblastic layer.
The sensitivity of the dentin must be explained by changes in the odontoblastic processes, possibly changes of surface tension and surface electrical charges, that in turn provide a stimulus for the nerve endings in contact on the surface of the cell body.
Fig. 873. Dentin: seconds. Electron microscope photograph X7000.
5. Age And Functional Changes
A discussion on the vitality of dentin is complicated by the fact that many investigators think of dentin as consisting only of ground substance. However, dentin consists of ground substance and the odontoblasts with their cytoplasmic processes. If vitality is understood to be the capacity of the tissue to react to physiologic and pathologic stimuli, dentin should be considered a vital tissue.
The intercellular substance of the dentin is permeated, as any other tissue, by tissue fluid, unnecessarily termed dental lymph.“ 3' 9’ 1”’ “- 15 The dentin owes to this tissue fluid its turgor that plays an important role in securing the connection between dentin and enamel.
shadowed replica. of ground section, etched with 1/10 N HCL for 5 (Courtesy Dr. .1. Kennedy.)
Under normal conditions, formation of dentin may continue throughout life. Frequently, the dentin formed in later years of life is separated from that previously formed by a darkly stained line. In such cases the dentinal tubules bend more or less sharply at this line (Fig. 88). In other cases the newly formed dentin shows irregularities of varying degree; the tubules are often wavy and less numerous per unit area of the dentin. The dentin, forming pulpward of the line of demarcation, is called secondary dentin. This dentin is deposited on the entire pulpal surface of the dentin. However, its formation does not proceed at an even rate in all areas. This is best observed in bieuspids and molars where more secondary dentin is produced on the floor and roof of the pulpal chamber than on the side walls (see chapter on Pulp).
Fig. 88. The dentlnal tubules bend sharply as they pass from the primary into the secondary dentin. Dentinal tubules are somewhat irregular in the secondary dentin. Ground section.
The change in the structure from primary to secondary dentin may be caused by the progressive crowding of the odontoblasts which finally leads to the elimination of some and to the rearrangement of the remaining odontoblasts.”
The pulp reacts to more severe stimuli by forming a. type of dentin which shows still greater diiferences from primary dentin than the secondary dentin. It forms in restricted areas of the pulpal wall as a reacnnnrm 117 tion to extensive wear, erosion, and caries, which by the exposure of odontoblast processes cause pulpal irritation; it is termed irregular dentin. Here, the course of the tubules is frequently twisted and their number greatly reduced (Fig. 89). Some areas of irregular dentin contain few or no tubules. Dentin forming cells are often included in the rapidly produced ground substance; such cells degenerate and vacate the spaces which they formerly occupied. Frequently, irregular dentin is separated from primary or secondary dentin by a deeply staining line.
Fig. 89. Irregular dentin stimulated by penetration of caries into the dentin. Dentinal tubules are irregular and less numerous than in regular dentin. Decalcifled section.
Stimuli of different nature not only induce additional formation of dentin (secondary or irregular) but lead to changes in the dentin itself.’ Calcium salts may be deposited in or around degenerating odontoblast
]_3odecker' introduced the term “protective metamorphosis” for the age changes in the dentin, characterized by decreased permeability oi.’ the dentin due to changes in the dental
Fig. 90. (For legend ace oppoaite page.) Irregular dentin
processes and may obliterate the tubules. The refractive indices of dentin in which the tubules are occluded are equalized, and such areas become transparent. Transparent dentin can be observed in teeth of old people, especially in the roots. On the other hand, zones of transparent dentin developed around the dentinal part of enamel lamellae of Type B (Fig. 48) and under slowly progressing caries (Fig. 90). In such cases the blocking of the tubules may be considered as a defensive
Fig. 91.Dead tracts in the dentin of a vital tooth. due to attrition and exposure or
a group oi.’ dentinal tubules. Corresponding to the dead _tract irregular dentin formation 11: 1:12; glib. Dead tracts appear dark in transmitted light (A) and white in reflected g .
reaction of the dentin. Roentgen ray absorption tests“ a11d permeability studies“ have shown that such areas are denser and hardness tests“ have demonstrated that they are harder than normal dentin.“°= 3’
Transparent dentin is seen only in ground sections. It appears light in transmitted (Fig. 90, A1) and dark in reflected light (Fig. 90, B) be Fig. 90.—'l‘mnsps.rent dentin under a carious area viewed by A. transmitted light, B, reflected light. and G, Grenz rays. Normal dentinal tubules are filled with air in dried und sections and appear dark in transmitted light (A) and white in reflected light ( ). Transparent dentin shows the opposite behavior because the tubules are filled with calcium salts. In a Grenz-ray picture (0 transparent dentin appears more white because of its higher degree of radiopucity. ( renz-ray picture courtesy E. Applebaum. Columbia. University.)
cause the light passes through the transparent dentin, but is reflected from the normal dentin. Zones of dentin, decalcified by caries, normal dentin, and transparent dentin can be differentiated by the examination of ground section with soft roentgen rays (grenz rays) .
Fig. 92.—Dead tracts in the dentin or a. vital tooth, due to crowding and degeneration of odontoblasts in narrow pulpal horns. and exposure or dentinal tubules in erosion. Well-fixed. ground section (not dry!).
The composition of dentin does not change with age,“ *3 though increase
of specific gravity of dentin with advancing age and reduction of its strength was reported.‘ ‘
In dried ground sections of normal dentin the odontoblastie processes disintegrate, and the empty tubules are filled with air. They appear black in transmitted and white in reflected light (Figs. 90, A and B). Disintegration of odontoblastic processes may also occur in living teeth as a result of the irritation of caries, attrition, abrasion, or ‘erosion (Figs. 91 and 92). Degeneration of odontoblasts is frequently observed in the narrow pulpal DENTIN 121
horns (Fig. 92) due to crowding of odontoblasts. Irregular dentin seals these tubules at their pulpal end. In all these cases, dentinal tubules in vital teeth are filled with gaseous substances; in ground sections such groups of tubules appear black in transmitted and white in reflected light. Dentin areas characterized by degenerated odontoblast processes have been called dead tracts.“ They are areas of decreased sensitivity.
6. Development
The first sign of dentin development is a thickening Q:f the basement membrane (membrana preformativa) between the inner enamel epithelium and the mesodermal primary pulp. The thickening is first visible at the tips of cusps or incisal edges of the tooth germs, progressing in the direction of the ultimate apex (Fig. 93). The basement membrane which is derived from the mesenchyme of the pulp consists of fine intercrossing reticular argyrophil fibrils. The next staggin dentin development is characterized by the formation of irregularl s iralin fibers ori ' atin ' $13, which merge_with the fibrils of the l')aseme.n_’_r. me.m}n_-amp. (Fig. 94). These spiraling fibersqstainblack with silver (argyrophil fibers), a reaction which is characteristic of precollagenous substance. At their pulpal origin they are continuous with many fine fibrils of the pulp; they are known as Korff’s fibers. Each consists of a large number of fine fibrils cemented together to form an optically homogenous structure.
While the Korff’s fibers appear, the spindleshaped mesenchymal ggl_l closest to the asement membrane, assume a high columnar sham (see chapter on Enamel, Fig. 61). These cells are arranged in a continuous layer and are termed odontoblasts. They are linked to one another by intercellular bridges. A protoplasmic process of the odontoblast extends toward the future dentino-enamel junction; it elongates and branches as dentin deposition takes place.
The formation of dentin starts with a spreading of the parts of l_§orE’s Qers nearest to the baselnent membrane (Fig. 94). This spreading may be due to an expansion or swelling of the interfibrillar cementing substance in Korfl?’s fibers. At the same time, the substance of the KorfE’s fibers undergoes a striking E_eg1_g_Ll1apge which caus_es__ gem to pass gain a_ precollagenous_t_Q_a collagenous sta.ge_(Fig. 95). The change is best detected— by the reactions which this substance exhibits to certain specific stains. The substance which was formerly argggphg '3; no longer stains black in silver preparation, but assumes a reddish-brown glgr which is characteristic n'F nn11ageno11s_m%w Its collagenous nature can also be detected by Mallory-Azan staining; in this preparation the collagenous ground substance appears blue (Fig. 95).
The 2roumLsnJ_a.s_Lmce is 3'“ firS£—uI&1°ifi&<1 and» i predentin. The dentina] fibrils seem to diflerentiate in the primarily.
While the apposition of a current layer of predentin is taking place, the previous layer is undergoing calcification. The formation of predentin and calcification follow an incremental pattern. Calcification lags behind the formation of the ground substance in such a way that the last increment always remains uncalcified predentin as long as formation of dentin proceeds (Fig. 74). At the same time the K01-ff’s fibers elongate and extend farther into the areas between the receding odontoblasts.
Fig. 93. Tooth germ with beginning dentin formation. Silver impregnation. lander.)
(Beve The transformation of the pulpal fibrils into Kortf’s fibers, the transformation of the argyrophile Korff’s fibers into the collagenous predentin, the difierentiation of the fibrils in the predentin, and finally the calcification of the cementing substance are in all probability brought about by the enzymatic activity of the odontoblasts. Dentinogencsis is correlated with the presence of alkaline phosphatase in the subdontoblastie region and in the odontoblasts and their processes.‘
The calcification of dentin matrix follows a varied pattern. The crystals of calcium salts (apatite) are deposited around the collagenous fibrils in the cementing substance; the fibrils thcniselves remain uncalcified. In some cases, globules of different sizes are formed which later fuse and give the ground substance a homogeneous appearance. In other cases, the calcium salts are deposited in layers. Sometimes, areas can be observed in which globular and linear calcification are combined (Fig. 96). These phenomena are similar to those occurring during precipitation of crystals in colloidal media (Liesegang’s rings). When calcified, predentin constitutes the mature dentin ground substance.
Fig. 94. Thickening of the basement membrane between pulp and enamel epithelium. Development ot Korlfs fibers and their transrormation lnto dentin matrix Bevelander. )
7. Clinical Considerations
The vitality of dentin is dependent upon the presence of odontoblasts and their processes, and thus upon the vitality of the pulp. The vitality of the dentin constitutes the basis of defense reactions in response to normal and pathologic stimuli (irregular dentin, transparent or sclerotic dentin).
As a vital tissue, dentin should be treated with the utmost care in operative procedures."’v 257 3‘ Whenever dentin is cut or heat or drugs are applied, a reaction occurs as a result of irritation of the odontoblasts through their processes. The exposed dentin should not be insulted by strong drugs, undue operative trauma, unnecessary thermal changes, or irritating filling materials. Contact of exposed dentin with saliva should be avoided. It should be borne in mind that, by exposing one square millimeter of dentin, about 30,000 dentinal tubules are likewise exposed. The surface may be treated with astringent drugs, such as phenol, silver nitrate,‘*"‘- ‘° to coagulate the protoplasm of the exposed odontoblast processes. It is advisable to cover the exposed dentin surface by a nonirritating insulating substance formed.
Fig. 95. Due to a chemical change the argyrophilic Korfi’s fibers become trflfls mto the collagenous ground substance or the dentin.
Fig. 96. Linear and globular calcification of dentin. A. Silver impregnation (Korfl's fibers, black) and hems.toxy1in—azure+eosin (collagenous dentin ground substance, brownish red). B. Silver impregnation (Korffs fibers, black) and staining by Heidenhain's non of Mallol-y’s method. (Coflagenous dentin ground substance. blue.) (Orb$
The rapid penetration and spreading of caries in the dentin is due to the high content of organic substances in the dentin matrix. The enamel may be undermined at the dentino-enamel junction, even when caries in the enamel is confined to a small area. The dentinal tubules form a passage for invading bacteria which may, thus, reach the pulp through a thick dentinal layer.
The sensitivity of the dentin varies considerably in difierent individuals. In most cases, it is greater close to the outer surface of the dentin, and diminishes in the deeper layers. The sensitivity of the dentin, therefore, is not a warning signal to avoid exposure of the pulp. The operations in the dentin can be rendered less painful by avoiding heat and pressure by the use of water and sharp instruments.
References
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Cite this page: Hill, M.A. (2024, April 18) Embryology Book - Oral Histology and Embryology (1944) 4. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_Oral_Histology_and_Embryology_(1944)_4
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