Book - Oral Histology and Embryology (1944) 4: Difference between revisions

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==2. Chemical Composition==
==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.
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.
First draft submitted by Getrit iéevelander.




The dentin consists of a fibrillar calcified ground substance which
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
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.”
 
“JIg:v  J g.f|
.1 1 9..


E. .$ ." .‘ '_ Q. 0 ;
i ‘ . 9


’ , ' ‘ 9 l ‘
Fig. 72. Collagcnous fibrils in the dentirml ground substance (decalclflcd longitudinal section ; silver impregnation).


_‘ p .__r I . s
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."


(E ‘xii’? 3 ,3‘? ii
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.”


\ 2
""§§ ex ‘1 I; .‘
5 " ’ \
a 4 (1 ‘ m.
we‘ '0 ' ' It‘?
Kile‘-1 .' DC 0,5‘
‘ ’. .. . Q 5


4 ‘cg


' ‘ 3.!
Fig. 73. Colls.genous fibrils in the dentinal ground substance‘ (decalcifled transverse section; Mallory-Azan). (Orbanfl)
i  Q 5 i W
.94 '3; ‘4* we‘. is "
5, __ nu


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.


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


Odontoblastic
Processes
and Dentinal Tubules
104 ORAL HISTOLOGY AND EMBRYOLOGY


during histologic preparation, due to shrinkage. The course of the dentinal
Fig. 74. 0dontob1a.stic processes (Tomes' fibers) lyin in dentinal tubules, extend from the odontoblasts into the entin.
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.


_ _—_" Calcifled dentin
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).


' '  ' " Unealcifled dentin
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.
(predentin)


" ' Odontoblastic processes


*_ '"— '3 '?—'—* Odontoblasts


Q Q
-  if i O
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).
DENTIN 105
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 far
Fig. 75.—Ground section of human incisor. Observe course of dentinal tubules.
Fig. 75.—Ground section of human incisor. Observe course of dentinal tubules.


ther 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


106 ole.-xi. HIS’i‘()I.()GY AND EMBRYOLOGY
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


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.


The formation and calcification of dentin begin at the tip of the cusps
Fig. 76.—Dentina.l tubules showing dichotomous branching close to the dentino-enamel junction.
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


_»~——»— A— Deniinal lubulo-s
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


Dentino-enamel - - - *Junction
Fig. 77. Seconda.ry branches of dentlnal tubules anastomosiug with those of neighboring as well as distant tubules. (Be\'ulancler.)


Enamel —
-~ W-*—“—“‘ Dentinal tubules
 
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
DENTI1\‘ 107
»-1
.
...\~ 9-t'~
1'4 ,_ma-»z,4."SL
.I _~
 
 
-‘x 1  ; x  r
’ ‘Alf bf-gl  ml?
Fig. 77.—Seconda.ry branches of dentlnal tubules anastomosiug with those of neighboring
as well as distant tubules. (Be\'ulancler.)
_f_  ,‘, I 1, V1‘ . .
_;:b fit ‘ . ~ /
Enamel .  / ' ’ I ’. '
Terminal branches "A
of dentinal tubules


   
   
 
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.
Splitting of
dentinal tubules
 
 
 
 
Splitting of
dentinal tubules
 
 
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
(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
Line 237: Line 75:
A and B.
A and B.


Fig. 80.-—Diag'rammatlc illustration of the inc ta! it‘ 1 tt
Fig. 80. Diag'rammatlc illustration of the inc ta! it‘ 1 tt ¢e°i'1“°“S central incisor) 5 m.i.u. = 5 montheilhelilitezzo.
¢e°i'1“°“S central incisor) 5 m.i.u. = 5 montheilhelilitezzo. ap(ps°§h$3?aan§aM§§§1ergémer
DENTIN 109
 
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
 
i.»~  x \ . -  .
__ . .
 
.
r
I
 
.
|
is’
>‘ (
l \
\ ' '3
. v_ —
C’
. . ‘..‘ \
5 ' \
‘l‘.‘l‘ "'1 V ‘ K
. \
‘:-:——_I .——T_


Fig. 81.—Incrementa.l lines in the dentin. Imbricatlon or incremental lines of v. Ebner
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
and Owen. Ground section.




Fig. 82.—Accentuated incremental lines in the dentin: Contour lines of Owen.
Fig. 81. Incrementa.l lines in the dentin. Imbricatlon or incremental lines of v. Ebner and Owen. Ground section.
Postnatal dentin ——~:——--————-—— v ~——-
Interglobular
Dentin


Prenatal dentin -
110 onnn msronooy AND EMBRYOLOGY


(Fig. 44), the prenatal and postnatal dentin are separated by an accentuated contour line, the so-called neonatal line“ (Fig. 83). It corresponds
Fig. 82. Accentuated incremental lines in the dentin: Contour lines of Owen. Postnatal dentin
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.


Neonatal line -
(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.


K’:


Fig. 83.—PostnataIly formed dentin is separated from the prenatally formed by an accentuated lncremental line: the neonatal llne. (Sehour and Poncher."-‘)
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
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.
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.
DENTIN 111
 
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
 
_  _ ___
 
J, J
,i '
 
 
 
6
5


.5
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.
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
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.
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
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.“
dentin is largely the result of calcium salt impregnation around the fibrils.“


Tomes’ Granular Layer
Submicroscopic
Structure
112 omn HISTOLOGY AND EMBRYOLOGY


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).
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).
Line 346: Line 107:
Subsequent studies with polarized light“ have shown another arrangement of inorganic salts, unique in the dentin. The polarized light reveals
Subsequent studies with polarized light“ have shown another arrangement of inorganic salts, unique in the dentin. The polarized light reveals


Dentlno-enamel
junction
 
Interglobular areas
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
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-.)


i
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
-—— Enamel


Dentin —~
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.
’ Cemento-enamel junction


Inter-globular dentin




Cementum


- - Tomes’ granular layer


Interglobular dentin --
Fig. 8s.'.l.‘omes’ granular layer lies in theectperipheral zone of the root dentin. Ground 3 on.
. Tomes’ granular layer


_.—%..——_ . A J


Fig. 8s.—'.l.‘omes’ granular layer lies in theectperipheral zone of the root dentin. Ground
==4. Innervation==
3 on.


from the inorganic parts. By such means it has been shown” that the
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.
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.
114 omu. HISTOLOGY AND EMBRYOLOGY


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.2* '-’*- “S


   
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.
   


(‘geld Pelalian lo
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.
Enamel Poi


54-rrulunar ar $-/tubal
Fig. 873. Dentin: seconds. Electron microscope photograph X7000.


n./.,r...1...,. my ;. denlirjl
==5. Age And Functional Changes==


Dentn-u1I'TubuIes _ l
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.


Una  jcneml
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.
.Dir:ci/on fiamllel la d-e
ju..=:;ny o/I‘76rr'A'figl Tomes)


Denfi.-I.
shadowed replica. of ground section, etched with 1/10 N HCL for 5 (Courtesy Dr. .1. Kennedy.)


8”--:°,.*,:%;i:'::8:.i°mi:L‘;:%::.“3;* aztusiesaztztaszaez :51a::*:;'a*‘%:§::°P‘° “W


The pulp contains numerous unmyelinated and myelinated nerve fibers.
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).
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
DENTIN 115


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:
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.
seconds. Electron microscope photograph X7000.


5. AGE AND FUNCTIONAL CHANGES
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.


A discussion on the vitality of dentin is complicated by the fact that
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.
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.)


vitality of
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.
Dentin
secondary
Dentin


Irregular
Dentin


116 03.1.1. nrsromev AND EMBRYOLOGY


Under normal conditions, formation of dentin may continue throughout
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
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).


Demarcation line
]_3odecker' introduced the term “protective metamorphosis” for the age changes in the
 
 
 
’-- Secondary dentin
 
Fig. 88.—’l.‘he 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
 
 
‘lit
 
8-. £15‘
 
tlxrd‘ 3 i
1, S "' 1
‘ ' ls‘
 
Irregular dentin
 
   
 
~ 1 in A” . Demarcation line
 
 
.~ .3»;
 
I
 
Dentin
 
’.
5%’
 
A
 
.,i
4'
 
 
Fig. 89.—Irreg'ular dentin stimulated by penetration of caries into the dentin. Dentinal
tubules are irregular and less numerous than in regular dentin. Decalcifled section.
 
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.
 
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
dentin, characterized by decreased permeability oi.’ the dentin due to changes in the dental


lymph.
Transparent
(sclerotic)
Dentin
- Dentinal tubules
filled with air
v Carious dentin
9
‘I ' Transparent dentin
Dentinal tubules
filled with air
~~A~—~A ~—~— ——--~——~ Curious dentin
r — ‘—-—~——‘~v"’-W“-*-' ~ " Transparent dentin
.
Dentin
 
__ _ WW, — . carious dentin
9 ,' ‘.
— --“.—’..... Transparent dentin
Fig. 90.-—(For legend ace oppoaite page.)
Irregular
dentin
DENTIN 119
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
I
9


A. B.


Fig. 91.——Dead tracts in the dentin of a vital tooth. due to attrition and exposure or
Fig. 90. (For legend ace oppoaite page.) Irregular dentin


a group oi.’ dentinal tubules. Corresponding to the dead _tract irregular dentin formation
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
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
Fig. 91.Dead tracts in the dentin of a vital tooth. due to attrition and exposure or
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.)


Irregular
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 .
dentin
Dead Tracts


120 omu. HISTOLOGY AND EMBRYOI.-OGY
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’


cause the light passes through the transparent dentin, but is reflected
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.)
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) .1


4:4
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) .


fiffl


Fig. 92.—Dead tracts in the dentin or a. vital tooth, due to crowding and degeneration
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!).
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
The composition of dentin does not change with age,“ *3 though increase


of specific gravity of dentin with advancing age and reduction of its
of specific gravity of dentin with advancing age and reduction of its strength was reported.‘ ‘
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 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
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
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.
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.14, 34
==6. Development==


6. DEVELOPMENT
The first sign of dentin development is _a thicke_nin.«z__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.


The first sign of dentin development is _a thicke_nin.«z__Q:f the basement
While the Kor1f’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.
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.
The formation g£_dentin sta3ts_ 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).


While the Kor1f’s fibers appear, the spindleshaped mesenchymal ggl_l§
The 2roumLsnJ_a.s_Lmce is 3'“ firS£—uI&1°ifi&<1 and» i predentin. The dentina] fibrils seem to diflerentiate in the primarily.


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
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.
intercellular bridges. A protoplasmic process of the odontoblast extends


toward the future dentino-enamel junction; it elongates and branches as
Fig. 93.—'J.‘ooth germ with beginning dentin formation. Silver impregnation. lander.)
dentin deposition takes place.


The formation g£_dentin sta3ts_ with a spreading _of the parts of_ l_§orE’s
(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 pi-edentin, 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.
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
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.
predentin. The dentina] fibrils seem to diflerentiate in the primarily.
homogeneous predenE'n._
Beginning dentin


formation


Enamel organ


122
Fig. 94.—'.l‘hickening of the basement membrane between pulp and enamel epithelium. Development ot Korlfs fibers and their transrormation lnto dentin matrix Bevelander. )


ORAL HISTOLOGY AND 1«:1\1mn'0L0uY


_ 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.—'J.‘ooth germ with beginning dentin formation.
==7. Clinical Considerations==


Silver impregnation.
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).
lander.)


(Beve
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 ubstance. formed _ Fig. 95.—Due to a chemical change the argyrophilic Korfi’s fibers become trflfls mto the collagenous ground substance or the dentin.
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 pi-edentin, 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
DENTIN‘ 123


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


   


Ameloblasts ,  _
Fig. 96.—Linear and globular calcification of dentin.
5%,, i "E". 3”‘
‘In » Wu!»
, I ..
.»-~«; “
' . 44-.3:
‘ k  KorfE's
E‘ flbers


Thickening of Ebasement
membrane


Fig. 94.—'.l‘hickening of the basement membrane between pulp and enamel epithelium. Development ot Korlfs fibers and their transrormation lnto dentin matrix
A. Silver impregnation (Korfl's fibers, black) and hems.toxy1in—azur¢+eosi1'I (collagenous dentin ground substance, brownish red).
Bevelander. )


phenomena are similar to those occurring during precipitation of crystals
B. Silver impregnation (Korffs fibers, black) and staining by Heidenhain's non of Mallol-y’s method. (Coflagenous dentin ground substance. blue.) (Orb$
in colloidal media (Liesegang’s rings). When calcified, predentin constitutes the mature dentin ground substance.


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
124 omu. I-IISTOLOGY AND EMBRYOLOGY


of the dentin constitutes the basis of defense reactions in response to
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.
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
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.
their processes. The exposed dentin should not be insulted by strong drugs,
undue operative trauma, unnecessary thermal changes, or irritating filling


Linear
==References==
calcification


Globular
1. Applebaum. E., Hollander, FL, and Bodecker, C. F.: Normal and Pathologital Variations in Calcification of Teeth as Shown by the Use of Soft X-rays_ Dental Cosmos 75: 1097, 1933.
calcification


Fig. 96.—Linear and globular calcification of dentin.
Berkelbach van der Sprenkel, H.: Zur Neurologie des Zahnes (The Neurology of the Tooth), Ztschr. f. mikr.-anat. Forsch. 38: 1, 1935.


materials. Contact of exposed dentin with saliva should be avoided. It
Bevelander, G.: The Development and Structure of the Fiber System of Dentin, Anat. Rec. 81: 79, 1941.
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
ubstance.
formed
_ Fig. 95.—Due to a chemical change the argyrophilic Korfi’s fibers become trflfls
mto the collagenous ground substance or the dentin.


A. Silver impregnation (Korfl's fibers, black) and hems.toxy1in—azur¢+eosi1'I
Bevelander, G., and Johnson, P. L.: Odontoblasts and Dentinogenesis, J. Dent. Research 25: 381, 1946.
(collagenous dentin ground substance, brownish red).


_ B. Silver impregnation (Korffs fibers, black) and staining by Heidenhain's
Bevelander, G., and Amler, M. H.: Radioactive Phosphate Absorption by Dentin and Enamel, J. Dent. Research 24: 45, 1944.
non of Mallol-y’s method. (Coflagenous dentin ground substance. blue.) (Orb$
 
stalninz
 
f;x‘9dlfl(:8.fl. ‘)
 
DENTIN 125


The rapid penetration and spreading of caries in the dentin is due to
Black, G. V.: Investigation of the Physical Characters of the Human Teeth, D. Cosmos 37: 353, 469, 1895.
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,
Bodecker, C. F.: The Soft Fiber of Tomes, a Tubular Structure: Its Relation to Dental Caries and the Desensitization of Dentin, J. Nat. Dent. Assn. 9: 281, 1922.
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
Bodeeker, C. F.: Dangers of Extensive Operative Procedure on Recently Erupted Teeth, J. A. D. A. 28: 1598, 1941.


1. Applebaum. E., Hollander, FL, and Bodecker, C. F.: Normal and Pathologital
9. Bodecker, C. F., and Gies, Wm. J .: Concerning the Character of the Age Changes in Enamel and Dentin and Their Relation to the Vital Dental Pulp, J. Am. Coll. Dentists 9: 381, 1942.  
Variations in Calcification of Teeth as Shown by the Use of Soft X-rays_
Dental Cosmos 75: 1097, 1933.


Berkelbach van der Sprenkel, H.: Zur Neurologie des Zahnes (The Neurology
10. Bodecker, C. F., and Lefkowitz, W.: Concerning the “Vitality” of Calcified Dental Tissues, J. Dent. Research 16: 463, 1937.  
of the Tooth), Ztschr. f. mikr.-anat. Forsch. 38: 1, 1935.
 
Bevelander, G.: The Development and Structure of the Fiber System of Dentin,
Anat. Rec. 81: 79, 1941.
 
Bevelander, G., and Johnson, P. L.: Odontoblasts and Dentinogenesis, J. Dent.
Research 25: 381, 1946.
 
Bevelander, G., and Amler, M. H.: Radioactive Phosphate Absorption by Dentin and Enamel, J. Dent. Research 24: 45, 1944.


Black, G. V.: Investigation of the Physical Characters of the Human Teeth,
11. Cape, A. T., and Kitchin, P. C.: Histologic Phenomena of Tooth Tissues as Observed Under Polarized Light, J. A. D. A. 17: 193, 1930.  
D. Cosmos 37: 353, 469, 1895.


Bodecker, C. F.: The Soft Fiber of Tomes, a Tubular Structure: Its Relation
12. Crowell, D. C., Hodge, H. C., and Line, W. 15%.: Chemical Analysis of Tooth Samples Composed of Enamel, Dentin and Cementum, J. Dent. Research 14: 251, 1934. '
to Dental Caries and the Desensitization of Dentin, J. Nat. Dent. Assn. 9:
281, 1922.


. Bodeeker, C. F.: Dangers of Extensive Operative Procedure on Recently
13. Ebner, V. v.: Ueber die Entwicklung der leimgebenden Fibrillen im Zahnbein (Development of Collagenous Fibrils in the Dentin), Sitzungsb. d. k. Akad. d. Wissensch. Vienna 115: 281, 1906; and Anat. Anz. 29: 137, 1906.


Erupted Teeth, J. A. D. A. 28: 1598, 1941.
14. Fish, E. W.: An Experimental Investigation of Enamel Dentin and the Dental Pulp, London, 1932, John Bale Sons & Danielsson.  
9. Bodecker, C. F., and Gies, Wm. J .: Concerning the Character of the Age Changes
in Enamel and Dentin and Their Relation to the Vital Dental Pulp, J. Am.
Coll. Dentists 9: 381, 1942. ‘
10. Bodecker, C. F., and Lefkowitz, W.: Concerning the “Vitality” of Calcified
Dental Tissues, J. Dent. Research 16: 463, 1937.
11. Cape, A. T., and Kitchin, P. C.: Histologic Phenomena of Tooth Tissues as Observed Under Polarized Light, J. A. D. A. 17: 193, 1930.
12. Crowell, D. C., Hodge, H. C., and Line, W. 15%.: Chemical Analysis of Tooth
Samples Composed of Enamel, Dentin and Cementum, J. Dent. Research
14: 251, 1934. '
13. Ebner, V. v.: Ueber die Entwicklung der leimgebenden Fibrillen im Zahnbein
(Development of Collagenous Fibrils in the Dentin), Sitzungsb. d. k.
Akad. d. Wissensch. Vienna 115: 281, 1906; and Anat. Anz. 29: 137, 1906.
14. Fish, E. W.: An Experimental Investigation of Enamel Dentin and the Dental
Pulp, London, 1932, John Bale Sons & Danielsson.
15. Fish, E. W.: The Circulation of Lymph in Dentin and Enamel, J. A. D. A. 14:
1, 1927.
16. Gebhardt, W.: Ueber den funktionellen Bau einiger Ziihne (The Functional
Structure of Some Teeth), Arch. f. Entwcklngsmechn. d. Organ. 10: 135,
1900.


P’
15. Fish, E. W.: The Circulation of Lymph in Dentin and Enamel, J. A. D. A. 14: 1, 1927.


°°>'S"?‘!'°‘P’
16. Gebhardt, W.: Ueber den funktionellen Bau einiger Ziihne (The Functional Structure of Some Teeth), Arch. f. Entwcklngsmechn. d. Organ. 10: 135, 1900.


_17. Grurley, W. B., and Van Huysen, Gr.: Histologic Response of the Teeth of Dogs
17. Grurley, W. B., and Van Huysen, Gr.: Histologic Response of the Teeth of Dogs


to Operative Procedures, J. Dent. Research 19: 179, 1940. _
to Operative Procedures, J. Dent. Research 19: 179, 1940. _


18. Hodge, H. C.: Microhardncss Studies of Transparent Dentine, Brit. D. J. 63:
18. Hodge, H. C.: Microhardncss Studies of Transparent Dentine, Brit. D. J. 63: 181 1937.
181 1937.


19. Keil, A.’: Ueber Doppelbrechung und Feinbau des menschlichen Zahnbeins (On
19. Keil, A.’: Ueber Doppelbrechung und Feinbau des menschlichen Zahnbeins (On Birefringence and the Minute Structure of Human Dentin), Ztschr. :E. Zel1forsch. u. Mikr. Anat. 21: 635, 1934. 126 ORAL HISTOLOGY AND EMBRYOLOGY
Birefringence and the Minute Structure of Human Dentin), Ztschr. :E. Zel1forsch. u. Mikr. Anat. 21: 635, 1934.
126 ORAL HISTOLOGY AND EMBRYOLOGY


20. Keil, A.: Ueber den Feinbau der harten Zahnsubstanz nach Untersuchungen in
20. Keil, A.: Ueber den Feinbau der harten Zahnsubstanz nach Untersuchungen in polar-isiertem Licht (On the Minute Structure of the Hard Tissue of the Teeth Studied Under Polarized Light), Deuische Zaln1- Mun(l- 11. l{ieferheilk. 2: 741, 1935.
polar-isiertem Licht (On the Minute Structure of the Hard Tissue of the
Teeth Studied Under Polarized Light), Deuische Zaln1- Mun(l- 11. l{ieferheilk. 2: 741, 1935.


2]. K0rfl', K. v.: Wa.chstun1 der Dentingrundsulrstanz verschiedener Wirhelliorc
2]. K0rfl', K. v.: Wa.chstun1 der Dentingrundsulrstanz verschiedener Wirhelliorc ((z‘rroWth of the Dentin Matrix of Difi"erent Vertebrates), Ztschr. f. 1nikr.anat. Forsch. 22: -1-45, 1930.
((z‘rroWth of the Dentin Matrix of Difi"erent Vertebrates), Ztschr. f. 1nikr.anat. Forsch. 22: -1-45, 1930.


22. Korif, K. v.: Die Entwicklung der Zahnbein Grundsubstanz der Saugetiere
22. Korif, K. v.: Die Entwicklung der Zahnbein Grundsubstanz der Saugetiere (The Development of the Dentin Matrix in Mammals), Arch. f. mikr. Anat. 67: 1 1905.
(The Development of the Dentin Matrix in Mammals), Arch. f. mikr. Anat.
67: 1 1905.


23. LeFevre, ’1\I. L., and Hodge, H. 0.: Chemical Analysis of Tooth Samples, J.
23. LeFevre, ’1\I. L., and Hodge, H. 0.: Chemical Analysis of Tooth Samples, J. Dent. Research 16: 279, 1937.
Dent. Research 16: 279, 1937.


24. Lehner, J., and Plenk, H.: Die Ziihne (The Teeth), Moel1endorif’s Handb. der
24. Lehner, J., and Plenk, H.: Die Ziihne (The Teeth), Moel1endorif’s Handb. der Mikrosk. Anat., vol. 5, pt. 3, p. 449, 1937.
Mikrosk. Anat., vol. 5, pt. 3, p. 449, 1937.


25. Manley, E. G.: Traumatic Effect of the Drill During Cavity Preparation Brit.
25. Manley, E. G.: Traumatic Effect of the Drill During Cavity Preparation Brit. D. J. 70: 329, 1941.
D. J. 70: 329, 1941.


26. Muntz, J. A., Dorfman, A., and Stephan, R. M.: In Vitro Studies on Sterilization of Carious Dentin J. A. D. A. 30: 1893, 1913.
26. Muntz, J. A., Dorfman, A., and Stephan, R. M.: In Vitro Studies on Sterilization of Carious Dentin J. A. D. A. 30: 1893, 1913.
Line 903: Line 297:
27. Orban, B.: The Development of the Dentin, J. A. D. A. 16: 1547, 1929.
27. Orban, B.: The Development of the Dentin, J. A. D. A. 16: 1547, 1929.


28. Schmidt, W. J.: The Elements of the Animal Body in Polarized Light, Bonn.
28. Schmidt, W. J.: The Elements of the Animal Body in Polarized Light, Bonn. 192-1., quoted by Kitchin, P. (3.: Beyond the Microscope, J. Dent. Research 17: 275, 1938.
192-1., quoted by Kitchin, P. (3.: Beyond the Microscope, J. Dent. Research
17: 275, 1938.


28a. Scott, D. B., and Wycoif, R. NY. G.: Electron Microscopy of Human Enamel,
28a. Scott, D. B., and Wycoif, R. NY. G.: Electron Microscopy of Human Enamel, J. 1). Res. 29: 556, 1950.
J. 1). Res. 29: 556, 1950.


29. Schour, I., and Hofiman, M. M.: The Rate of Apposition of Enamel and Dentin
29. Schour, I., and Hofiman, M. M.: The Rate of Apposition of Enamel and Dentin in Man and Other Animals, J. Dent. Research 18: 161, 1939.
in Man and Other Animals, J. Dent. Research 18: 161, 1939.


30. Schour, I., and Massler, M.: The Neonatal Line in Enamel and Dentin of the
30. Schour, I., and Massler, M.: The Neonatal Line in Enamel and Dentin of the Human Deciduous Teeth and Ifirst Permanent Molar, J. A. D. A. 23: 19-16, 1936.
Human Deciduous Teeth and Ifirst Permanent Molar, J. A. D. A. 23: 19-16,
1936.


31. Schour, I., and Massler, M.: The Growth Pattern of Human Teeth. II,
31. Schour, I., and Massler, M.: The Growth Pattern of Human Teeth. II, J. A. D. A. 27: 1918, 1940.
J. A. D. A. 27: 1918, 1940.


32. Schour, 1., and Poncher, H. G.: The Rate of Apposition of Human Enamel and
32. Schour, 1., and Poncher, H. G.: The Rate of Apposition of Human Enamel and Dentin as Measured by the Efiects of Acute Fluorosis, Am. J. Dis. Child. 54: 757, 1937.
Dentin as Measured by the Efiects of Acute Fluorosis, Am. J. Dis. Child.
54: 757, 1937.


33. Sicher, H..- The Biology of Dentin, Bur 46: 121, 1946.
33. Sicher, H..- The Biology of Dentin, Bur 46: 121, 1946.


34. Thomas, B. O. A.: Protective Metamorphosis of the Dentin: Its Relationship
34. Thomas, B. O. A.: Protective Metamorphosis of the Dentin: Its Relationship to Pain, J. A. D. A. 31: 459, 1944.
to Pain, J. A. D. A. 31: 459, 1944.


35. Van Huysen, G-., and Gurley, W. B.: Histologic Changes in the Teeth of Dogs
35. Van Huysen, G-., and Gurley, W. B.: Histologic Changes in the Teeth of Dogs Following Preparation of Cavities at Various Depths, J. A. I). A. 26: S7, 1939.
Following Preparation of Cavities at Various Depths, J. A. I). A. 26: S7,
1939.


36. Van Huysen, G., Hodge, H. C., Warren, S. L., and Bishop, F. W.: Quantitative
36. Van Huysen, G., Hodge, H. C., Warren, S. L., and Bishop, F. W.: Quantitative Roentgen-Ray Study of Certain Pathological Changes in Dentin, Dental Cosmos 75: 729, 1933.
Roentgen-Ray Study of Certain Pathological Changes in Dentin, Dental
Cosmos 75: 729, 1933.


37. Van Huysen, Gr., Bale, W. F., and Hodge, H. C.: Comparative Study of the
37. Van Huysen, Gr., Bale, W. F., and Hodge, H. C.: Comparative Study of the Roentgen-Ray Absorption Properties of Normal and Pathological Dentin, Dental (‘osmos 77: 14-6, 1935.
Roentgen-Ray Absorption Properties of Normal and Pathological Dentin,
Dental (‘osmos 77: 14-6, 1935.


38. Wasserman, F.: The Innervation of Teeth, J. A. D. A. 26: 1097, 1939.
38. Wasserman, F.: The Innervation of Teeth, J. A. D. A. 26: 1097, 1939.


39. Weidenreieh, F.: Ueber den Ban und die Entwicklung des Zahnbeines in der
39. Weidenreieh, F.: Ueber den Ban und die Entwicklung des Zahnbeines in der Reihe der Wirbeltiere (Structure and Development of the Dentin of the Vertebrates), Ztschr. f. Anat. u. Entwcklngsgesch. 76: 218, 1925.
Reihe der Wirbeltiere (Structure and Development of the Dentin of the
Vertebrates), Ztschr. f. Anat. u. Entwcklngsgesch. 76: 218, 1925.


40. Zander, H. A., and Burrill, D.: Penetration of Silver Nitrate Solution Into
40. Zander, H. A., and Burrill, D.: Penetration of Silver Nitrate Solution Into Dentin, J. Dent. Research 22: S5, 1943.  
Dentin, J. Dent. Research 22: S5, 1943.
ormative
utritive


isensory


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Orban B. Oral Histology and Embryology (1944) The C.V. Mosby Company, St. Louis.

Orban 1944: 1 Development of the Face and Oral Cavity | 2 Development and Growth of Teeth | 3 Enamel | 4 The Dentin | 5 Pulp | 6 Cementum | 7 Periodontal Membrane | 8 Maxilla and Mandible (Alveolar Process) | 9 The Oral Mucous Membrane | 10 Glands of the Oral Cavity | 11 Eruption Of The Teeth | 12 Shedding of the Deciduous Teeth | Temporomandibular Joint | The Maxillary Sinus | 15 Technical Remarks


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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 thicke_nin.«z__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 Kor1f’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 g£_dentin sta3ts_ 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.—'J.‘ooth 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 pi-edentin, 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.—'.l‘hickening 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 ubstance. 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—azur¢+eosi1'I (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

1. Applebaum. E., Hollander, FL, and Bodecker, C. F.: Normal and Pathologital Variations in Calcification of Teeth as Shown by the Use of Soft X-rays_ Dental Cosmos 75: 1097, 1933.

Berkelbach van der Sprenkel, H.: Zur Neurologie des Zahnes (The Neurology of the Tooth), Ztschr. f. mikr.-anat. Forsch. 38: 1, 1935.

Bevelander, G.: The Development and Structure of the Fiber System of Dentin, Anat. Rec. 81: 79, 1941.

Bevelander, G., and Johnson, P. L.: Odontoblasts and Dentinogenesis, J. Dent. Research 25: 381, 1946.

Bevelander, G., and Amler, M. H.: Radioactive Phosphate Absorption by Dentin and Enamel, J. Dent. Research 24: 45, 1944.

Black, G. V.: Investigation of the Physical Characters of the Human Teeth, D. Cosmos 37: 353, 469, 1895.

Bodecker, C. F.: The Soft Fiber of Tomes, a Tubular Structure: Its Relation to Dental Caries and the Desensitization of Dentin, J. Nat. Dent. Assn. 9: 281, 1922.

Bodeeker, C. F.: Dangers of Extensive Operative Procedure on Recently Erupted Teeth, J. A. D. A. 28: 1598, 1941.

9. Bodecker, C. F., and Gies, Wm. J .: Concerning the Character of the Age Changes in Enamel and Dentin and Their Relation to the Vital Dental Pulp, J. Am. Coll. Dentists 9: 381, 1942.

10. Bodecker, C. F., and Lefkowitz, W.: Concerning the “Vitality” of Calcified Dental Tissues, J. Dent. Research 16: 463, 1937.

11. Cape, A. T., and Kitchin, P. C.: Histologic Phenomena of Tooth Tissues as Observed Under Polarized Light, J. A. D. A. 17: 193, 1930.

12. Crowell, D. C., Hodge, H. C., and Line, W. 15%.: Chemical Analysis of Tooth Samples Composed of Enamel, Dentin and Cementum, J. Dent. Research 14: 251, 1934. '

13. Ebner, V. v.: Ueber die Entwicklung der leimgebenden Fibrillen im Zahnbein (Development of Collagenous Fibrils in the Dentin), Sitzungsb. d. k. Akad. d. Wissensch. Vienna 115: 281, 1906; and Anat. Anz. 29: 137, 1906.

14. Fish, E. W.: An Experimental Investigation of Enamel Dentin and the Dental Pulp, London, 1932, John Bale Sons & Danielsson.

15. Fish, E. W.: The Circulation of Lymph in Dentin and Enamel, J. A. D. A. 14: 1, 1927.

16. Gebhardt, W.: Ueber den funktionellen Bau einiger Ziihne (The Functional Structure of Some Teeth), Arch. f. Entwcklngsmechn. d. Organ. 10: 135, 1900.

17. Grurley, W. B., and Van Huysen, Gr.: Histologic Response of the Teeth of Dogs

to Operative Procedures, J. Dent. Research 19: 179, 1940. _

18. Hodge, H. C.: Microhardncss Studies of Transparent Dentine, Brit. D. J. 63: 181 1937.

19. Keil, A.’: Ueber Doppelbrechung und Feinbau des menschlichen Zahnbeins (On Birefringence and the Minute Structure of Human Dentin), Ztschr. :E. Zel1forsch. u. Mikr. Anat. 21: 635, 1934. 126 ORAL HISTOLOGY AND EMBRYOLOGY

20. Keil, A.: Ueber den Feinbau der harten Zahnsubstanz nach Untersuchungen in polar-isiertem Licht (On the Minute Structure of the Hard Tissue of the Teeth Studied Under Polarized Light), Deuische Zaln1- Mun(l- 11. l{ieferheilk. 2: 741, 1935.

2]. K0rfl', K. v.: Wa.chstun1 der Dentingrundsulrstanz verschiedener Wirhelliorc ((z‘rroWth of the Dentin Matrix of Difi"erent Vertebrates), Ztschr. f. 1nikr.anat. Forsch. 22: -1-45, 1930.

22. Korif, K. v.: Die Entwicklung der Zahnbein Grundsubstanz der Saugetiere (The Development of the Dentin Matrix in Mammals), Arch. f. mikr. Anat. 67: 1 1905.

23. LeFevre, ’1\I. L., and Hodge, H. 0.: Chemical Analysis of Tooth Samples, J. Dent. Research 16: 279, 1937.

24. Lehner, J., and Plenk, H.: Die Ziihne (The Teeth), Moel1endorif’s Handb. der Mikrosk. Anat., vol. 5, pt. 3, p. 449, 1937.

25. Manley, E. G.: Traumatic Effect of the Drill During Cavity Preparation Brit. D. J. 70: 329, 1941.

26. Muntz, J. A., Dorfman, A., and Stephan, R. M.: In Vitro Studies on Sterilization of Carious Dentin J. A. D. A. 30: 1893, 1913.

27. Orban, B.: The Development of the Dentin, J. A. D. A. 16: 1547, 1929.

28. Schmidt, W. J.: The Elements of the Animal Body in Polarized Light, Bonn. 192-1., quoted by Kitchin, P. (3.: Beyond the Microscope, J. Dent. Research 17: 275, 1938.

28a. Scott, D. B., and Wycoif, R. NY. G.: Electron Microscopy of Human Enamel, J. 1). Res. 29: 556, 1950.

29. Schour, I., and Hofiman, M. M.: The Rate of Apposition of Enamel and Dentin in Man and Other Animals, J. Dent. Research 18: 161, 1939.

30. Schour, I., and Massler, M.: The Neonatal Line in Enamel and Dentin of the Human Deciduous Teeth and Ifirst Permanent Molar, J. A. D. A. 23: 19-16, 1936.

31. Schour, I., and Massler, M.: The Growth Pattern of Human Teeth. II, J. A. D. A. 27: 1918, 1940.

32. Schour, 1., and Poncher, H. G.: The Rate of Apposition of Human Enamel and Dentin as Measured by the Efiects of Acute Fluorosis, Am. J. Dis. Child. 54: 757, 1937.

33. Sicher, H..- The Biology of Dentin, Bur 46: 121, 1946.

34. Thomas, B. O. A.: Protective Metamorphosis of the Dentin: Its Relationship to Pain, J. A. D. A. 31: 459, 1944.

35. Van Huysen, G-., and Gurley, W. B.: Histologic Changes in the Teeth of Dogs Following Preparation of Cavities at Various Depths, J. A. I). A. 26: S7, 1939.

36. Van Huysen, G., Hodge, H. C., Warren, S. L., and Bishop, F. W.: Quantitative Roentgen-Ray Study of Certain Pathological Changes in Dentin, Dental Cosmos 75: 729, 1933.

37. Van Huysen, Gr., Bale, W. F., and Hodge, H. C.: Comparative Study of the Roentgen-Ray Absorption Properties of Normal and Pathological Dentin, Dental (‘osmos 77: 14-6, 1935.

38. Wasserman, F.: The Innervation of Teeth, J. A. D. A. 26: 1097, 1939.

39. Weidenreieh, F.: Ueber den Ban und die Entwicklung des Zahnbeines in der Reihe der Wirbeltiere (Structure and Development of the Dentin of the Vertebrates), Ztschr. f. Anat. u. Entwcklngsgesch. 76: 218, 1925.

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