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Quantitative studies in the development of the rat molar. I. The growth pattern of the primary and secondary dentin (from birth to 500 days of age)

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QUAKTITATIVE STUDIES I N T H E DEVELOPMEXT
O F THE RAT MOLAR
I. THE GROWTH PATTERN OF THE PRIMARY AND SECONDAR,Y
DEXTIX-' (FROM BIRTH TO 500 DAYS OF AGE)
M. M. HOFFMAN AND I. SCHOUR
Dcpartment of Histology, The University of Illinois College of Dentistry, Chicago
FOUR TEST FIQURES AND ONE PLATE (FOUB FIGURES)
INTRODUCTION
The purpose of this investigation was to determine the rate
of apposition and growth gradients of the dentin of the molar
of the albino rat from birth to 500 days of age. Previous
quantitative growth studies of the enamel and dentin of various species including man by means of injections of alizarine
red S or sodium fluoride suggested this study (Schour and
Hoffman, '39). It was felt that a knowledge of the growth
pattern of the rat molar would be useful in the analysis of
the pliysiologic and pathologic effects recorded in the molars
of experimental animals.
A careful search of the literature failed to reveal any quantitative measurements on the rate of dentin apposition in the
rat molar.
NETHODS AND MATERIAL
Animals. This study is based on sixty-three albino rats from
birth to 502 days of age (table 1). These were obtained from
four different sources.
Vital iwjectioszs. Multiple intraperitoneal or subcutaneous
injections of a sterile solution of 2% alizarine red S (sodium
This investigation was aided in part by grants from the Carnegie Corporation
of New York and from Mead Johnson and Company.
233
sulphalizarate) of color index 1034 (C'olc'maii Bell arid Company) were given at iiitcrvals of 2 claps to 1 weeks. The dosage
was about 100 mg. pcr kilograiii. The earliest iiijectioiis of
alizarine red S were given on the first cia:- of postnatal life,
and the last iiijectioiis were giveii on tlic 495th day. The csperimental animals can be divided iiito four groups on the
basis of the tiiiie of the injections and death (table 1).
.Iy)'pAI,
Ar.g
NUXBBR
' YOFA LNIM IN
('R"rP
STB-
T3ME 03%
LNJF( T I O l S
AGE I S
DAYS
GROUPS
I
21
I1
27
In
8
IV
r
II
SACRI-
~
FrCm
TFIK FIRST H O U R
1,8,22,336
7
9,23,30
40
2
00, 34
40
10
8
40, 54
60
77
0
100,114
12.5, 142
2
2
2
150,175
184,202
2l.i, 245,
275,302
2
2
3
303,329,
370, 402
403,443,
465,195
OF ApposllY1ohA1' ""oWTH
12
62,FS
,~~,
93
RANGE OF DAILY
D4Y8
WHEI
20-40
12.7
14*
177
209
309
409
~
1.OCUS OF EFFECTS
IN THE DENTIN OF
~
TIIE FIBW M O I d P
The effects of injectious given at 1
15.8-4.3
day of age were at
(Cusps-apex) the cuspal tips and
alwaded in aninials
samifieed at 40 days
The dentin immedi9.8-1.25
ately over the pulpal
( C ~ 1 ~ p - a l ) ~horns
~ ) showed no n l i
aariiie lines
3 2-1 *?
( Pulp-midcllaml~erroot)
K O primary
dentin
growth
after
333 days
Effects produced
after 135 days seen
only in secondary
dentin
502
The animals were weighed before the beginning of the esperiment and at regular intervals (usually weekly) during
the course of the alizarine red S injections. Three control
animals were injected with sterile physiologic saline solution.
Preparation of mattrial. Upon sacrifice of the animals the
inandibles and maxillae were dissected and fixed in 10%
formalin. Only ground sections were prepared since the alixa-
?
~
rine is lost during decalcification. The entire molar segment
(three molars) (fig. 5 ) or single section (fig. 6) was first
ground to the inid-pulpal plane on a revolving carborundum
stone moiintcd on a laboratory lathe till the pulp chamber and
pulp canals appeared. The tooth or segment was easily held
against the revolving stone by finger pressure. Three minutes
were usually sufficient for this procedure.
The section was then placed ground face down on the flat
surface of an unused cork 9 inch in diameter, and then ground
to the desired thickness. This was done by holding the cork
x-ith its flat surface parallel t o the face of the lathe stone and
oscillating it upward and downward against the direction of
the rotation of the stone. The section was then cleaned, dehydrated, cleared in xylol and mounted in gum damar.
At the beginning of this study the sections were mounted
011 the cork writhi cellulose cement, but this step was later found
to be unnecessary.
M i c r o m e t e r measurements. SIeasurements between succcssive alieariiie red S lines in the primary and secondary dentin
were made alon,p the dentinal tubules which mark the path of
growth. A i q single measurement was arrived at by taking
tlie average of three or more standardized readings. This
average w-as divided by the number of days which elapsed
between the two successive lines or between the last one and
death. A total of 1221 micronieter readings were madc. Measurements were taken a t the following anatomic landuiarks
(table 3 and fig. 1): 1, euspal tip; 2, incline of cusp; 3, midcrown ; 4,cemento-enamel juiiciion ; 5, mid-root ; and 6, apical
third of root.
All measurements in each age group and those falling within
the separate zones (table 3 ) were subjected to statistical
evaluation (Rrognian, '34). The mean values, standard clcviations and mean errors were computed (table 3).
OBBERVBTIOSS
Pre$zataE a d cjarly postlzatal developmc& of the rat molar
A brief review of the early development of the rat molar
mill help in tlie orientation of the findings and is given in
236
31. N. HOFFMAN A N D I. SCHOUR
table 3. According t o Addison and Appletoii ( '22), the enamel
organ of the first molar is present in the 18-day fetus. Vascularization begins in the 20-day fetus. I n the newborn rat the
deposition of dentin has just begun and the first indications
of enamel are present. The summits of the cusps of the rat
molar are not covered by enamel. The dentin, therefore, is
exposed at these sites at the time of clinical eruption, and is
subject to abrasion at a very early period.
Fig. 1 Semidiagrammatic reeonstruetion of an antero posterior section of a
lower first molar of an albino rat 100 days of age. Note the direction of the
gradientq the approximate rates of apposition from birth (euspal tip) to the
100th day of life, and the levels at which the measurements were taken. The
secondary dentin fills in the pulpal horn. The figures are in microns. X 26.6.
GROWTH I'hTTER,N O F DENTIN O F RAT MOLAR
237
Gross findings
In the rats that were injected during the first 3 weeks, the
molars were red in color as a result of the vital staining in the
early dentin. Animals less than 30 days of age showed a listlessness and discomfort for about 4 hour after each injection.
TABLE 2
Clwonology of the rat molars dziring prenatal and early postnatal dez;elopment
YOURS
DETELOPMEINTAL PROCESS
First appearance of dental
lamina
Beginning of differentiation
of ameloblasts
Beginning of dentin apposition
(enamel apposition occurs
12-24 hours later)
Beginning of calcification
Completion of appositional
growth i n crown
Beginning of bifurcation of
roots
First appearance in oral cavity
First occlusal contact
functioaal occlusion)
(not
13th day in utero
14th-15th day
20th day
in utero
in utero
20th day i n utero Birth t o first day 8th-10th dav
postnatal
postnatal
20th day in utero
1st-2nd day
13th-14th day
to birth
postnatal
postnatal
Birth to first day
postnatal
10th-11th day
postnatal
11th-13th day
postnatal
19th day
postnatal
23rd day
postnatal
2nd-3rd day
postnatal
12th-13th day
postnatal
14th-16th day
postnatal
21st-2211d day
postnatal
25th-28th day
postnatal
15th day
postnatal
19th-21st day
postnatal
24th day
postnatal
35th day
postiiatal
39th-40th day
Postnatal
l The data given in this table are based upon the findings of: Addison and
Appletoii ('22), S. Glasstone ('38), Hoffman and Schour ('38), H. Mellanby
( '39), and the results of this investigation.
* Lower molars. The upper molars are chronologically about 24 hours behind
the lowers.
Older animals (100-500 days of age) as a rule showed no
untoward effects and their weight was not significantly different from the normal (fig.3).
First molar
Microscopic findings i.n primary derztilz. The effects of the
injections of alizarine red S were seen in the primary and
238
X. M. HOPFMAS AND I. SCHOUE
secoriclary dentin as red lines that u7erc superposed on t l ~ c
normal incremental rings which were f oriiiiiig and calcifying
at the time of the injections. The effects in the calcifying
enamel were diffuse and coulcl riot be measured.
Figures 2-5 show the incremental pattern of thc primary
dentin. The apposition of dentin occurred only along the
pulpal surfaces. Kone mas foniid along the clentino-enamel or
clentino-cementa1junctions.
Fig. 2 Tracing of a photomicrograph of nu antero-posterior ground section
shoiring the secondary dentin in the pulpal horn of thr disto-buccal eusp of the
lower first molar of ail albino rat whicli was given ten injections of alizarine red 8
from the 59th to the Y3rd day and sawificed on the 100th day. Note the tpn
slizarinc lilies produced in the secondary dentin and the very close proximity of
the latter to the abradiiig surface. The inert.mental lines f o l l o ~a conical pattern
with the apex faring the pulp. X 64.5.
The int raperitoneal injections produced nioi'e intense effects
in color than the subcutaneous ones. Injections givcn when
tlie animal was 8 or 9 days old produced sharper effects than
did similar injections of tlie same dose given 1or 2 weeks later.
The location of the effects naturally varied with the age of
the animals at the time of the iiijections (table 3, fig. 1).
Rates mid gradieizts o f growth. The maximum daily rate,
dose to 16 p, was observed during the first meek of life and
occurred in the cuspal region. The minimum daily rate, about
L 2 p , was found in the apical region between the 70th and 80th
clays, and in the roof of the pulp chamber in the 80th-135th
clay.
I
Grns
100
95
90
85
3 "375
1
;
80
---Weight:
15
10
350.9
325.
U
:'
300k275 250-
'3 225
65
60.G
3
55
506
a
-
45
40
;
2003 173-
35
3 150 -
30
25
20
15
OJ
.
5
10
5
10
15
20
23
30
Days after birth
35
40
0
Pig. 3 Graph showing t h t the rate of dentin apposition is independent of
ciianges in body weight. The rate of dentin growth in animal B was normal in
spite of the fact that it did not gain and even lost m i g h t as :I result of an accidental
snppurating infection at tlie site of the injections.
Table 3 gives the more detailed findings on the rate of primary dentin apposition in the first iiiolar from birth to 135
days of age. The vertical columns show tlie decrease of the
mte with age (age gradient). The horizontal columns show
the decrease of the rate with the distance from the growth
center (locus gradient). The rates show the following decelerating gradients of appositional growth :
I+
0
10.47 k 0.74 f 0.371
10.30 rt 0.49 f 0.112
10.08 rt 0.44 f 0.220
8.02 rt 0.51 f 0.285
8.50 -C 0.77 f 0.352
8.29 f 0.62 f 0.188
6.34 f0.24 f0.173
5.72 -C 0.79 & 0.441
14-20 12.01 f 0.11 10.015
20-27
11.50 & 0.75 f 0.226
27-30
11.36 k 0.63 f 0.252
30-35
9.88 2 0.47 k 0.235
3.AO
9.81 -C 0.42 f 0.210
40-45
9.63 f 0.36 k 0.136
50-70
9.39 k 0.61 10.114
70-80
80-1 00
9.88 f 0.22
9.60 2 0.31
9.24 2 0.38
7.46 rt 0.16
7.27 f 0.43
7.29 rt 0.66
5.14 2 0.52
4.43 2 0.61
1.14 2 0.32
2.41 2 0.27
1.06 2 0.04
2.18 2 0.16
f 0.098
f 0.190
f 0.045
f 0.119
f 0.209
f 0.214
f 0.322
f 0.714
& 0.522
f 0.121'
f 0.227:
k 0.123
10.46 -+ 0.79 f 0.336
!0.60 2 0.44 f 0.133
Mid crown
8.69
8.32
8.16
8.80
6.43
6.00
4.47
3.24
2.36
f 0.69
f 0.68
f 0.47
k 0.55
k 0.49
f 0.60
f 0.73
f 0.34
f0.41
2 0.219
2 0.237
f 0.332
10.122
f 0.192
f 0.251
2 0.331
f 0.278
A 0.374
3omn not completed
until 10th day
9.33 -t 0.57 e 0.051
Cemento-enamel
junction
I
Apical third of root
Root does not begin to form
until 10th day
Root does not begin to form
until 3 ;h day
6.88 f 0.68 2 0.178 4.30 2 0.57 +- 0.381
6.72 k 0.79 2 0.250 5.02 & 0.45 k 0.150
6.64 & 0.67 f 0.335 4.83 -+ 0.71 -C 0.177
5.07 f 0.54 & 0.183 4.26 rt 0.81 +- 0.270
4.81 f 0.73 f 0.103 4.04 f0.34 -C 0.237
4.77 f 0.78 -+ 0.260 4.74 rt 0.70 4.0.233
2.85 10.79, f 0.416 2.37 & 0.87 -C 0.324
2.34 f 0.24 -+ 0.471 1.26 2 0.73 k 0.422
1.44 f 0.62 2 0.434
No appositional
growth
No appositional
1.21 f 0.43 rt 0.243
growth
NIid root level
* Measurements are expressed in microns. The figures listed in this chart are the mean values plus the standard deviations and the mean
errors, based on a total of 1221 micrometer readings. Each mcasuremciit indicated is the average from each of the six age groups in table 1.
* Roof of pulp chamber.
a Floor of pulp chamber.
1013-13 5
I
12.32 k 0.64 f 0.291
15.61 k 0.11 -t 0.317
12.67 2 1.31 k 0.396
Incline of cusp
ANATOMICAL ZONES SELECTED FOR MEASVBEMEXTS A N D DAILY RATES O F APPOSITION
T-1.1
Cuspal tip
TABLE 3
the growth gradients of the y i i ~ u qtlciiliri of t k a fiist molurs in s i x t y - t h r c c albino rut8
15.80 rt 0.46 k 0.159
I
ciitil
1-7
DAYS
AGE I N
TIM rate of appositioiL
GROWTH PATTERN OF DENTIN O F BAT MOLAR
241
(1) From the tip of the cusps to the apices of the roots
(locus gradient) (figs. 7 and 1).
(2) From the dentino-enamel or dentino-cementa1 junction
to the pulp (age gradient) (figs. 7 and 1).
(3) From the periphery of the tooth toward the center, that
is, at any given root level the rate decelerates as the center of
the tooth is approached (radial gradient) (figs. 7 and 1).
A comparison of the rates in the first, second and third
molars shows a fourth gradient which is the antero-posterior
gradient (table 6).
Secomdary dentiw. The findings in secondary dentin are
different in the pulpal horns and in the pulpal floors and roofs,
and therefore will be described separately (table 4).
TABLE
4
Bummnry of findings of secondarv dentin
INITIAL
TION OR
TYPE
AND
YAXIXAL
FO&MATION
DAILY
ST1AR,TS
RATE IN
XICRA
Pulpal
horn
35th-45th
day
15.73
Pulpal
roof vr
floor
135th day
2.52
IACA-
TIME W H E N
1XCR.EXENTAL
PATTERN
toward pulp
tal pattern of
primary dentin
HISTOLOGIC
CHARACTF.RISTICS
as well calcified ac
primary dentin
TIME W H E N
IWRMATION
RTOPS
Continues
beyond
BOO days
Continues
beyond
500 days
In the horns of the pulp the formation of secondary dentin
begins at about the thirty-fifth t o the forty-fifth day. The
increments follow a conical pattern with the apices facing the
pulpal surface, and thus present in a limited sense a mirror
image of the incremental pattern of the preceding primary
dentin (figs. 1 and 2). The tubules are irregular and the calcification is less homogeneous than in primary dentin.
The rate of apposition of secondary dentin shows similar
gradients to those of the primary dentin. The initial daily
rate approximates 16 p per 24 hours in the pulpal horns, the
same as that of primary dentin (table 1 and fig. 4).
I n the roof and floor of the pulp formation begins at the
125th-135th day a t a faster daily rate (2.5 p) than that of the
242
M. M. HOFFMAR AWD I. SCHOUR
immediately preceding primary dentin. This rate decelerates
to 0.69 p at 500 days of age (table 5).
There are tlins two types of secondary dentin: The pulpalhorn typp wliicli is characterized by early initiation, rapid
Secondary dentin
apposition begun
200
I50
100
50
"OJ
' 2 0 ' 4 b ' 6 0 ' s b ' I d o
Days
lio'140'rdo'l
Fig.4 Graph showing the total increments of dentin growth from birth to
200 days at the level of thc cuspal tip. Note the abrupt rise of the curve upon
the initiation of the formation of secondary dentin.
TABLE 5
Tlrr rate of apposition and Ilrc growth gmdtrnts of thc secoiidary drntin of tlic
f k . 5 1 nrc;lw.s in fort?j-fa.o albino Yats
AVEJLIGE DAILY RATE
AGE INTERVALS
IN DAYS
3.5-4.5
50-60
60-80
80-100
100-125
125-200
200-300
300-400
400-500
I
I>&
rate in
pulps1 horns
15.73
12.37
10.61
5.18
5.97
2.63
1.74
1.42
1.25
I
IN 361CEONS
Anatomical znnw
F1yzb$p
1
I
2.52
2.44
2.29
2.23
2.18
1.65
1.31
1.17
1.03
Roof of pulp
chamber
2.49
2.40
3.11
2.06
1.96
1.54
1.16
1.08
0.83
243
GROWTH PATTERN OF DENTIN O F 1;AT &IOLBlt
rate and ail incremental pattern that is different from tlic
primary dentin; and the pulpal roof or floor type which lias
a later beginning, a slower rate and a pattern similar to that
of primary dentin (table 5).
Second nlzd third rnolavs
I n this report emphasis is placed on the findings in the first
molar. Examination of the second aid third molars sliows
that the appositional growth processes of the primary a i d
TABLE 6
Cornpurison of findings in first, s r r o ~ dmid third niolnrs
YTNI)INO
31,
X.,
>I
Bcgiiining of apposition of priinarp dciitin
Rirtli
?lld-3rd day
13tli-14th
an
lnitial daily rate of pyiinarp dentin in
microns
Teriiiiiial daily rate of primary dentin in
mierous
Initial :il)pearancr i i i o w l cavity
1 Ti.80
15.61
3 4.iO
1.17
19th d:ly
1.l6
2Lt-2211d
0.97
:i.?tll day
day
Cessation of root elongation by means of 80th-83rd
primary dentin
d:1$
973
Illaximal length of
Tip of c u y s
dentinal tubules i n
AIid root level
microns
l)i furcn tioii
1i4
130
Calculated approximate life-span of dcntiiiforming cells in days
Beginning of secondary dentin growth i n 35tli-k0th
1mIp:il horns
clny
!)llth-95tli
775
11 ntli-113tll
cln T’
720
212
122
127
120
d;l>
1otil-4Y~~d .??iid-.iSth
d;l).
d:1?.
244
M. M. HOFFMAN AND I . SCHOUR
tion to : (1)Time of beginning of the appositional process of
primary dentin; (2) initial rates of apposition of primary
dentin ; (3) time at which the appositional process of primary
dentin ceases to add to root elongation; (4)time of eruption;
(5) time at which the apposition of secondary dentin begins
(table 6).
DISCUSSION
dlixarine red S as a%imdicator of d e w h appositiom. Cameron ( '30) and Cottlieb ('14) deiiionstrated that alizarine,
when used as a vital stain, is taken up by those tissues in
which calcium is being deposited at the time the alizarine is
administered. Dentin is a tissue rich in calcium and therefore
takes the stain readily. The response to each injection of
alizarine red S is acute aud is manifested by an intensely
stained red incremental line. Since calcification of the dentin
follows its apposition in close succession, alizarine red S is
not only an indicator of the calcification processes but also an
accurate indicator for the assessment of the rate of dentin
apposition (Schour and Hoffman, '39).
Abseace of alixarime red S lirzcs irz the eaamel. The diffuse
effect in the enamel may possibly be explained on the basis
that the enamel calcifies en masse. The animals injected later
than 10-14 days showed no coloration in the enamel. The fact
that the incremental layers of the dentin reach the dentinocementa1 junction on the tenth day indicates that the enamel,
and thus the crown, is completed at that time.
Dose o f alixarine red 8 arzd its relation t o the staining reaction. Injections given on the eighth or ninth day produced
sharper effects than those given 2 weeks later. This difference
may be explained on the basis of dosage. The first and second
injections were of the same volume and the same percentage.
Since the weight of the animal at the time of the second injection was three times greater than the weight at the time of the
first injection, the dosage of the second injection in terms of
milligrams per kilogram was one-third that given at the beginning of the second week. It is probable that with the increase
in volume of the calcio-receptive matrix in the oIder animaI
GROWTH PA4TTERXOF DENTIN O F RAT XOLAR
245
the relatively smaller dosages were not sufficient to give an
intense stain.
Lack of correlation between body weight and rate of dentin
appositiolz. Despite the normal variations in weight between
the various colonies, the rate of dentin apposition was not
different. Three animals which, as a result of a complicating
infection, developed suppurating ulcers at the sites of injection, concurrent with listlessness and malaise, showed a sharp
decline in body weight for about 1 week following a single
injection (fig. 3). The rate of dentin apposition in these animals remained normal. These findings indicate a lack of correlation between body weight and dentin growth, and that
dentin appears to have a growth potential which is not readily
modified. Similarly, Outhouse and Mendel ( ’33) found that
the time of eruption of the third molar in the rats was uninfluenced by their weight or rate of growth.
Significame of appositiolzal rates and gradielzts. The finding of the growth gradients in the rat molar confirms Huxley
( ’32) who demonstrated mathematically that dermoid structures in general possess growth gradients. I n discussing the
logarithmic spirals of dermoid structure, Huxley states,
“There is a constant ratio between the increments at the two
ends of the growing structure with regular (though not necessarily uniform) gradients of growth-rate between the high
and the low points.” The tooth is ideally constructed with a
high point (cusp) and a low point (apex) so that it may be
used histologically to demonstrate Huxley ’s hypothesis in regard to growth gradients.
Secondary dentin. The findings contribute to our knowledge
of the biology of secondary dentin. While the latter is somewhat irregular in structure, it nevertheless follows definite
laws of growth. The rate of apposition of secondary dentin
shows the characteristics of a secondary growth center (fig.4).
The initial daily rate is greater than the terminal rate of the
preceding primary dentin. This is especially prominent in the
pulpal horn where the primary dentin which started with a
daily rate of 16 p slows down to 9 p per day and then is fol-
246
hi.
nx.
I ~ O F Y M A NAND I. SCHOUK
lowed by an initial daily rate of approxiniately 16 ~1 in the
secondary dentin (fig. 4).
It is interesting that in the horns where abrasion begins
earlier and proceeds at a more rapid rate the secondary dentin
forms earlier and faster than at other pulpal surfaces.
There is an additional factor that may be responsible for
the early initiation of secondary dentin in the region of pulpal
horns. Measurements show that the dentin-forming cells which
have a calculated life-span of 130 days (table 4) function for
only one-half of their life-span in the region of the pulpal
horns. It appears that the cells in this region become crowded
tinct lack nutrition, and therefore undergo premature atrophy.
Secondary dentin then steps in to protect the pulp and compensate for the cessation of primary dentin.
Tt is at first sight surprising that the cells responsible for
primary dentin function f o r only about 40 clays while those of
the sccoizdary dentin continue to function for a much longer
period. One might expect that the crowding in the pulpal horn
which was responsible for the cessation of primary dentin
formation would also lead to more limited duration of secondary dentin deposition. A consideration of the incremental
pattern of the secondary dentin in the pulpal-horn region
tlirows interesting light on this question. The crowding of the
cells active in the formation of secondary dentin is obviated
by the fact that the incremental cones become reversed in position with the apex toward the pulp so that the surface area
exposed to pulpal nutrition is increased and cellular activity
is favored (figs. 1 and 2).
The continuous growth of secondary dentin beyond 500 days
of age is a response to the continuous process of attrition. A
comparison of the rate of attrition and rate of secondary
dentin apposition will be reserved for a subsequent paper
(Hoffman and Schour, '40).
The constancy of the eatermal diantcter o f t h e rat molar.
Our findings do not confirm the conclusions of Donaldson and
French ('27) that the antero-posterior as well as the buccolingual diameters of the molar crowns increased with age after
G R O W T H PATTERN OF DENTIN O F BAT MOLAR
247
eruption. The growth of enamel in the molars is completed at
the time the cemento-enamel junction is reached (about 10
days of age in the first molar). The ameloblasts cease their
appositional activity and are soon completely atrophied and
lost. The dentino-enamel junction is established early and
remains unaltered. All appositional growth of the crown after
completion of enamel proceeds inward toward the pulp. The
size of the pulp is thus gradually reduced but the external
diameter is unchanged.
dpplica,tiolz of the alixarifie red S method i i t otlter studies.
The alizarine red S method permits an analysis of the growth
pattern of the cementum and bone and of the eruption pattern
of teeth (Hoffman and Schour, '40) and call be employed in
the Macacus rhesus and other animals as well as in the rat.
It would be interesting to extend the investigatioii of Gurlep
and Van Huysen ('37) and others on the experimental production of secondary dentin by supplementing their experiments with injections of alizarine red S. The employment of
this method would permit a quantitative evaluation of their
experimental results.
The rat molar as a% indicaior of experinterdal conditions.
The findings in this study show that the rat molar will record
effects in the primary dentin for a longer period (up to about
100 days) than had been previously assumed.
SUMMARY AND CONCLUSIOSS
The rate of apposition of primary and secondary dentin
was studied in sixty-three albino rats, 1to 502 days old, which
were gven multiple injections of 2% alizarine red S. The
dosage averaged 100 mg. per kilogram of body weight. Ground
sections of molars showed a red line in the dentin which was
calcifying at time of each injection.
In the primary dentin:
1. The apposition of primary dentin starts at birth and continues up to approximately 135 days of age. After 80 days of
age growth ceases along the long axis of the pnlpal surface
248
M. 31. HOFFMAN AX’D I. SCHOUR
and is confined to the roof and floor of the pulp chamber. The
findings permit a chronological charting of each growth level.
2. The daily rate of apposition of primary dentin ranged
from approximately 16 p to 1 p. It decreased according to four
main gradients :
a. Age gradient-from dentinal surfaces to pulp.
b. Locus gradient-from tips of coronal cusps to apices
of roots.
c. Spiral gradient-from root surface toward bifurcation.
ci. Antero-posterior gradient-from first to third molars.
3. The ineremental pattern of the primary dentin is conical
with the apex toward the cuspal growth center.
4. Rate of dentin growth was independent of body weight.
I n the secondary dentin:
1. The apposition begins at 35-40 days of age concomitant
with the clinical occlusion and continues as late as 500 days
of age.
2. I n the pulpal horn the apposition shows a maximal rate
of 16 p and a minimal rate of 1.3 p.
3. The incremental pattern of secondary dentin in the pulpal
horn is conical, with the apex toward the pulp.
The following conclusions may be drawn :
1. Vital staining with alizarine red S permits a quantitative
study of the entire growth pattern of the rat molar and offers
a valuable method for similar studies in higher animals.
2. Dentin growth is not uniform but proceeds along definite
gradients.
3. The primary dentin of the rat molar can be used as an
indicator of experimental conditions in animals up to 100 days
of age.
LITERATTJRE CITED
ADDISON,W. H. F., AND J. L. APPLETON 1921 On the development of the ameloblasts of the molars of the albino rat with special reference t o the
enamel-free areas. Anat. Rec., vol. 21, p. 43.
CAYE&ON, G. R. 1930 The staining of calcium. J. Path. and Bact., rol. 33,
p. 929.
GROWTH PbTTNEN O F DENTIX OF BAT MOLAR
249
H. H., AND H. 32. J?R)N(~H 1927 O n the growth in the diameters of
the albino rat, after eruption. Ailat. Rec., vol. 34, pp. 277-299.
GLASSTONE, S. 1935 A comparative study of the development in viro and in
vitro of rat and rabbit mohis. Proc. Roy. Hoe. London, Series B,
NO. 844, VOI. 126, pp. 315-330.
GOTTLIEB, B. 1914 M e Vitalo Fiirbung der 1Gilkhaltigcn Gewcbc. Anat. Rnz.,
V O ~ . 46, PI). 179-192.
GURLEY,W. B.,AND Q . VAN H U Y ~ E N1937 Histologic changes in teeth due to
plastic filling materials. J. Am. Dcnt. Assoc., vol. 24, pp. 1806-1816.
HUXLEY,J. S. 1!)32 Problems in Relative Growth. The Dial Press, London.
1938 Rate and gradients of growth in the int
HOWMAN,
M. If.,AND 1. SVHOUR
molar as demonstrated by injections of alizarine red S. J. Dent. Res.,
~ 0 1 .17, pp. 307-308.
- ._ __._ 1940
Quantitative studios in the devrloynwit of tlrc rat molar.
TI. Alveolar hone, cementum and eniption. Am. J. Ortli. and Oral
Surg. (in press).
KR OG~ UN,
W. hf. 1934 Kole of biometry in orthodontic research. J. Am. Dent.
h S O C . , v01. 21, pp. 986-996.
MELLANBY,H. 1939 The development of tooth in the albino rat. Erit. Dent. J.,
rol. 66, pp. 76-86.
OUTHOI'SB,J., AND 1,. A. MENDEL 1933 Rate of growth. I. I t s influence on the
rkelctal development of the albino rat. .J. Exp. Zool., vol. 64, p. 257.
SCHOIJR,
I., AND M. M. HOFFMAN 1939 Studies in tooth dcvelopment. 11. The
rate of apposition of enamel and dentin in man and other mammals.
J. Dent. Res., vol. IS, no. 2, pp. 161-175.
nONAI,DSON,
PLATE 1
h1PLAN.kTION
OF FIGTJRES
5 Photogiapli of a niid-s:igitt:il ground section of tlie molar segnieiit of the
niandiblo of a normal rat 92 days of age. Kote the three molars, M,, M2,M,. The
section passes through the proximal rnd of tlie lower incisor. Al.R, alveolar bone;
Crm, secondary cementum; D, dentiii ; En, enamel; P.D.M., periodontal memhr:tne.
X 8.
ogr:iph of :in niitero-posterioi yroiind section of the lowcr first
molar of a r a t which was given three injections (1, 2, 3 ) of alizarine red S a t the
38th, 45th and 53rd days and sacrjficcd on the 100th day. C.E..T., cemento-enamel
junction ; Cem., secondary cementum; P.D.M., periodontal rnemhrane. X 18.
7 High-poiwr view of field A indicatrd in figure 2. Notc the three injection
clffects (1, 2, 3 ) in tlic deiitiii, and thr growth gradients which are also shown i n
figure 5 . En., en:imel. The locus gradient is evidenced by the fact that the inteivals between the injection effects are widrr i n the coronal portion than i n the
npical portion of the distal dentin. Tlic spiral or radial gradient is evidenced by
the fact that the intervals betwren the injection effects are shorter in the dentin
facing thc bifurcation than in the distal dentin. Legends same as in figure 2.
x 49.
8 Seiriidiayiainiriatic thi e r dinirnsional reronstruction of lower first molar of
a rat 100 d a j s of age sl~ownin figure 2. Note the iiicremeiital pattern of the
primary dentin and the seeoiidary rementum. The incremental lines in the dentin
rrprrscnt interwls of approximately 14 d:ijs. X 9.
250
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development, dentin, birth, growth, secondary, primary, rat, days, quantitative, patterns, molar, age, 500, studies
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