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The Development of the Bifurcation of Multirooted Teeth
TH E DEVELOPMENT OF TH E BIFURCATION OF M ULTIROOTED TEETH By BALINT ORBAN, M. D., and EMIL MUELLER, D.D.S., M. S., Chicago, 111.
U C H research w ork has been done on tooth development in general, but, so far, little consideration has been given the development o f the bi furcation.
M
apical end of the enamel epithelium pointing toward the center is H ertw ig’s epithelial sheath. T h e arrows indicate the direction in which the epithelium is thought to grow. B shows how these
F ig . 1.— D i a g r a m s h o w in g the o r ig in a n d th e d e v e lo p m e n t o f th e b ifu r c a t io n o f m u ltir o o te d teeth .
A , t o o t h -g e r m f r o m the s id e . T h e h a tc h e d a r e a s a r o u n d th e to o th g e r m in d ic a t e the
e n a m e l e p ith e liu m , th e e n d s o f w h ic h , m a r k e d b y a r r o w s , a r e H e r t w i g ’ s e p ith e lia l sh ea th .
B, th e to o th g e r m v i e w e d f r o m th e b a s a l o p e n in g . T h e s h a d e d a r e a is H e r t w i g ’ s e p ith e lia l s h e a th . T h e a r r o w s in d ic a te th e d ir e c t io n in w h ic h th e e p ith e lia l sh e a th is s u p p o s e d t o g r o w .
It is assumed that after the crown portion of the tooth develops, certain points of H ertw ig’s epithelial sheath commence horizontal growth toward the center o f the pulp. This process is diagrammatically illustrated in Figure 1. A shows the crow n portion of the tooth covered with the enamel epithelium. T h e Jour. A . D . A ., February, 1929
297
processes were to grow together to form the bifurcation when viewed from the bottom. Later in development, dentin is laid down inside the sheath, and, in this way, the outline o f the bifurcation develops. N o accounts of investigations show ing exactly the details o f this process
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have thus far been published. W eski and Contreras1 attempted to explain bifurca tion development upon the basis of a few histologic pictures. T heir material as well as their methods was inadequate. T heir diagrammatic drawings which they called reconstructions o f the his tologic specimens were only reconstruc tions of the authors’ conceptions, and not reconstructions of the true specimens.
F ig . 2.— S e ctio n t h r o u g h a m o la r o f a ra t.
sary to conduct investigations covering this phase of the development. T h e high point of this newer observa tion is that the epithelium does not grow into the depth to form the dental lamina and tooth germs, but certain portions of the epithelium remain in relatively fixed positions, while the surrounding tissues grow. Measurements from extensive human material verify this observation.
a, th e p u lp a l o r c e n tr a l en d o f th e e p ith e lia l
sh e a th , b, p e r ip h e r a l en d o f th e sa m e w h e r e it io in s th e e n a m e l e p ith e liu m . these p o in ts h a v e b een m a r k e d th ro u g h o u t the e n tire serie s.
D ev e lo p m e n ta l problem s can not be solved with single specimens and the gaps filled in from the imagination. Because of recent newer observations in tooth development,2 we felt it neces-
W h ile conducting these investigations, it was found that the development of the root does not proceed as it was formerly thought. T he former view assumed that 2.
I.
W e s k i, O ., a n d C o n t r e r a s , R .:
In th e t r a c in g s ,
B e fu n d e o f
O rb a n , B a lin t:
T ooth
1 5 :1 0 0 4
G erm s
(J u n e )
u n d V o r g ä n g e im I n t e r r a d ik u lä r r a u m e M e h r w u r z e li g e r Z ä h n e ; V ie r te l]a h r s c h r . f . Z a h n -
E m b r y o lo g y ,
h e ilk ., 1924, P a r ts 2 -3 , p . 229.
1928.
and
G row th and M ovem en t T eeth ,
J. A .
D.
A.
1 9 2 8 ; D e n ta l H i s t o l o g y a n d
C h ic a g o : R ogers
P r in t in g
C o .,
Orban and M u e lle r — D evelop m en t o f Bifurcation
H ertw ig’s epithelial sheath grew into the depth to precede the development o f the root. T h e studies o f one of us (B . O .) have shown this view to be incorrect. H ertw ig’s epithelial sheath during the
299
this point peripherally. These observa tions are incompatible with the former th e o re tica l co n ce p tio n o f b ifu rca tion d ev elop m en t. T h is in com p a tib ility prompted the present investigation.
F ig 3.— T h e outline o f H e r tw ig ’s epithelial sheath o f the upper first m ola r o f a rat 8 days old . T h e inner heavy line corresp on d s to a in F igu re 2 ; the outer h ea v y line, to b. E ach horizontal line betw een the tw o h e a v y lines represents the epithelial sheath. D indicates the d irection d ista lly ; M , m e sia lly ; L, lingu ally, and B, bu ccally.
entire development o f the tooth marks the relatively fixed point. T h e whole growth of the tooth takes place from
T he study of bifurcation development on human material is almost impossible, Embryos over 200 mm. in length are
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T h e Journal o f the American D en ta l Association
F ig. 4.— T o o th germ o f the upper first m olar o f a rat 8 days old . T h e section w a s cut m esially o f the basal op ening, and so the epithelial sheath is continuous.
Orban and M u e ller — D evelopm en t of Bifurcation
301
Fig. 5.— T o o th g e rm o f the upper first m ola r o f a rat 8 days old, the section bein g m ade a little m ore d ista lly fr o m the location o f that in F igu re 4, co rre sp o n d in g to the h orizon tal line in the corn er d ia g ra m . T h e epithelial sheath ex-tends equ ally on both sides.
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F ig. 6.— T o o th germ o f the upper first m olar o f a rat 8 days old. T h e section w as m ade still fu rth e r distally than in F igu re 5. A com parison with the corn er d iag ra m explains the lateral position o f the ba sal op en in g and the g reater length o f the epithelial sheath on one side.
Orban and M u e lle r — D evelopm en t of Bifurcation
303
F ig. 7.— T o o th germ o f the upper first m o la r o f a rat 8 days old , the section bein g m ade through the tooth germ co rre sp o n d in g to the h orizon tal line in the diagram . T w o basal op enings are present, separated fro m each other by a lon g epithelial island.
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F ig. 8.— T o o th germ o f the up p er first m olar o f a rat 8 days old . T h r e e openings lead to the pulp, and tw o epithelial islands are to be seen. O nly the serial recon stru ction as given in the d ia g ra m can expla in this occu rren ce.
Orbati and M u e lle r — D evelopm en t of Bifurcation
305
F ig. 9.— T o o th germ o f the u p p er first m olar o f a rat 8 days old. T h e section is fr o m near the center o f the tooth germ . T h e r e is o n ly one op en in g in the center, and tw o lo n g H ertw ig ’ s epithelial sheaths.
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necessary for this work, and these are very difficult to obtain in series o f age. W e therefore decided to conduct our investigations on material from rats. W e assumed that the biologic processes that take place during development are
saw that the bifurcation development was already completed. A fter thoroughly studying the com plete series o f sessions, we concluded that some definite form of reconstruction was absolutely essential on account o f the ex
F ig. 10.— T o o th germ o f the upper first m ola r o f a rat 8 days old , the section b ein g m ade fu rth er distally than in F igure 9. T h r e e openings again separated by epithelial islan ds lead to the pulp.
similar to those in the human being. For this work, we obtained complete serial sections from rat jaws, the rats ranging in age from a few days before birth to twenty days after birth. A t this age, we
treme complexity o f the material. Only through reconstruction can the exact development be visualized. T w o means o f reconstruction were available: ( 1 ) the construction o f models and ( 2 ) the
Orban and M u e lle r — D evelopm en t of Bifurcation
making o f serial microscopic drawings. T h e latter seemed better suited for our purpose. W ith the presentation of Figure 2, a description of our method will be given. T his is a photomicrograph from a section through a molar of a rat
307
pheral end where it joined the enamel epithelium b. W e repeated this with each section o f the series. T h e single tracings were spaced 1 mm. apart be cause o f the thickness o f our specimens, 20 microns. W e enlarged the specimens
'ImUm X.
F ig. 11.— T o o th germ o f the upper first m olar o f a rat 8 days old . T h e changed picture corresp on d s to the chan ge in the shape o f H e r tw ig ’ s epith elial sheath. One sm all and one la rg e op en in g indicate the basal op e n in g o f the tooth germ .
8 days old. T h e serial sections were pro je cte d on to m illim e te r-ru le d paper with a micro-projecting apparatus. H ert w ig’s epithelial sheath was traced from the pulpal or central end a to the peri-
fifty times. T h e series of lines were con nected at both ends, and, in this way, the outline of the entrance to the pulp, i. e., the basal opening of the tooth germ as well as the exact shape of the entire
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H ertw ig’s epithelial sheath, was ob tained. In order to ascertain the true anatomic shape of our tracings, it was necessary for us to observe the topo graphic relationship of the tooth to other organs.
the-enamel organ, so that the outer heavy line has sometimes been set arbitrarily. W e would also stress the fact that we were not particularly concerned with the number, size or distribution of the root canals, but were primarily con-
IÉSS F ig. 12.— T o o th germ o f the u p p er first m o la r o f a rat S days old. T h e section is through the distal third o f the tooth germ . T w o root op enings explain the tw o la rg e spaces separated fr o m each other b y an ep ith elial island. T h is ep ith elial island, i f com p ared w ith the corn er d ia g ra m , is m anifest as a cross-section o f an epithelial p eninsula.
W e desire to call attention to the fact that it was sometimes difficult to determine exactly where to set the border line between H ertw ig’s epithelial sheath and
cerned with the order of development of the bifurcation, The outline o f H ertw ig’s epithelial sheath and the progress of the develop-
Orban and M u e lle r — D evelo p m en t o f Bifurcation
ment of the bifurcation o f the upper first molar of a rat 8 days old is shown in Figure 3. T h e inner heavy line out lining the irregular opening corresponds to a in Figure 2 ; the outer heavy line corresponds to b in Figure 2. Each hori-
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the basal opening o f the tooth. T h e low er part of the picture is the mesial side; the upper, the distal; the right, the buccal, and the left, the lingual side. T o show in more detail just how we ob tained the basal openings, w e will show
PH
F ig. 13.— T o o th germ o f the upper first m olar o f a rat 8 days o ld . T h is section is fr o m near the end o f the basal opening. T h e sin gle op e n in g is located bu ccally. T h is explains the lo n g e r lin gu al epithelial sheath.
zontal line between the tw o hea.vy lines represents epithelium; i. e., H ertw ig’s epithelial sheath. T h e space inside o f the inner heavy line is the entrance to the coronal pulp; that is, what we call
interrupted series of pictures from the specimens from which Figure 3 was con structed. In the left upper corner o f each photomicrograph, we present a re duced photograph o f Figure 3. T h e
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T h e Journal of the Am erican D en ta l Association
horizontal heavy line in this picture in dicates the location of the section in the series from which the corresponding photograph was taken. T h e section shown in Figure 4 was cut through the tooth germ at the mesial side at a point
diagram. W e see the epithelial sheath of the tooth extending toward the cen ter about equally on both sides. This position of H ertw ig’s epithelial sheath at right angles to the walls of the tooth is almost the typical position of the
'Vi«»
F ig. 14.— T o o th germ o f the upper first m olar o f a rat 8 days old , the section b ein g distad from the basal op ening. T h e epithelial sheath is ag ain continuous as it is in F igu re 4.
where H ertw ig’s epithelial sheath was continuous. Figure 5 was taken from a section a little more distally from the location of that in Figure 4, correspond ing to the horizontal heavy line of the
sheath in all stages of development. F ig ure 6 was taken still more distally. On one side, H ertw ig’s epithelial sheath is much longer than on the other, and the basal opening is correspondingly well
Orban and M u e lle r — D evelopm en t of Bifurcation
F ig. 15.— B asal op en in g o f the upper first m o la r o f a rat 5 days before birth.
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terrupted, and we now have on both sides of the tooth short epithelial sheaths. About midway between the two, there is an elongated island and two openings leading to the pulp. O nly complete serial sections afford the explanation for this occurrence. T he epithelial island is only a cross-section of a peninsula, a pro jection of H ertw ig’s epithelial sheath, as can readily be seen from tracings of all sections o f the series. T h e picture ap pears to be more complicated in Figure 8. Here, we have two short islands be tween the outer sheaths. In reconstruc tion, the longer island is shown to be
F ig. 16.— T r a c in g o f the basal op en in g o f the u p p er first m olar o f a rat 1 day old.
toward one side. T h e study of the dia gram serves to explain this. T h e section of Figure 7 was obtained further distally than the section o f Figure 6. T he former long lingual sheath is now in-
the end of the peninsula extending from the mesiolingual side. T h e shorter is the end o f another peninsula projecting from the buccal side toward the center. T h e left opening of the three is formed
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by one of the lingua roots. T h e small opening in the center is a space between tw o epithelial projections. The space on the right side, as w ill be seen later, marks the opening o f a separate root. Figure 9 is a section obtained from near the center of the tooth germ. T here is only one opening in the center and two long H ertw ig’s epithelial sheaths. For
11, the small opening on the buccal side has disappeared, and only a small open ing in the center and a larger opening on the lingual side remain. Figure 12 is a section through the distal third of the tooth germ. T w o root openings explain the two large spaces separated from each other by an epithelial island, a crosssection o f the distal peninsula. Figure
F ig. 17.— T r a c in g o f the ba sal op en in g o f the upper first m ola r o f a rat 4 days old .
further explanation, we refer to the dia gram. Figure 10 shows again a changed picture. W e see three openings and two epithelial islands. T h e two epithelial islands at the center are the cross-sections of projections o f epithelium dif ferent from those referred to in Figure 8. These projections in Figure 10 are epi thelial extensions from the buccal and distal sides of the tooth germ. In Figure
13 is a section more distally than Figure 12. One of the openings has disappeared and only one opening is located more buccally. T his explains the longer lin gual epithelial sheath. Figure 14 is a section distally from the last root open ing, and H ertw ig’s epithelial sheath is again continuous as in Figure 4. W ith this presentation, it should be evident how interpretations from single
Orban and M u e lle r — D evelopm en t o f Bifurcation
sections can lead us into error. U nin terrupted series and uniformly thick sec tions are absolutely essential for this kind of work. In the follow ing figures, we present
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tures are the mesial sides ( M ) ; the upper, the distal ( D ) ; the right, the buccal ( B ), and the left, the lingual sides ( L ) . Figures 15 to 20 are tracings from upper first molars. Figure 15
F ig. 18.— T r a c in g o f the basal op e n in g o f the upper first m olar o f a rat 11 days old.
our tracings o f upper and lower first molars from rats o f different ages. In all figures, the lower portions of the pic-
shows a tracing o f a basal opening o f a molar from a rat five days before birth, T h e opening is kidney-like in shape, with
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the long axis mesiodistally. T h e buccal side is somewhat concave, the lingual, convex. In Figure 16, we observe the basal opening o f a molar from a rat 1 day old. Here, already we find a radical
marked difference in size. T h e chief change in comparison with the former stage has taken place in the outline of the basal opening. In Figure 15, we n o t e t ha t the o u t l i n e o f the basal
F ig . 19.— T r a c in g o f the basal op e n in g o f the upper first m olar o f a rat 15 days old .
difference from Figure 15; which calls for an analysis. T h e gross shape is ap proximately the same as in the rat five days before birth. There is, however, a
opening (a, Fig. 2 ) is parallel to the o u t e r l i ne o f H e r t w i g ’ s e p i t h e l i a l sheath ( b, Fig. 2 ) . T h a t means that H ertw ig’s epithelial sheath is of the
Orban and M u e lle r — D evelopm en t o f Bifurcation
same length all around the basal open ing. In Figure 16, finger-like projections point toward the center o f the opening. W e can, at this early stage, detect the location of the later roots. This is a most remarkable fact, for we see that
315
about this demarcation, is not correct. W e see, in Figure 16, the basal opening of a molar o f a rat 1 day old. T his al ready shows a definite shaping toward the future root outline. In this stage of tooth development, no dentin has yet
Fig. 20.— T r a c in g o f the basal op en in g o f the u p p er first m ola r o f a rat 19 days old .
the former view, i. e., that the demarca tion for the roots commences after the crown has fully developed and that Hertw'ig’s epithelial sheath grows hori zontally toward the center to bring
been deposited. During this stage, we can hardly speak o f the finished develop ment o f the crown. T h e question arises whether the irregular outline of the basal opening is due to an inward growth of
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F ig. 21.— T r a c in g o f the basal op en in g o f the lo w e r first m ola r o f a rat five days b efore birth.
o f development, the formation o f the irregular outline is due to an eccentric enlargement o f the whole basal opening peripherally rather than to an active in ward growth of certain portions of the epithelium. T h e finger-like projections o f H ertw ig’s epithelial sheath are penin sulas o f epithelium which remained fixed while portions of the epithelium pulled away to make room for future roots. In a section from a rat 4 days old (F ig. 17), the development has further prog ressed. T h e outline o f four of the roots is definitely to be seen. On the buccal side, irregularities appear. In this stage, also, the development has been due
Fig. 22.— T r a c in g of the basal op en in g o f the lo w e r first m olar o f a rat 1 day old.
certain points of H ertw ig’s epithelial sheath, as it was formerly believed, or whether some other order o f develop ment takes place. From our serial trac ings, we must state that, up to this stage
largely to expansion of the organ. A slight thickening of the epithelial projec tions indicates a certain activity in this region. Dentin development has just begun in this stage. T h e next stage, a
Orban and M u e lle r — D evelopm en t o f Bifurcation
section from a rat 8 days old, has al ready been seen in Figure 3. The de velopment of the bifurcation has ad vanced considerably, and the outline o f the different roots is becoming more dis tinct. N o w the epithelial projections have not only thickened but also show definite inward growth. T hrou gh fu r ther thickening of the ends of the finger like projections, the division of the roots
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have called attention in Figure 17, has now developed into an area which will form a root. In a rat 15 days old (F ig. 19) , the channels formerly connecting the roots have largely disappeared, leav ing only a few small openings3 to indi cate their former location. T he direc tion of the small openings and the shapes o f the root openings point toward the lines of fusion o f the epithelium. Figure
F ig. 23.— T r a c in g o f the basal op e n in g o f the lo w e r first m olar o f a rat 4 days old .
w ill be brought about. T h e larger spaces, which w ill be the future roots, are still connected with each other through narrow channels. In a rat 11 days old (F ig. 18), the channels con necting the larger spaces have become very much more constricted. T h e irregu larity on the buccal side, to which we
20, from a rat 19 days old, shows pro gression o f the root development to the 3. W . H . F. A d d ison and J. L. T . A p p le ton con sid er these sm all openings the result o f fa u lty fu sion o f H e r tw ig ’s epithelial sheath. (P roceed in g s o f the Sixth A nn ual M ee tin g o f the International A ssociation fo r D en tal Research, J. D ent. Res., N o. 3, p. 403, [J u n e ] 1928.)
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point where the roots have become sep arated from each other.
ment to the upper molar from a rat 15 days old (F ig. 19).
Figures 21 to 25 are tracings from lower first molars from rats ranging in age from five days before birth to eleven days after birth. T h e same order o f de
This particular subject does not re quire any further description. The pic tures and tracings tell their own story. It is true that there are many more dif-
F !g. 24.— T r a c in g o f the basal o p e n in g o f the lo w e r first m olar o f a rat 8 days old.
velopment can be followed through this series.
T he
chief
difference
between
this and the upper series is that the de velopment of the lower teeth proceeds, apparently, more rapidly than that of the upper. Figure 25, obtained from a rat 11 days old corresponds in develop-
ferent points for discussion into which we did not enter. W e had selected for ourselves a certain definite problem and this we have covered. O ur conclusions are: T h e bifurcation is determined long before the crown development is com
Urban and M u e lle r — D evelopm en t of Bifurcation
pleted. T h e roots are first outlined by eccentric peripheral enlargement of the basal opening of the tooth germ. In this eccentric growth, certain portions of H ertw ig’s epithelial sheath remain rela-
319
epithelial sheath, composed o f a double layer of cells, and the determination of the later roots in the very early stages o f development preclude any possible epi thelial downward growth.
F ig. 25.— T r a c in g o f the basal op e n in g o f the lo w e r first m ola r o f a rat 11 days old.
tively fixed, though later these projec tions enlarge and unite to form the bi furcation which demarcates the roots. T he horizontal position o f H ertw ig’s
T he observation that H ertw ig’s epi thelial sheath is a fixed point from which the tooth germs grow peripherally is confirmed.
U C H research w ork has been done on tooth development in general, but, so far, little consideration has been given the development o f the bi furcation.
M
apical end of the enamel epithelium pointing toward the center is H ertw ig’s epithelial sheath. T h e arrows indicate the direction in which the epithelium is thought to grow. B shows how these
F ig . 1.— D i a g r a m s h o w in g the o r ig in a n d th e d e v e lo p m e n t o f th e b ifu r c a t io n o f m u ltir o o te d teeth .
A , t o o t h -g e r m f r o m the s id e . T h e h a tc h e d a r e a s a r o u n d th e to o th g e r m in d ic a t e the
e n a m e l e p ith e liu m , th e e n d s o f w h ic h , m a r k e d b y a r r o w s , a r e H e r t w i g ’ s e p ith e lia l sh ea th .
B, th e to o th g e r m v i e w e d f r o m th e b a s a l o p e n in g . T h e s h a d e d a r e a is H e r t w i g ’ s e p ith e lia l s h e a th . T h e a r r o w s in d ic a te th e d ir e c t io n in w h ic h th e e p ith e lia l sh e a th is s u p p o s e d t o g r o w .
It is assumed that after the crown portion of the tooth develops, certain points of H ertw ig’s epithelial sheath commence horizontal growth toward the center o f the pulp. This process is diagrammatically illustrated in Figure 1. A shows the crow n portion of the tooth covered with the enamel epithelium. T h e Jour. A . D . A ., February, 1929
297
processes were to grow together to form the bifurcation when viewed from the bottom. Later in development, dentin is laid down inside the sheath, and, in this way, the outline o f the bifurcation develops. N o accounts of investigations show ing exactly the details o f this process
298
T h e Journal of the American D en ta l Association
have thus far been published. W eski and Contreras1 attempted to explain bifurca tion development upon the basis of a few histologic pictures. T heir material as well as their methods was inadequate. T heir diagrammatic drawings which they called reconstructions o f the his tologic specimens were only reconstruc tions of the authors’ conceptions, and not reconstructions of the true specimens.
F ig . 2.— S e ctio n t h r o u g h a m o la r o f a ra t.
sary to conduct investigations covering this phase of the development. T h e high point of this newer observa tion is that the epithelium does not grow into the depth to form the dental lamina and tooth germs, but certain portions of the epithelium remain in relatively fixed positions, while the surrounding tissues grow. Measurements from extensive human material verify this observation.
a, th e p u lp a l o r c e n tr a l en d o f th e e p ith e lia l
sh e a th , b, p e r ip h e r a l en d o f th e sa m e w h e r e it io in s th e e n a m e l e p ith e liu m . these p o in ts h a v e b een m a r k e d th ro u g h o u t the e n tire serie s.
D ev e lo p m e n ta l problem s can not be solved with single specimens and the gaps filled in from the imagination. Because of recent newer observations in tooth development,2 we felt it neces-
W h ile conducting these investigations, it was found that the development of the root does not proceed as it was formerly thought. T he former view assumed that 2.
I.
W e s k i, O ., a n d C o n t r e r a s , R .:
In th e t r a c in g s ,
B e fu n d e o f
O rb a n , B a lin t:
T ooth
1 5 :1 0 0 4
G erm s
(J u n e )
u n d V o r g ä n g e im I n t e r r a d ik u lä r r a u m e M e h r w u r z e li g e r Z ä h n e ; V ie r te l]a h r s c h r . f . Z a h n -
E m b r y o lo g y ,
h e ilk ., 1924, P a r ts 2 -3 , p . 229.
1928.
and
G row th and M ovem en t T eeth ,
J. A .
D.
A.
1 9 2 8 ; D e n ta l H i s t o l o g y a n d
C h ic a g o : R ogers
P r in t in g
C o .,
Orban and M u e lle r — D evelop m en t o f Bifurcation
H ertw ig’s epithelial sheath grew into the depth to precede the development o f the root. T h e studies o f one of us (B . O .) have shown this view to be incorrect. H ertw ig’s epithelial sheath during the
299
this point peripherally. These observa tions are incompatible with the former th e o re tica l co n ce p tio n o f b ifu rca tion d ev elop m en t. T h is in com p a tib ility prompted the present investigation.
F ig 3.— T h e outline o f H e r tw ig ’s epithelial sheath o f the upper first m ola r o f a rat 8 days old . T h e inner heavy line corresp on d s to a in F igu re 2 ; the outer h ea v y line, to b. E ach horizontal line betw een the tw o h e a v y lines represents the epithelial sheath. D indicates the d irection d ista lly ; M , m e sia lly ; L, lingu ally, and B, bu ccally.
entire development o f the tooth marks the relatively fixed point. T h e whole growth of the tooth takes place from
T he study of bifurcation development on human material is almost impossible, Embryos over 200 mm. in length are
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F ig. 4.— T o o th germ o f the upper first m olar o f a rat 8 days old . T h e section w a s cut m esially o f the basal op ening, and so the epithelial sheath is continuous.
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Fig. 5.— T o o th g e rm o f the upper first m ola r o f a rat 8 days old, the section bein g m ade a little m ore d ista lly fr o m the location o f that in F igu re 4, co rre sp o n d in g to the h orizon tal line in the corn er d ia g ra m . T h e epithelial sheath ex-tends equ ally on both sides.
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F ig. 6.— T o o th germ o f the upper first m olar o f a rat 8 days old. T h e section w as m ade still fu rth e r distally than in F igu re 5. A com parison with the corn er d iag ra m explains the lateral position o f the ba sal op en in g and the g reater length o f the epithelial sheath on one side.
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F ig. 7.— T o o th germ o f the upper first m o la r o f a rat 8 days old , the section bein g m ade through the tooth germ co rre sp o n d in g to the h orizon tal line in the diagram . T w o basal op enings are present, separated fro m each other by a lon g epithelial island.
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F ig. 8.— T o o th germ o f the up p er first m olar o f a rat 8 days old . T h r e e openings lead to the pulp, and tw o epithelial islands are to be seen. O nly the serial recon stru ction as given in the d ia g ra m can expla in this occu rren ce.
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F ig. 9.— T o o th germ o f the u p p er first m olar o f a rat 8 days old. T h e section is fr o m near the center o f the tooth germ . T h e r e is o n ly one op en in g in the center, and tw o lo n g H ertw ig ’ s epithelial sheaths.
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necessary for this work, and these are very difficult to obtain in series o f age. W e therefore decided to conduct our investigations on material from rats. W e assumed that the biologic processes that take place during development are
saw that the bifurcation development was already completed. A fter thoroughly studying the com plete series o f sessions, we concluded that some definite form of reconstruction was absolutely essential on account o f the ex
F ig. 10.— T o o th germ o f the upper first m ola r o f a rat 8 days old , the section b ein g m ade fu rth er distally than in F igure 9. T h r e e openings again separated by epithelial islan ds lead to the pulp.
similar to those in the human being. For this work, we obtained complete serial sections from rat jaws, the rats ranging in age from a few days before birth to twenty days after birth. A t this age, we
treme complexity o f the material. Only through reconstruction can the exact development be visualized. T w o means o f reconstruction were available: ( 1 ) the construction o f models and ( 2 ) the
Orban and M u e lle r — D evelopm en t of Bifurcation
making o f serial microscopic drawings. T h e latter seemed better suited for our purpose. W ith the presentation of Figure 2, a description of our method will be given. T his is a photomicrograph from a section through a molar of a rat
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pheral end where it joined the enamel epithelium b. W e repeated this with each section o f the series. T h e single tracings were spaced 1 mm. apart be cause o f the thickness o f our specimens, 20 microns. W e enlarged the specimens
'ImUm X.
F ig. 11.— T o o th germ o f the upper first m olar o f a rat 8 days old . T h e changed picture corresp on d s to the chan ge in the shape o f H e r tw ig ’ s epith elial sheath. One sm all and one la rg e op en in g indicate the basal op e n in g o f the tooth germ .
8 days old. T h e serial sections were pro je cte d on to m illim e te r-ru le d paper with a micro-projecting apparatus. H ert w ig’s epithelial sheath was traced from the pulpal or central end a to the peri-
fifty times. T h e series of lines were con nected at both ends, and, in this way, the outline of the entrance to the pulp, i. e., the basal opening of the tooth germ as well as the exact shape of the entire
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H ertw ig’s epithelial sheath, was ob tained. In order to ascertain the true anatomic shape of our tracings, it was necessary for us to observe the topo graphic relationship of the tooth to other organs.
the-enamel organ, so that the outer heavy line has sometimes been set arbitrarily. W e would also stress the fact that we were not particularly concerned with the number, size or distribution of the root canals, but were primarily con-
IÉSS F ig. 12.— T o o th germ o f the u p p er first m o la r o f a rat S days old. T h e section is through the distal third o f the tooth germ . T w o root op enings explain the tw o la rg e spaces separated fr o m each other b y an ep ith elial island. T h is ep ith elial island, i f com p ared w ith the corn er d ia g ra m , is m anifest as a cross-section o f an epithelial p eninsula.
W e desire to call attention to the fact that it was sometimes difficult to determine exactly where to set the border line between H ertw ig’s epithelial sheath and
cerned with the order of development of the bifurcation, The outline o f H ertw ig’s epithelial sheath and the progress of the develop-
Orban and M u e lle r — D evelo p m en t o f Bifurcation
ment of the bifurcation o f the upper first molar of a rat 8 days old is shown in Figure 3. T h e inner heavy line out lining the irregular opening corresponds to a in Figure 2 ; the outer heavy line corresponds to b in Figure 2. Each hori-
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the basal opening o f the tooth. T h e low er part of the picture is the mesial side; the upper, the distal; the right, the buccal, and the left, the lingual side. T o show in more detail just how we ob tained the basal openings, w e will show
PH
F ig. 13.— T o o th germ o f the upper first m olar o f a rat 8 days o ld . T h is section is fr o m near the end o f the basal opening. T h e sin gle op e n in g is located bu ccally. T h is explains the lo n g e r lin gu al epithelial sheath.
zontal line between the tw o hea.vy lines represents epithelium; i. e., H ertw ig’s epithelial sheath. T h e space inside o f the inner heavy line is the entrance to the coronal pulp; that is, what we call
interrupted series of pictures from the specimens from which Figure 3 was con structed. In the left upper corner o f each photomicrograph, we present a re duced photograph o f Figure 3. T h e
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horizontal heavy line in this picture in dicates the location of the section in the series from which the corresponding photograph was taken. T h e section shown in Figure 4 was cut through the tooth germ at the mesial side at a point
diagram. W e see the epithelial sheath of the tooth extending toward the cen ter about equally on both sides. This position of H ertw ig’s epithelial sheath at right angles to the walls of the tooth is almost the typical position of the
'Vi«»
F ig. 14.— T o o th germ o f the upper first m olar o f a rat 8 days old , the section b ein g distad from the basal op ening. T h e epithelial sheath is ag ain continuous as it is in F igu re 4.
where H ertw ig’s epithelial sheath was continuous. Figure 5 was taken from a section a little more distally from the location of that in Figure 4, correspond ing to the horizontal heavy line of the
sheath in all stages of development. F ig ure 6 was taken still more distally. On one side, H ertw ig’s epithelial sheath is much longer than on the other, and the basal opening is correspondingly well
Orban and M u e lle r — D evelopm en t of Bifurcation
F ig. 15.— B asal op en in g o f the upper first m o la r o f a rat 5 days before birth.
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terrupted, and we now have on both sides of the tooth short epithelial sheaths. About midway between the two, there is an elongated island and two openings leading to the pulp. O nly complete serial sections afford the explanation for this occurrence. T he epithelial island is only a cross-section of a peninsula, a pro jection of H ertw ig’s epithelial sheath, as can readily be seen from tracings of all sections o f the series. T h e picture ap pears to be more complicated in Figure 8. Here, we have two short islands be tween the outer sheaths. In reconstruc tion, the longer island is shown to be
F ig. 16.— T r a c in g o f the basal op en in g o f the u p p er first m olar o f a rat 1 day old.
toward one side. T h e study of the dia gram serves to explain this. T h e section of Figure 7 was obtained further distally than the section o f Figure 6. T he former long lingual sheath is now in-
the end of the peninsula extending from the mesiolingual side. T h e shorter is the end o f another peninsula projecting from the buccal side toward the center. T h e left opening of the three is formed
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by one of the lingua roots. T h e small opening in the center is a space between tw o epithelial projections. The space on the right side, as w ill be seen later, marks the opening o f a separate root. Figure 9 is a section obtained from near the center of the tooth germ. T here is only one opening in the center and two long H ertw ig’s epithelial sheaths. For
11, the small opening on the buccal side has disappeared, and only a small open ing in the center and a larger opening on the lingual side remain. Figure 12 is a section through the distal third of the tooth germ. T w o root openings explain the two large spaces separated from each other by an epithelial island, a crosssection o f the distal peninsula. Figure
F ig. 17.— T r a c in g o f the ba sal op en in g o f the upper first m ola r o f a rat 4 days old .
further explanation, we refer to the dia gram. Figure 10 shows again a changed picture. W e see three openings and two epithelial islands. T h e two epithelial islands at the center are the cross-sections of projections o f epithelium dif ferent from those referred to in Figure 8. These projections in Figure 10 are epi thelial extensions from the buccal and distal sides of the tooth germ. In Figure
13 is a section more distally than Figure 12. One of the openings has disappeared and only one opening is located more buccally. T his explains the longer lin gual epithelial sheath. Figure 14 is a section distally from the last root open ing, and H ertw ig’s epithelial sheath is again continuous as in Figure 4. W ith this presentation, it should be evident how interpretations from single
Orban and M u e lle r — D evelopm en t o f Bifurcation
sections can lead us into error. U nin terrupted series and uniformly thick sec tions are absolutely essential for this kind of work. In the follow ing figures, we present
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tures are the mesial sides ( M ) ; the upper, the distal ( D ) ; the right, the buccal ( B ), and the left, the lingual sides ( L ) . Figures 15 to 20 are tracings from upper first molars. Figure 15
F ig. 18.— T r a c in g o f the basal op e n in g o f the upper first m olar o f a rat 11 days old.
our tracings o f upper and lower first molars from rats o f different ages. In all figures, the lower portions of the pic-
shows a tracing o f a basal opening o f a molar from a rat five days before birth, T h e opening is kidney-like in shape, with
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the long axis mesiodistally. T h e buccal side is somewhat concave, the lingual, convex. In Figure 16, we observe the basal opening o f a molar from a rat 1 day old. Here, already we find a radical
marked difference in size. T h e chief change in comparison with the former stage has taken place in the outline of the basal opening. In Figure 15, we n o t e t ha t the o u t l i n e o f the basal
F ig . 19.— T r a c in g o f the basal op e n in g o f the upper first m olar o f a rat 15 days old .
difference from Figure 15; which calls for an analysis. T h e gross shape is ap proximately the same as in the rat five days before birth. There is, however, a
opening (a, Fig. 2 ) is parallel to the o u t e r l i ne o f H e r t w i g ’ s e p i t h e l i a l sheath ( b, Fig. 2 ) . T h a t means that H ertw ig’s epithelial sheath is of the
Orban and M u e lle r — D evelopm en t o f Bifurcation
same length all around the basal open ing. In Figure 16, finger-like projections point toward the center o f the opening. W e can, at this early stage, detect the location of the later roots. This is a most remarkable fact, for we see that
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about this demarcation, is not correct. W e see, in Figure 16, the basal opening of a molar o f a rat 1 day old. T his al ready shows a definite shaping toward the future root outline. In this stage of tooth development, no dentin has yet
Fig. 20.— T r a c in g o f the basal op en in g o f the u p p er first m ola r o f a rat 19 days old .
the former view, i. e., that the demarca tion for the roots commences after the crown has fully developed and that Hertw'ig’s epithelial sheath grows hori zontally toward the center to bring
been deposited. During this stage, we can hardly speak o f the finished develop ment o f the crown. T h e question arises whether the irregular outline of the basal opening is due to an inward growth of
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F ig. 21.— T r a c in g o f the basal op en in g o f the lo w e r first m ola r o f a rat five days b efore birth.
o f development, the formation o f the irregular outline is due to an eccentric enlargement o f the whole basal opening peripherally rather than to an active in ward growth of certain portions of the epithelium. T h e finger-like projections o f H ertw ig’s epithelial sheath are penin sulas o f epithelium which remained fixed while portions of the epithelium pulled away to make room for future roots. In a section from a rat 4 days old (F ig. 17), the development has further prog ressed. T h e outline o f four of the roots is definitely to be seen. On the buccal side, irregularities appear. In this stage, also, the development has been due
Fig. 22.— T r a c in g of the basal op en in g o f the lo w e r first m olar o f a rat 1 day old.
certain points of H ertw ig’s epithelial sheath, as it was formerly believed, or whether some other order o f develop ment takes place. From our serial trac ings, we must state that, up to this stage
largely to expansion of the organ. A slight thickening of the epithelial projec tions indicates a certain activity in this region. Dentin development has just begun in this stage. T h e next stage, a
Orban and M u e lle r — D evelopm en t o f Bifurcation
section from a rat 8 days old, has al ready been seen in Figure 3. The de velopment of the bifurcation has ad vanced considerably, and the outline o f the different roots is becoming more dis tinct. N o w the epithelial projections have not only thickened but also show definite inward growth. T hrou gh fu r ther thickening of the ends of the finger like projections, the division of the roots
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have called attention in Figure 17, has now developed into an area which will form a root. In a rat 15 days old (F ig. 19) , the channels formerly connecting the roots have largely disappeared, leav ing only a few small openings3 to indi cate their former location. T he direc tion of the small openings and the shapes o f the root openings point toward the lines of fusion o f the epithelium. Figure
F ig. 23.— T r a c in g o f the basal op e n in g o f the lo w e r first m olar o f a rat 4 days old .
w ill be brought about. T h e larger spaces, which w ill be the future roots, are still connected with each other through narrow channels. In a rat 11 days old (F ig. 18), the channels con necting the larger spaces have become very much more constricted. T h e irregu larity on the buccal side, to which we
20, from a rat 19 days old, shows pro gression o f the root development to the 3. W . H . F. A d d ison and J. L. T . A p p le ton con sid er these sm all openings the result o f fa u lty fu sion o f H e r tw ig ’s epithelial sheath. (P roceed in g s o f the Sixth A nn ual M ee tin g o f the International A ssociation fo r D en tal Research, J. D ent. Res., N o. 3, p. 403, [J u n e ] 1928.)
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point where the roots have become sep arated from each other.
ment to the upper molar from a rat 15 days old (F ig. 19).
Figures 21 to 25 are tracings from lower first molars from rats ranging in age from five days before birth to eleven days after birth. T h e same order o f de
This particular subject does not re quire any further description. The pic tures and tracings tell their own story. It is true that there are many more dif-
F !g. 24.— T r a c in g o f the basal o p e n in g o f the lo w e r first m olar o f a rat 8 days old.
velopment can be followed through this series.
T he
chief
difference
between
this and the upper series is that the de velopment of the lower teeth proceeds, apparently, more rapidly than that of the upper. Figure 25, obtained from a rat 11 days old corresponds in develop-
ferent points for discussion into which we did not enter. W e had selected for ourselves a certain definite problem and this we have covered. O ur conclusions are: T h e bifurcation is determined long before the crown development is com
Urban and M u e lle r — D evelopm en t of Bifurcation
pleted. T h e roots are first outlined by eccentric peripheral enlargement of the basal opening of the tooth germ. In this eccentric growth, certain portions of H ertw ig’s epithelial sheath remain rela-
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epithelial sheath, composed o f a double layer of cells, and the determination of the later roots in the very early stages o f development preclude any possible epi thelial downward growth.
F ig. 25.— T r a c in g o f the basal op e n in g o f the lo w e r first m ola r o f a rat 11 days old.
tively fixed, though later these projec tions enlarge and unite to form the bi furcation which demarcates the roots. T he horizontal position o f H ertw ig’s
T he observation that H ertw ig’s epi thelial sheath is a fixed point from which the tooth germs grow peripherally is confirmed.