Int J Oral-Med Sci 9(2):75-87,2010 Original Article

Developmental Pattern of Human Milk Teeth

Kazuya Ohzeki,Rumi Ota-Yokota,and Hayato Ohshima

Departments ofOral Diagnostics,and Histology,Cytology and Developmental Anatomy,Nihon University School of

Dentistry at Matsudo,Matsudo,Chiba 271-8587,Japan Divisions of Anatomy and Cell Biology of the Hard Tissue,Niigata University Graduate School of Medical and Dental

Sciences,Niigata,Niigata 951-8510,Japan

Correspondence to: Kazuya Ohzeki The development of mammalian tooth cusps or ridges has been ex-

E-mail: ohzeki.kazuya＠ nihon-u.ac. plained by phylogenetic homology,such as the‘Tritubercular theory’ jp (Gregory 1934).However,descriptions of the incisor and canine are

lacking, and many questions remain unanswered. In this paper, the

authors examine crown development in human teeth in embryos and

skulls of young children by micro computed tomography(μ-CT),and

discuss these problems.The results were as follows: 1)In milk incisors and canines,the first swelling developed into the

central marginal ridge,and new swellings became mesial and distal

sites and also buccal and mesial sites. Mesial and distal promi- nences developed into mesial and distal mamelons,but labial and

lingual swellings formed cingulums. 2)In upper first milk molars,the first swelling developed into a buccal

cusp and new swellings around it became buccal and lingual,and

also mesial and distal sites symmetrically.The lingual prominence

formed a lingual cusp,and the mesial and distal prominences for- med mesial and distal cones or marginal ridges,but only the labial

prominences developed into a mesio-buccal ridge. 3)In lower first milk molars,the first swelling developed into a buccal

cusp,and new swellings became mesial and distal,and also buccal

and lingual sites symmetrically.The lingual prominence developed

into a lingual cusp.The mesial ridges and conules developed from

the mesial prominence,and the distal prominence developed into

three cones known as hypoconids. The buccal prominence devel- oped into a mesio-buccal ridge similar to the first upper milk

molars. 4)In upper second milk molars, the first swelling developed into a

mesio-buccal cusp, and new swellings formed in the mesial and

distal,and also buccal and lingual sites symmetrically.The lingual

prominence developed into a mesio-lingual cusp, and the distal

swelling into a disto-buccal cusp.The mesial prominence developed

into mesial cones or mesial ridges, and the buccal prominence

developed into a cingulum or a mesio-buccal ridge.Next,the disto -lingual cusp formed from a distal extension of the mesio-lingual

and disto-buccal cusps. 5)In lower second milk molars, the first swelling developed into a

mesio-buccal cusp,and new swellings spread mesially and distally, and also to buccal and lingual sites.The lingual prominence devel- oped into a mesio-lingual cusp,the mesial prominence into mesial

ridges,and the buccal prominence formed a cingulum or a mesio- buccal ridge.Subsequently,new swellings added two more mesial

cusps,which developed into three hypoconids. Int J Oral-Med Sci 9(2):75-87,2010

6)Many swellings appeared temporarily and then disappeared during

the development process between and around those main cusps. 7)Stable main cusps or mamelons developed in wide spaces formed

during jaw development. However, small swellings and invagina- tions of the crown,such as cingulums,marginal ridges and other

structures,formed in narrow spaces of the jaw.The tooth and jaw

morphogenesis have a close relationship. 8)Those developments suggested that the crown developed with sym- metrical divergence patterns such as widening of concentric circu- lar ripples.However,some of the new structures were inhibited by

other factors.The authors dubbed this the‘Divergence’and‘Conver- gence’processes of tooth morphogenesis.These patterns originated

in mammalian evolution. Keywords: 9) This fundamental developing pattern of the crown is the same in tooth crown development, human both the upper and lower,and also the right and left sides of jaws. tooth, micro computed tomography, 10)All structures could be understood from the symmetry in ‘Diver- deciduous tooth gence’and ‘Convergence’factors.

study.These were specimens stored for educational

Cusps and ridges of human molar tooth have been purposes at the Nihon University School of Dentistry named based on phylogenic homology,but this termi- at Matsudo,Niigata University Faculty of Dentistry nology has not been applied to other teeth and small and Tsurumi University School of Dental Medicine. swellings (1-4).On the other hand, there are many These specimens were analyzed by μ-CT,(Elescan; reports on the development of cusps(5-15).However, Nittetsu Elex, Tokyo, Japan, 80 kV, 70-80μA and the order of differentiation of cusps does not corre- TOSCANER-31300μhd; Toshiba X-Ray Inspection spond with the phylogenic terminology. Though System, Tokyo, Japan, 60-85 kV, 80-110μA, slice there are many swellings and ridges on the crown, width: 25μm,slice pitch: 0.20),and with a software their mechanism of development and phylogenic program (NDTView; Sony,Tokyo,Japan,3D bone purpose are unclear. for WinNT ; Ratoc System Engineering Co., Ltd., Many questions regarding crown formation and Tokyo,Japan and Tri/3D-Bon; Ratoc System Engi- development remain unanswered. What is the pur- neering Co.,Ltd.,Tokyo,Japan). pose of these small swellings on the tooth crown? How can one determine the number,position and size of these swellings Why does the mesial marginal Initial calcification stage of crown(4-to 5-month ridge of P1 and M1 protrude to the mesial side?How -old fetus)(Fig.1). can we explain the purpose of these small structures Wide alveolar foramina were observed in the phylogenically? upper jaw,but only an alveolar groove was seen in

In this paper,the authors aimed to answer these the lower jaw,where the interalveolar septa started questions regarding the crown development in initial calcification. human teeth by micro computed tomography(μ-CT) Early calcification stage of crown(4-to 5-month- observation of these structures,because this method old fetus)(Fig.2). is much more useful than X-rays,the Alizarin-red Milk incisors,milk canines,and first milk molars staining method,and serial sections. were observed in wide alveolar foramina. On the

upper and lower milk incisors, a large central

mamelon, and small mesial and distal mamelons

Eight human fetuses (4.5-8 months old) and 10 were observed,and resembled the shape of a flying skulls, about 6-9 months of age were used in this bird.First,milk molars appeared as a small swelling Int J Oral-Med Sci 9(2):75-87,2010

Fig.1. Sample 1 (approx.4-to 5-month-old fetus) a: upper jaw,b: lower jaw

Fig.2. Sample 2 (approx.4-to 5-month-old fetus) a: upper jaw,b: lower jaw Int J Oral-Med Sci 9(2):75-87,2010

(mesio-buccal cusp) with a shoulder on the distal the mesial side of the ridge.The mesio-lingual cusp side.Milk canines appeared only as a small cone. developed into an isolated lingual part,following a

Sample 3 (5-6 months old)(Fig.3,Sample 3) pyramidal pattern. First incisors to second milk molars were obser- On upper second milk molars, there were three ved. mesio-buccal cusps,a mesio-lingual cusp and a disto The upper one-third of the crown of upper milk -buccal cusp.The large mesio-buccal cusp had three incisors had formed, which had a central mamelon ridges on the medial, distal and lingual sides. A and two mamelons on either side.The upper three- mesial angle developed on the mesial side ridge.The fourths of the crown of lower incisors had formed, mesio-lingual cusp had a pyramidal shape.The disto with three mamelons on the cutting edge,and also -buccal cusp appeared as a small initial calcified central,distal and mesial lingual ridges.Only the top point separate from those two structures. of the cuspid of milk canines had appeared, which Lower first milk molars exhibited four parts, of had a triangular pyramidal shape. which three, a large mesio-buccal cusp, a mesio- Upper first milk molars exhibited two structures. lingual cusp and a buccal side containing two

The mesio-buccal cusp developed in the large part, hypoconids(disto-buccal cusp and distal cusp),were which had four ridges on the mesial,distal,lingual connected to each other.A central ridge was obser- and buccal sides. The swelling on the distal ridge ved between the mesio-buccal cusp and mesio-lin- developed into a disto-buccal cusp.The mesio-buc- gual cusp, and the mesial marginal ridge extended cal angle and mesial marginal ridge were observed in from the mesio-buccal cusp. A large swelling was

observed on the buccal surface of the mesio-buccal

- cusp. A small calcified cone (hypoconid, disto lin- gual cone)was isolated from other structures. On lower second milk molars, there were four

parts,two mesial (mesio-buccal and mesio-lingual) cusps,and two distal(disto-buccal and disto-lingual) cusps. A mesial marginal ridge extended from the

largest mesio-buccal cusp. About 5-7 months old (Fig.4,Sample 4) The entire developing milk dentition was obser- ved. The upper half of the crown of upper first milk

incisors was observed. Mamelons and angles were

observed on the cutting edge,and some cones were

on the labial and lingual surfaces.There were two

ridges,a mesial and distal marginal ridge,but there

was no central ridge on the lingual surface. The

upper one-third of the crown was observed on the

upper second milk incisor.It had a big central and

two small mamelons on the cutting edge and angles

were lacking on both sides. Lower first milk incisors exhibited almost all

crown forms.It had two or three mamelons on the

cutting edge,a lingual central ridge and a cingulum

Fig.3. Sample 3 (approx.5-6 months old) on the cervical area.Three-fourths of the crown of Int J Oral-Med Sci 9(2):75-87,2010

the mesial (mesial marginal ridge and cone),distal, lingual (central ridge) and buccal (mesial-buccal

ridge)sides.The ridge to the buccal side,continued

to the lingual ridge from the mesio-buccal cusp. Lower first milk molars exhibited an almost com- plete occlusal surface.There were two cones on the

mesio-buccal cusp and mesio-lingual cusp. Those

two cusps were continuous with two central ridges (distal trigonid ridge),and mesial and distal mar- ginal ridges,which surrounded the mesial and distal

fossae. A remarkable notch (distal trigonid notch) was recognized on the mesial marginal ridge near the

lingual cusp.Two or three cones(disto-buccal cusp, distal cusp and disto-lingual cusp)were observed on

the distal ridge. Only the lower-right second milk molar was obser- ved. Five cusps could be seen completely,but the

distal three were isolated from the mesial two cusps. The mesio-buccal and mesio-lingual cusp merged at

a mesial marginal ridge.It remarkably projected to

the mesial side. About 6-7 months old (Fig.5,Sample 5)

Fig.4. Sample 4 (approx.5-7 months old) An entire developing milk dentition was observed, but lower first incisors were missing. The upper half of the crown of the milk incisor had the lower second milk incisors appeared. A large formed. On the first upper milk incisor,mamelons, mamelon was observed on the cutting edge,and also distal and mesial angles were observed on the cutting a distal marginal ridge and central ridge on the edge.Mesial and lingual marginal ridges were seen lingual surface. on the lingual surface,and labial and lingual central

Upper and lower milk canines exhibited a small ridges were observed on both surfaces.The second triangular pyramidal structure. upper milk incisor had a mamelon and central ridges

The developing occlusal surface on the upper first on both labial and lingual surfaces. milk molar was similar to a completed tooth crown. Three mamelons were observed on the cutting

The buccal cusp developed into a large and high edge of the lower second milk incisor. Mesial and structure having marginal mamelons,and disto-and distal marginal ridges were recognized on the labial mesio-angles in both sides,from which mesial and surface,and central ridges were also seen on both distal marginal ridge ran to the lingual side.A lin- labial and lingual surfaces. gual (palatal) cusp, developed at the lingual side Upper and lower milk canines appeared as a small continued with the buccal cusp through the central triangular pyramid. ridge. Upper first milk molars exhibited almost all oc- There were three parts on the upper second milk clusal surfaces.It consisted of two main buccal and molar,which formed a trigon,a mesio-buccal cusp,a distal parts.The buccal and lingual cusps had mesial mesio-lingual cusp and a disto-buccal cusp. Four and distal mamelons and angles, from which mar- ridges ran from the top of the mesio-buccal cusp to ginal ridges ran to the inside of the occlusal surface. Int J Oral-Med Sci 9(2):75-87,2010

protruded to mesial side. The disto-lingual cone

appeared as a small point. Lower second milk molars exhibited five cusps, mesio-buccal cusp,mesio-lingual cusp,disto-buccal

cusp,disto-lingual cusp and distal cusp.Two large

mesial cusps and the disto-buccal cusp were continu- ous with the marginal ride.The disto-lingual cusp

and distal cusp appeared as a small dot separated

from other structures in the distal area. About 6-9 months (Fig.6,Sample 6) Crown formation of the upper first milk incisor

was almost complete, and the cutting edge had

mamelons and angles. Distal and mesial marginal

ridges surrounded the labial pit,but a buccal spine

could not be observed in these.Upper second milk

incisors also exhibited mamelons,and the labial pit

was surrounded by distal and mesial marginal ridges. Lower first and second milk incisors exhibited

nearly completed crown shape,in which the cutting

Fig.5. Sample 5 (approx.5-7 months old)

Both cusps had a central ridge and two accessory ridges,which divided into four pits (central groove) on the occlusal surface.Three ridges were observed on both the lingual and mesial surfaces,but the mesio -buccal ridge could not be recognized. There were three cusps,a mesio-buccal,a mesio- lingual (palatal)and a disto-buccal cusp on upper second milk molars.These were continuous with the marginal ridge forming the trigon.The disto-lingual cusp was observed separate from other cusps as a small dot.The mesial marginal ridges protruded to the mesial side. On lower first milk molar,there were four cusps on the occlusal surface-mesio-buccal cusp, mesio-lin- gual cusp and two distal hypocones of a disto-buccal and disto-lingual cone.The three cusps,except the disto-lingual cone,were continuous with each other forming a circular united structure. A remarkable notch was observed on the mesial margin, which Fig.6. Sample 6 (approx.6-9 months old) Int J Oral-Med Sci 9(2):75-87,2010 edge had three remarkable mamelons, and central ridges ran to the cervical region on labial and lingual Histological overview of crown formation surfaces. The mamelons of the first incisor were The terminology of cusp, ‘tritubercular theory’ clearer than those on the second incisor. based on paleontological research, is considered

Upper and lower canines exhibited a sharp pyrami- phylogenic terminology.The theory was originally dal shape.Mamelons and angles were observed on advocated by Cope-Osborn (1-4, 16, 17), but the both mesial and distal shoulders of the ridge. original cusp name of ‘protocone’changed from

Remarkably large cones were observed around the mesio-lingual cusp to mesio-buccal cusp based on cuspid. research conducted afterwards.However,Cope-Os- Upper first milk molars exhibited buccal and lin- born’s‘tritubercular theory’is popular not only in gual cusps continuous with distal and mesial mar- paleontology but also in developmental biology,so it ginal ridges, on which there were mamelons and is still used by many scientists. angles.Those structures surrounded a central fossa Calcification of tooth crown or crown development and a groove.A mesio-buccal ridge on the buccal has been widely studied(5-15,18-22).Early develop- surface was remarkable. Notably, many swellings mental biology of teeth was examined with macro- were observed around these structures. scopic anatomy for dissection and histological

Upper second milk molars had four cusps.Three of methods (13,14,22,23).These results were different them,a mesio-buccal cusp,a mesio-lingual cusp and from paleontological or phylogenic ‘Tritubercular a disto-buccal cusp,formed the trigon, which were theory’on the upper molars. surrounded by the marginal ridge that continued In the early 19th century, radiographs showed with those three cusps. more detailed data regarding calcification and devel- These marginal ridges had cones on them,and the opment of human dentition in addition to those tech- mesial marginal ridge protruded into the mesial side. niques (7-10, 15).These data were obtained from

Notably,the mesio-buccal cusp comprised 2-3 cones. over 100 human embryos. The developing cusp of Carabelli was noted on the Nomata (12)also examined tooth development by lingual(palatal)surface of the mesio-lingual cusp.A 3D-reconstruction with serial sections of more than disto-lingual cusp was observed at the distal site 100 embryos. He concluded that the mesial cusps continued to other three cusps with thin bridges. were calcified earlier than the distal,and the buccal

Lower first milk molars had main two cusps, a cusps earlier than the lingual.However,he did not mesio-buccal cusp and mesio-labial cusp,and three suggest a relationship between the development and distal cones (hypoconid, disto-buccal cone, disto- phylogenetic homology. labial cone and distal cone).There were two or three On the other hand,Kraus et al.(20)reported crown cones on the mesial marginal ridges, and trigonid development by Alizarin red staining in several notch was observed on the mesial marginal ridge hundred embryos and newborns.He confirmed that near the labial cusp. the mesio-buccal cusp was the first calcification

Lower second milk molars had five main cusps,a point in both upper and lower milk molars. In the mesio-buccal cusp, a mesio-lingual cusp, a disto- next stage,a mesial marginal ridge including three buccal cusp,a disto-lingual cusp and a distal cusp. cones and a mesio-lingual cusp, and also a disto- Mesial two main cusps had two or three cones on the buccal cusp appeared.Finally,distal cusps developed top. after these formations. He also suggested that the

Lower first molars appeared as three small cones, incisor and canine calcification started at the central a mesio-buccal cusp, a mesio-lingual cusp and a cones. These findings are identical to those in the disto-buccal cusp. report by Nomata (12).However,Nomata explained

that these phenomena match the phylogenic theory. Int J Oral-Med Sci 9(2):75-87,2010

The same developmental processes are observed in there are many temporary structures during develop- other primates (24-27),and also in some mammals ment. (25-30).It shows that development of cusp in these Upper first milk molar (Figs.3-6) animals is identical to the process of milk molar Calcification starts at the buccal cusp, and it development in humans. From these comparative diverges to mesial and distal sides to form mesial examinations, it was concluded that the mesio-lin- and distal ridges and angles, and the development gual cusp is the original cusp, or protocone in also extends to the mesial buccal ridge,and to the odontological phylogeny. lingual cusp. Subsequently, the crown develops to

In some mammals,the calcification first occurs on more mesial and distal areas to form marginal ridges the disto-buccal cusp or the mesio-lingual cusp (29, and many small swellings.Thus,the crown differenti- 30).Moreover,Suncus molar development starts at ates in symmetric directions from an original point. the mesio-buccal cusp (3rd molar) and the disto- The development of the labial cusp is concerned with buccal cusp (1st and 2nd molar).This suggests that palate differentiation. the developmental process of teeth can be quite Many swellings are small and ridges are seen in the variable,but, Kondo (29) and Hanamura et al.(30) intra cusp region temporarily,because those struc- concluded that the phylogenetic law is reflected in tures disappear during development. the cusp development. Why do these variations Lower first milk molar (Figs.3-6) occur? And why is there no explanation on the Calcification starts at the buccal cusp, and then developmental relationship between phylogeny and spreads out to form the lingual cusp, the mesial ontogeny?These questions remain to be answered. marginal ridge,the disto-buccal cone and the mesio Furthermore,there is no theory explaining the evolu- -buccal ridge.This is symmetric divergence from the tion of the incisor and canine except Bolk’s‘Dimer original point to opposite sides.Though large stable theory’(18,19). structures, such as the lingual cusp and the disto- buccal cone, develop in large spaces in jaws, the

Crown formation pattern differentiation of other small structures,the mesial

Milk incisor and milk canine(Figs.1-5) ridge and the mesio-buccal ridge,are inhibited by

On the milk incisor and milk canine, the crown mesial teeth and the buccal alveolar bone.The disto formation starts and differentiates from the central -lingual cone and distal cone differentiate after com- mamelon,or the cuspid,sideways to both the mesial pletion of the mesial structures.This also suggests and distal mamelons.This is related to jaw growth, continuous symmetric development. by which the right and left width of the incisal area The buccal cusp sometimes consists of two cones, increases. Moreover, buccal and lingual central and the mesio-buccal ridge also has two or three ridges appear. The development extends to form cones. Many temporary structures appear around distal and mesial angles,labial and lingual central those solid structures during crown development. ridges, and then distal and medial marginal labial This phenomenon suggests that differentiation of ridges. These phenomena suggest that the crown temporary structures is a divergence process, but develops from an original point symmetrically in those are converged by many factors to form a solid directions corresponding to jaw growth. tooth crown as in other teeth. These milk incisors have three mamelons and a The mesial trigonid notch and distal trigonid ridge central ridge, as in permanent incisors, and many are original structures because they are stable struc- cones appear during the development process.Some tures in early developmental stages. cones are also observed on the cuspid of milk Upper second milk molar (Figs.3-6) canines.However,most of these structures disappear The mesio-buccal cusp appears first.The mesio- in the final form. These phenomena suggest that lingual cusp and disto-buccal cusp then develop to Int J Oral-Med Sci 9(2):75-87,2010 form the trigon. The mesial marginal ridge which gence pattern which forms the cusp in opposite protrudes to the mesial side differentiates to form the directions,and the convergence pattern. mesial marginal ridge,and the buccal ridge.Next, the disto-buccal cusp, cusp of Carabelli and many Patten of crown formation temporary small swellings develop. The development of milk incisor and canine

This indicates symmetric divergence from the crowns is initiated in the central mamelon and cuspid original cusp,and the convergence process. (5,6,11-14,20,22,23),next the differentiation widens

Lower second milk molar (Figs.3-6) toward mesial and distal,and also buccal and distal

The mesio-buccal cusp appears initially.Then,the directions(Figs.2-6).The distal and lingual mamelons mesio-lingual cusp, mesial marginal ridge, disto- and angles develop relatively wider corresponding buccal cusp, and buccal ridges develop on buccal with the jaw growth,thus increasing the width of the surface. During this process, the mesial marginal incisal area in the right and left directions.However, ridges remarkably protrude to the mesial side as in small lingual ridges and cingulums form on the the upper second molar.This is the result of symmet- lingual and labial sides because a narrow space ric divergence in the first step of development.The develops between the labial and lingual alveolar two or three distal cusps arise in the next stage,and plates. many temporary swellings appear during develop- First milk molars arise in the corner of jaws ment. between incisor and molar regions (Figs.7-8).The

This shows the symmetric and continuous diver- original point is the buccal cusp and it differentiates

Fig.7. Schema of developing swelling

Swellings increase symmetrically from initial calcification point to mesial and distal,and also to buccal (labial)and lingual (palatal)directions.Swellings develop into large stable structures,such as cusps,mamelons and cuspids,in a large space,which corresponded with jaw growth.On the opposite side of these structures,small ridges and cingulums develop. Black point: original swelling

Gray point: secondary swelling

White point: tertiary swelling Int J Oral-Med Sci 9(2):75-87,2010

Kraus et al.in about 800 embryos and fetuses (20) with histological and Alizarin red staining. They

also discussed the relationship between development

and phylogenic terminology. Their reports are

almost the same as this study. Before their report,the initial cusp formation was

already reported using approximately 100 samples of

human fetuses and young children (e.g.,7-9, 12,15). Our results were also in agreement with these

studies. They suggested that initial calcification

occurs in the center (central mamelon) of incisors

and canines and the mesio-buccal cusp on molars. They also suggested that the ontogenic differentia- tion of crowns differs from the phylogenic‘trituber- cular theory’(18,19, 35).Nomata (12)reported the

calcification occurred from mesial to distal in

molars.However,he also could not find the relation- ship between tooth and jaw development in this

report. Fig.8. Relationship between crown and jaw development.An upper and lower crown structure develops symmetrically. From the results of the present study,it is conclud- Black point: original swelling ed that all coronal structures differentiate from an

Gray point: secondary swelling of large stable structure

White point: small structure initial point symmetrically in opposite directions,

Large stable structures develop in a large space,but small mesial and distal,and buccal and lingual (palatal) structures form in narrow spaces on the opposite side in sides in all teeth with many temporary swellings and relation to jaw growth. cones(Fig.7).The divergence continues from a new

cusp to further structures, but stable structures in the buccal (mesio-lingual ridge),mesial (mesial finally develop in given sites according to jaw devel- marginal ridge), lingual (lingual cusp) and distal opment (Fig.8). (mamelons and angles)directions.The stable cusps Swellings and cones arise throughout crown devel- and cones develop in the wide buccal and distal opment (Figs.2-6).This process is called the‘Diver- spaces, but small structures, such as the mesial gence’process.However,some of it differentiates to marginal ridge and mesio-lingual ridge at the buccal stable cusps in wider sites during jaw growth,while side,arise in narrow spaces.This suggests that the others form small ridges and cones in cramped areas alveolar regions develop towards lingual and both by many inhibitors.This is called the‘Convergence’ mesial and distal sides as the result of outward jaw process. growth. The authors propose the concepts of‘Divergence’ In second milk molars(Figs.3-6),the mesio-buccal and ‘Convergence’in dental morphogenesis. cusp appears initially, then the mesio-lingual and Another more important suggestion is given from disto-buccal cusps,the mesial marginal ridge and the the‘Divergence’and ‘Convergence’phenomena cusp buccal cingulum develop. The other distal cusps development can occur everywhere in relation to jaw differentiate after these developments,which suggest development. The crown formation pattern of all that this is a result of jaw growth;as in the first milk teeth in this study was nearly identical between molar. upper and lower,and right and left jaws(Figs.2-6). Detailed aspects of tooth crown were described by This would explain the dorso-ventral (upper-lower Int J Oral-Med Sci 9(2):75-87,2010 jaws),and right-left symmetry in body organization. malian species,because each tooth crown has its own

This suggests that developmental patterns in mam- pattern of differentiation. malian dentitions are variable,and thus this develop- mental process applies only to mankind or primates. Artifact factor

In the present study,it is possible that the results

Phylogenetic view included many artifacts, because cracks or clots

The relationship between cusp formation and appear during the drying process due to loss of phylogenetic homology has been controversial (1,2, organic matrix and water from these samples.This

16).As a result of arguments,cusps were named as damage was avoided as much as possible when proto-cone (-conid),para-cone (-conid),meta-cone analyzing the morphology,but we could not exclude

(-conid)and hypo-cone(-conid).A phylogenic termi- artifacts completely. These should be avoided in nology was considered because of the theory based further investigations using more embryos and on the molar evolution of fossils of early mammalian young skulls. dentitions.However,it fails to explain the evolution- ary theory in incisors and canines and the tooth root, and also the relationship between the dentition and The results of μ-CT images of human embryos jaws.It was already suggested earlier by Gregory(1, and skulls on the tooth development were as follows, 2)and Butler (31,32),that all teeth are one set in 1)In milk incisors and canines, the calcification jaws. started at the central mamelon and cuspid. It

The results of this study show that all teeth have spread symmetrically towards the opposite same developing pattern,leading to the proposal of directions, mesial, distal, lingual, and labial

‘Divergence’and ‘Convergence’.The differentiation sites.Solid structures develop on the mesial and of swellings has a strong relationship with jaw devel- distal sites,and small structures like cingulum opment.This interaction between tooth and jaws is and ridges develop on the labial and lingual characteristic in each group of teeth.Butler(31,32) surfaces. called the group ‘field’of jaws,and Osborn (3)also 2)In the first milk molar,calcification started at named it‘clone’.They suggest it belongs to the fam- the buccal cusp,and diverged symmetrically in ily of incisor,canine and molar.However,Hiraga et opposite directions, as in the incisor. Stable al.(33)reported that every tooth has a native field; cusps developed at buccal and lingual sites in as in the present results. both jaws,and a distal site in the lower jaw.On

The difference between phylogenic terminology mesial and buccal sites of these structures, and the present results can be explained by the inter- marginal ridges developed. pretation of ‘Biogenetic Law’(34).Many odontolo- 3)In the second milk molar,calcification started gists have thought that the order of cusp develop- at the mesio-buccal cusp.It spread out symmet- ment is reflected by the phylogenic pattern of crown rically in opposite directions,but the main cusps formation (1,2,4,16,17,35).However,the develop- developed at the mesio-lingual, disto-buccal, mental pattern of the crowns on both upper and disto-lingual and the distal sites.Opposite sides lower,and also right and left side of the jaws,show of these structures formed marginal ridges and the same symmetric pattern of‘Divergence’and‘Con- cingulums. vergence’.This is reflected by the organization of a 4)The development of crown shows a symmetric body.The phylogenic pattern is maintained in this pattern in both upper and lower,and right and development. left dentitions. The findings of this study suggest that the tooth 5)Stable cusps develop in the given large spaces developmental pattern is variable in each mam- corresponding with jaw development. Int J Oral-Med Sci 9(2):75-87,2010

6)The differentiation of crown diverges from the kaibougaku-teki kennkyu sono-3.nihonshikagakukai

initial point to symmetric directions continuous- -zasshi,23: 709-759, 1930.(in Japanese) 10.Arai S: Embryological study on the teeth of Japanese ly to form mature tooth crown. Hence, it is fetus.Sikagaku zasshi,6: 223-232,1946.(in Japanese) regulated by other factors,such as jaw develop- 11.Gantz SJZ : Studies on the fetal development of the

ment.The authors propose that these processes human jaws and teeth. Dental Cosmos, 64: 131-140,

are‘Divergence’and ‘Convergence’. 1922. 12.Nomata N : A chronological study on the crown for- 7)However, the interaction of ‘Divergence’and mation of the human deciduous dentition.Bull Tokyo

‘Convergence’is variable in each tooth in jaws, Med Dent Univ,11: 55-76,1964. because initial calcification points are different. 13.Rose C : Ueber die entwicklung der zghne des mens- 8)These suggest that the phylogenic reflection, chen.Archiv fur Mikroskopische Anatomie,38: 447- Biogenetic Laws may explain the symmetric 491,1891. 14.Rose C :̈ Uber die entstehung und formahanderungen

‘Divergence’and ‘Convergence’processes. der menschlichen molaren. Anat Anz, 7: 392-421, 1892. 15.Takehisa S, Koike C : Shiga no kaibougakuteki ken- We are grateful to Prof.Y Kozawa(Department of kyu sono 1 resenzo yori mita sekkaika ni tsuite. Tokyo shika daigaku kaibogakukyoushitu gyouse- Histology,Cytology and Developmental Anatomy), kisyu,8: 1-3,1958.(in Japanese) for his valuable advice,Mrs.Uemura,who helped us 16.Fujita T : Ha no kaibougaku, Ishiyaku pub, Tokyo, prepare the schemas,and Kenmotsu(Niigata Univer- 2007.(in Japanese) sity)for data analysis.This work was supported by 17.Sakai T : Ha no Keitai to Shinka, Ishiyaku Pub, Tokyo,1989. (in Japanese) a Grant-in-Aid for Strategic Research of Nihon 18.Bolk L : Fourth essay,On the relation between reptil- University School of Dentistry at Matsudo from the - ian and mammalian teeth.J Anat,56: 107 136,1922. Ministry of Education,Culture,Sports,Science and 19.Bolk L : Fifth essay,On the relation between reptilian

Technology,2008-2012. and mammalian dentition.J Anat,57: 55-75,1922. 20.Kraus BS, Jordan RE : The human dentition before

birth,Lea& Febiger,Philadelphia,1965. 21.Zuckerkandl E:̈ Uber das epitheliale Rudiment eines

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