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Developmental Pattern of Human Milk Teeth

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Developmental Pattern of Human Milk Teeth 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.
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