Developmental Pattern of Human Milk Teeth

The development of mammalian tooth cusps or ridges has been explained by phylogenetic homology, such as the ‘Tritubercular theory’ (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 prominences 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 formed 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 developed 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 developed 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.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 invaginations 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 symmetrical divergence patterns such as widening of concentric circular ripples. However, some of the new structures were inhibited by other factors. The authors dubbed this the ‘Divergence’ and ‘Convergence’ processes of tooth morphogenesis. These patterns originated in mammalian evolution.9) This fundamental developing pattern of the crown is the same in both the upper and lower, and also the right and left sides of jaws.10) All structures could be understood from the symmetry in ‘Divergence’ and ‘Convergence’ factors.