Sequence polymorphism in prenatal dental development.

Sequence polymorphism is a well-known phenomenon of postnatal dental development. Individual sequence polymorphisms include the dm2-c/c-dm2* orders of eruption in the deciduous dentitions, and the M2-P2/P2-M2 and C-M2/M2-C eruption sequences in the permanent dentition (S. M. GARN and A. B. LEWIS, J Dent Res 36:992-995, 1957; S. M. GARN and A. B. LEWIS, in BROTHWELL, D.R. (ed) Dental Anthropology, London: Pergamon Press, 1963; S. M. GARN, A. B. LEWIS, and J. H. VICINUS, J Dent Res 41:717, 1962). Sequence polymorphism in prenatal human and infrahuman dental development has been explored only slightly, however, and the extent of sequence polymorphism in early prenatal development has not been quantified hitherto. However, numerous dental sequence polymorphisms can be demonstrated in first trimester human embryos. Of 52 specimens between 14 and 58 mm in crown-rump length, considered normal by both gross and histologic inspection, departures from the il-i2-c-dml-dm2 "textbook" sequence of development could be demonstrated in 25% of the sides considered. As shown in the table, such polymorphisms ranged from the i2-il order to the extreme dml-il order, and included sequential variations in c and dm2 as well. Left-right concordance averaged 65% for the i2-il and c-il sequences and between 33 and 42% for the rarer c-i2 and d-il formation sequences. As extrapolated from both size and stage-atdevelopment, "atypical" orders of early prenatal dental development were increasingly common in older and longer specimens (CC = 0.40), which is consistent with a wider range of dental "stages". Since the il-i2-c-dml-dm2 order was assigned in the absence of evidence