PERSPECTIVES ON GENETIC-ASPECTS OF DENTAL PATTERNING

Nearly a century of speculation and experimentation has gone into tryi ng to understand the mechanisms that establish the pattern of the diff erentiated heterodont dentition. Regionally differing qualitative (com binatorial) expression of regulatory genes appears to be involved in t his process. Work by our laboratory and others shows that the six memb ers of the mammalian Dlx family of homeobox genes are expressed (a) at multiple times during dental development, (b) differently at differen t stages, (c) in a way that is related to their genomic organization a s gene-pairs linked to three of the four Hox clusters of positional pa tterning genes. The expression appears to be involved in jaw regionali zation, tooth initiation, and tooth development. However, this express ion correlates with no single aspect of dental patterning or tooth dev elopment, involves redundant and complementary function, and developme ntal differences between the maxillary and mandibular dentition sugges t that other elements remain to be identified. For example, the possib ility that quantitative aspects of gene expression specify development al fields in the dentition has not yet been investigated, Although the maxilla and mandible develop differently, indirect evidence suggests that, especially in the future midline (incisor) regions, both jaws ma y be patterned by a consistent process that occurs before neural crest migration takes place, and we hypothesize that signaling factors like Sonic hedgehog and Pax transcription factors may be involved.