Evolution and development of the dentition.

The genetic code that produces human teeth began to develop in primitive vertebrates around 500 million years ago. Some parts of the information appear to have been very stable, particularly the mineralized matrices, while others have evolved. The development of tooth shape and tooth number are very rigidly controlled by genes in each species and are responsive to relatively rapid genetic selection by the environment, as are bone shape and associated soft tissues. Their developmental independence is reflected in the ability of the embryonic tooth bud to develop in vitro. Part of the genetic control of tooth size and shape is correlated with the genetic control of size and shape of the jaws, but the jaws are more responsive to environmental variables modifying individual development than are the teeth. There are recognized mutations producing changes in teeth which act at all levels of control, the development of the embryonic bud, the morphogenesis of the bell stage, the production of enamel and dentin and the formation of the roots. The mechanisms of this genetic control are at the molecular and submolecular levels which are just beginning to be examined. Tooth germs are a good system for study of these processes, and changes in our knowledge will lead to increased understanding of the variation of teeth and its relation to the structure of other tissues both normal and abnormal.