Developmental Mechanisms Regulating Heterodonty and Diphyodonty of Mammalian Dentition

Mammalian dentition is characterized by regional differentiation into incisors, canines, premolars and molars on each jaw quadrant (heterodonty), and single tooth replacement during lifetime (diphyodonty). Despite their significance in mammalian biology and paleontology, little is known about the developmental mechanisms regulating tooth type determination and diphyodont tooth replacement. The mouse, the most popular laboratory animal, is not appropriate for the investigation of heterodonty and diphyodonty, because of its highly specialized dentition. The house shrew, Suncus murinus, has been implicated to be potentially an excellent model organism to study the mammalian basal condition of tooth development. Using this model organism, the tooth-forming regions for all tooth types have been precisely located in the developing jaws. The expression domain of Bmp4/Msx1 prior to odontogenesis corresponds to the incisor-forming region, whereas that of Fgf8/Barx1 corresponds to the molar-forming region. The model for tooth type determination established in the lower jaw of the mouse, will be applicable to both upper and lower jaws of the house shrew. In addition, the developmental sequence of tooth germs in the house shrew indicates that two adjacent primary enamel knots do not develop simultaneously, but with a constant time lag. This mode of tooth replacement can be explained by a sequential inhibitory cascade from developmentally preceding adjacent tooth germs. On the basis of the new knowledge on tooth development of the house shrew, this article discusses the developmental mechanisms regulating mammalian heterodonty and diphyodonty, and considers future prospects in the field of evolutionary developmental biology