Dental anomalies are major features of several human syndromes. For example, supernumerary teeth are found in cleidocranial dysplasia, and hypodontia is seen in hypohydrotic ectodermal dysplasia. Additionally, supernumerary teeth and hypodontia are found as isolated anomalies in a sizable part of the population. Studies in mice have provided important insights into mammalian tooth development. This process occurs through a series of reciprocal signaling interactions between oral epithelium and neural crest-derived ectomesenchymal cells. A central theme in this process is the role of signaling pathways, such as that initiated by Fibroblast Growth Factor (FGF) gene family products. Members of the Sprouty family, which was first identified in Drosophila melanogaster, are inhibitors of receptor tyrosine kinase signaling, and in particular FGF function. There are four mammalian orthologs, Spry1-4. We have generated mice null for Spry1, 2 and 4, the three Sprouty genes known to be widely expressed in the embryo. Spry2 and Spry4 null mice have abnormal molar cusp morphology, whereas Spry1 null mouse cusps appear grossly normal. Preliminary three-dimensional scanning analyses of Spry2 knockout teeth using geometric morphometric and GIS (Geographic Information Systems) techniques have shown bucco-lingual compression of cusp tips and extra lobes in the anteroconid (front part of the first molar). Ongoing studies of gene expression in the wild-type and mutant teeth should provide molecular explanations for the cusp abnormalities. Both Spry2 and Spry4 null mice have supernumerary teeth in the lower jaws, whereas Spry1 null animals appear to have the normal number of teeth. In mice, a toothless gap known as a diastema is present between the incisor and molar regions. The diastemal region contains vestigial tooth primordia that cease developing and undergo apoptosis during late gestation. The extra tooth in Sprouty knockout mice likely reflects an abnormal continuation of development of this tooth germ, and experiments are in progress to determine how loss of Sprouty gene function causes this effect. Interestingly, the supernumerary teeth in Spry4 null mice are always on the right side, suggesting a previously unknown lateral asymmetry in responsiveness to signals for tooth development. The results of our analysis will provide insights into mechanisms that control normal tooth development and that may lead to disease when perturbed. The supernumerary teeth in the Spry2 and Spry4 mouse diastema may correspond to human premolars, and study of these extra teeth may yield clues about the differences between mouse and human dentition.