Slit1 is specifically expressed in the primary and secondary enamel knots during molar tooth cusp formation

The shape and diversity of the mammalian molar teeth is suggested to be regulated by the primary and secondary enamel knots, which are putative epithelial signaling centers of the tooth. In search of novel molecules involved in tooth morphogenesis, we analyzed mRNA expression of Slit1, -2 and -3, earlier characterized as secreted signals needed for axonal pathfinding and their two receptors Robo1 and -2 (Roundabout1 and -2) in the developing mouse first molar. In situ hybridization analysis showed that Slit1 mRNAs were expressed in the primary enamel knot of the bud and cap stage tooth germ and later the expression continued in the secondary enamel knots of the late cap and bell stage tooth. In contrast, expression of Slit2 and -3 as well Robo1, and -2 was largely restricted to mesenchymal tissue components of the tooth until the bell stage. At the late bud stage, however, Robo1 transcripts were evident in the primary enamel knot, and at the cap stage a pronounced expression was noted in the middle of the tooth germ covering the primary enamel knot and dental papilla mesenchyme. During the bell stage, Robo1 and Slit2 expression became restricted to the dental epithelia, while Slit3 continued in the dental mesenchyme. Prior to birth, Robo1 and -2 were co-localized in the predontoblasts. These results indicate that Slits and Robos display distinct, developmentally regulated expression patterns during tooth morphogenesis. In addition, our results show that Slit1 is the second known gene specifically located in the primary and secondary enamel knots.