Fig. 7. Summary of the expression patterns of guidance molecules involved in the zone-to-zone targeting of vomeronasal axons. The upper part depicts schematically the zone-to-zone projection in the vomeronasal system. Axons from the apical zone of the VNO terminate in the anterior AOB, whereas basal axons terminate in the posterior AOB. Note that apical and basal axons enter the AOB at its medial margin. The lower part summarises the expression patterns of three families of axon guidance molecules described so far in the vomeronasal projection. The expression patterns provided in this study suggest that Robos and Slits act predominantly on basal axons. Robo2 appears to be the principal guidance receptor for basal axons, whereas Robo1 is uniformly expressed and later downregulated, leaving only faint expression on both apical and basal axons (broken line). This suggests a model in which basal axons expressing Robos more strongly than apical axons navigate to the posterior AOB due to repulsive interactions, with Slit proteins secreted from the anterior AOB. Ephrin A proteins and neuropilin 2 instead operate mainly on the apical subpopulation of VNO axons. Apical axons with higher ephrin A expression levels than basal axons project to the anterior AOB, with express higher concentrations of Epha proteins than the posterior AOB. In this scheme, Epha/ephrin A interactions would guide VNO axons on the basis of an attractive guidance mechanism, consistent with data from the stripe assay (Knöll et al., 2001). In turn, apical axons expressing neuropilin 2 are repelled by class 3 semaphorins, which are uniformly localised in the AOB. Neuropilin 2 in the anterior AOB might sequester semaphorins, thus rendering apical axons sensitive to only the semaphorins in the posterior AOB (Cloutier et al., 2000; Walz et al., 2002).