Mitral cell projection and possible guidance models. A, B, Schematic drawings of the rostral view (A) and lateral view (B) of E14.5 mouse telencephalons. The dorsal aspect is to the top in both views. The medial and rostral aspects are to the left in A andB, respectively. Mitral cells (Mt) in the olfactory bulb (OB) project axons into a lateral direction, avoiding the septum, and form the LOT on the lateral surface of the telencephalon. In the lateral view (B), the septum sits behind the telencephalon wall. C, Four possible models of the mitral cell guidance. Green circles symbolize Slit proteins, andred arrowheads symbolize LOT cues. In theSlit-Inactivation model, the LOT cue downregulates repulsive activity of Slit by processes, such as degradation, trapping, and neutralization, and allows the axons to grow along lot cells. In the Slit-Tolerance model, the LOT cue acts on mitral cell axons and makes them refractory to Slit, so that the axons can elongate in the presence of Slit proteins. In theSlit-Determinant model, Slit controls the distribution of the LOT cue. In the Parallel Model, Slit and LOT cue independently act on mitral cell axons. Green arrows andred arrows show directions of actions of Slit and the LOT cue, respectively.