Olfactory bulb and olfactory epithelium cocultures. OB explants were labeled with DiI and placed near an unlabeled explant of olfactory epithelium (OE,dotted line). Labeled axons grow almost exclusively on the distal side of the explant opposite to the olfactory epithelium, indicating that the OE releases a diffusible repulsive factor. Scale bar, 200 μm.