Fig. 3. Chemorepulsion of olfactory tract axons caused by Slit1 or Slit3 is inhibited in the presence of soluble Robo1/Fc. E14.5-15 olfactory bulb explants were co-cultured with aggregates of cells expressing either Slit1 (A,B) or Slit3 (C,D) in collagen gels in the absence (A,C) or presence (B,D) of Robo1/Fc conditioned medium. The cultures were incubated at 37°C for 24-48 hours and examined by phase contrast microscopy for chemorepulsion of olfactory tract axons.