Fig. 4

Slit and Robo remain in position to regulate floor plate crossing on E14.5. Coronal sections through the caudal oculomotor nucleus on E14.5 were labeled with Robo1 or 2 antibodies, or hybridized to Robo or Slit mRNA probes, followed by antibody labeling for Islet1/2 of the same section (for Robo antibodies), or adjacent sections (for in situ hybridization). A, B Within the nucleus, Robo1 and Robo2 was localized to Islet1/2+ cells indicating Robo expression by oculomotor neurons. C-D. In the midline of the caudal midbrain, Robo1 antibody labeling could be seen on some migrating neurons, while Robo2 labeling was less intense and variable. E, F In situ hybridization for Robo1 and 2 mRNAs showed labeling that overlapped with the nuclei, and also bridged across the midline in the area of migrating neurons. G-I. Slit mRNA expression by in situ hybridization, compared to Islet antibody labeling in adjacent sections. Slit1 and 2 continued to be expressed by floor plate cells on E14.5, while there was very little Slit3. Slit2 and Slit3 transcript was localized to the oculomotor nuclei as well as overlapping with the motor neurons migrating across the midline (H, I). Scale bars: D’, 50 μm, applies to Robo antibody labels; I’, 100 μm, applies to in situs