Fig. 5

Motor neuron cell bodies migrate prematurely into and across the floor plate in Slit and Robo mutants. A-D. On E10.5, DiI and DiO were applied to the left and right (respectively) peripheral oculomotor nerves to back-label the oculomotor nucleus, as well as the leading process and somata directly connected to the peripheral nerve. Open book preparation of the mouse midbrain, with anterior up. A, C On E10.5 in wild type littermate controls, oculomotor somata and axons remained ipsilateral to the nucleus. Leading projections are rarely seen projecting from the oculomotor nuclei (asterisk in A, C). B, D Slit1 ^-/-,-2^-/- mutants or Robo1 ^-/-,2^-/- mutants had numerous leading processes projecting into and across the floor plate. Cell bodies were found in the ventral region of the contralateral nucleus (yellow, arrows in B, D). Bulges in leading processes appeared to be cell bodies migrating across the floor plate (arrows in B’,D’). Leading processes looped into and across the floor plate in Slit mutants (B”). Robo mutants displayed more fasciculation by leading processes traversing through the floor plate (yellow color in box (D”). (wild type, n = 6; Slit1 ^-/-, 2^-/-, n = 8; Robo1 ^-/-,2^-/-, n = 9) Scale bars, 100 μm