Fig. 3. Ectopically positioned motoneurons at r5 level in reeler mutants. (A,B) Vibratome sections after wholemount in situ hybridization with an islet 1 riboprobe at E13.5. Level of the vibratome section is indicated on the whole-mount-stained hindbrain to the right. (A) At r5 level, a compact group of cells is clearly detectable at the pial surface in control embryos (arrow, FVM). (B) In reeler mutants, a strong islet 1 labeling is observed at the ventricular side of the hindbrain (arrow) and a more diffuse group of islet 1-positive cells is localized in a more medial position (arrowhead). Abducens nucleus (ab) is labeled near the ventricular zone. Note the absence of islet 1-positive cells at the pial surface in reeler mutants (circle). (C-E) Vibratome sections after whole-mount in situ hybridization with a Ret riboprobe on control and reeler embryos at E12.5 indicates that ectopic r5 MNs retained FVM markers. (C) Ret is expressed at the end of FVM migration in controls (arrowhead) and in the abducens nuclei near the floor plate (arrow). (D,E) In reeler hindbrains, ectopically positioned Ret-positive cells are observed at levels r4/r5 near the ventricular zone and at intermediate levels (dashed circles), but are absent at the pial surface (circle). v, ventricular surface; p, pial surface. (F,G) Cryostat sections at r5 level of E13.5 embryos labeled with an islet 1 riboprobe. (G) Islet 1 labeling is comparable to results obtained from vibratome sections in reeler mutants: a lack of pial surface labeling (stipple circle), and ectopic groups of cells within the neural tube and close to the ventricular side.