Fig. 5. Positioning of olivocochlear (OC) precursors is affected in reeler mice. (A) In control embryos at E12.5, Gata3 is expressed in the reticular formation at levels r4-6 and in a dense column of cells positioned rostrally to the facial nucleus and close to the pial surface (circled). (B) In reeler mutants at E12.5, the Gata3 expression pattern was similar to controls except for the dense column of cells (black arrowhead) which was localized on the ventricular side, as visualized in vibratome sections of the same embryos (C-F). Results corresponding to two different levels (r4, C,D; and r5, E,F) are shown. (C,E) A compact group of cells is observed in control embryos at the edge of the reticular formation (arrow). (D,F) In reeler embryos this compact group was absent (circles), but cells were observed in a dorsal position (arrowheads). At the r5 level, Gata3 labeling is observed in a small group of cells at the pial surface (unfilled arrowhead, F; compare with B). (G-L) Retrograde labeling from the VIIIth ganglion (right side) and photoconversion of the DiI signal (brown precipitate) were performed and subsequently followed by Tbx20 ISH (blue precipitate). Photoconversion provided a strong labeling at the VII/VIIIth nerve bundle on two consecutive sections from control embryos (G,H) and reeler mutants (I,K). Using this landmark, we were able to superimpose the ISH (I,K) and fluorescent pictures (J,L). (G,H) At E12.5, Tbx20 is expressed by facial branchial motoneurons (VII), vestibular (VEN) and OC efferents. OC neurons are localized in a ventral position as a compact group (circled). (I,K) In reeler mutants, Tbx20 is strongly expressed in facial motoneurons (red circle) and in a compact group of cells in a dorsomedial position (black circle). No labeling was observed at the ventral side, except for few scattered cells (I, unfilled arrowhead). Comparison with DiI labeling (compare I with J and K with L) indicated that ectopic Tbx20-positive cells accumulate at the position where they should migrate ventrally (dashed circles).