Fig. 2. The superficial and olivary migrations in vitro. (A-C) BrdU immunostained bulbar explants viewed from the pial surface. The explants incubated for 30 minutes with BrdU at E11.5 were analyzed after increasing time intervals in culture. The BrdU-labeled cells progress from their origin in the rhombic lip towards the floor plate (arrowheads) as schematized in (E). (D) The neurons of the superficial migration express TAG-1 in vitro. (F-G) E12.5 explants incubated for 30 minutes with BrdU maintained in culture for 3 days. TAG-1 transcripts are detected in blue and BrdU immunoreactivity is shown in brown. (F) The TAG-1-labeled cells form thin rows dorsally and arrange in a more diffuse pattern around the ventral midline. (G) Higher magnification of the area framed in F. The arrows point to TAG-1-labeled cells, which have incorporated BrdU at the beginning of the culture period. (H-M) Whole mounts (H,K) and transverse vibratome sections (I,J,L,M) of bulbar explants dissected at E11.5 (H-J) or E12.5 (K-M) and cultured for 3 days in vitro. Except in (J,M), where axons are labeled by immunostaining for L1, the brown label marks the olivary neurons detected by in situ hybridization of Brn3.2 transcripts. In E12.5 explants (K,L), as in vivo, the inferior olives on both sides are separated by the floor plate whereas they fuse ventrally in E11.5 explants (H,I). This behavior fits in with the presence of a ventral gap at the pial end of the floor plate in E11.5 (J) but not in E12.5 (M) explants. Arrowheads, floor plate.