Fig. 4. Influence of the floor plate on the olivary and superficial migrations in vitro. All panels except (E) show bulbar explants dissected at E11.5, cultured for 3 or 4 days, treated for the detection of Brn3.2 transcripts, and viewed from their pial surface. (E) Similar explant dissected at E12.5, cultured for 3 days, and treated for the detection of TAG-1 transcripts. The drawings schematize additional manipulations. The caudal part of the endogenous floor plate was ablated in most cases and both sides of the explants have fused together. (A) Ablation of the floor plate marked in red (F-spondin transcripts) did not prevent the migration of olivary neurons (arrow) towards the midline. (B-D) The olivary neurons accumulate along floor plate transplants dissected from the medulla oblongata (B,D) or the spinal cord (C) of E11.5 embryos. The migration of olivary (D) and superficial (E) neurons is deflected towards a floor plate apposed to the sectioned caudal hindbrain. The inset in E illustrates a higher magnification of the ectopic TAG-1 migration. (F,G) Caudal rhombic lip fragments were transplanted posteriorly (F) or anteriorly (G). They produced Brn3.2+ neurons, which migrated in the ectopic environment (F, spinal cord and G, pons) and accumulated ipsilaterally against the endogeous floor plate. (H,I) Grafts containing floor plates (delineated with dashed lines) were placed at different distances from the rhombic lip (RL). The endogenous midline (asterisk) fused with the additional bulbar tissue. The midline was crossed by olivary neurons but they did not migrate beyond an ectopic floor plate placed closer (H) or farther (I) than the endogenous one. Arrowheads point to the grafted floor plates and arrows to the ectopic ION. Bars, 500 µm (A-E); 300 µm (F-I).