Dab2 immunoreactivity in the neural tube. In whole mount preparations (A-D), tissues surrounding the neural tube, including the otic vesicle (asterisks in C) had been removed by microdissection. At E8.5, the neural tube (A: dorsal view; A': lateral view) already shows Dab2 expression in rhombomeres 5 and 6 (arrowheads). At E9.0 (B: lateral view) and E9.5 (C: dorsal view; C': lateral view), expression within rhombomeres 5 and 6 (r5 and r6, arrows in C') remains strong, and spreads along the floor plate of the neural tube (arrowheads in B). The lack of immunoreactivity in the pre-adsorption control (D) confirms the specificity of the Dab2 immunoreaction. An oblique section (E) and longitudinal sections (F, G, G") through E9.5 embryos demonstrate strong Dab2 expression in the whole span of r6 and weak immunoreactivity in the ventral part of r5 (G" shows the high magnification of the immunoreactive r5 and r6 in G). The pre-adsorption control section (G') adjacent to the section shown in G does not exhibit any immunoreactivity. The ventral expression of Dab2 in the floor plate of the neural tube extends rostrocaudally from the mesencephalic flexure (arrow, F) to the caudal spinal cord (arrow, G). At E10.5, Dab2 immunoreactivity becomes weak in r6 (arrow, H) (Hb; hindbrain; 4V: fourth ventricle) and absent in r5. Dab2 immunoreactivity along the floor plate of the spinal cord (arrowheads, I to M) is also observed in transverse sections through the rostral (I, J, K, L, M) and caudal spinal cord (I', J', K') at different developmental stage. Also note the weak Dab2 staining in the notochord (arrow, I'). Insets in I to M represent the high magnification of the floor plate shown in corresponding figures. The schematic diagram (N) summarizes changes in Dab2 expression along the floor plate of the neural tube from E9.5 to E12.5. The dotted line indicates the axial level of the mesencephalic flexure and the shading represents Dab2 immunoreactivity observed in the floor plate of the neural tube. Arrowheads in N indicate regions of the neural tube from which sections shown in I, I', K, K', L, M were taken. Note that the onset and down-regulation of Dab2 expression follow a rostral-to-caudal sequence. Scale bar: 100 μm for I; 50 μm for G' and I', 25 μm for insets of I-L, and 200 μm for other panels.