<i>fgf5</i> from mature neurons terminates the phase of neuroblast specification.

<p>The icon at the top of the figure indicates that analysis focuses on initial formation of neuroblasts. Experimental manipulations in groups IV and V are briefly summarized at the tops of the corresponding data panels. (A–H) Expression of <i>neurog1</i> in control embryos (A–C), a <i>hs:fgf8</i> embryo (D), <i>fgf5</i> morphants (E–G), and a <i>hs:</i>fgf8 embryo injected with <i>fgf5</i>-MO (H) at the indicated stages. Transgenic embryos (D, H) were heat shocked for 30 minutes at 39°C beginning at 24 hpf. Vertical lines in (A, C–E, G, H) indicate the plane of transverse sections in (B, F, and insets in C, D, G and H). (I–L) Expression of <i>isl2b:Gfp</i> at 22 hpf (I, J) and <i>neurog1</i> at 30 hpf (K, L) in a specimen in which mature (<i>fgf5</i>-expressing) neurons were laser-ablated. The same specimen is shown in all panels. Mature SAG neurons expressing <i>isl2b:Gfp</i> were serially ablated on the left side at 22 hpf (J) and 25 hpf (not shown), and the embryo was fixed and sectioned at 30 hpf to examine <i>neurog1</i> expression (L). Images of the unablated right side (I, K) were inverted to facilitate comparison. The surface of the otic vesicle is outlined in all panels. Arrows in sections indicate the edges of <i>neurog1</i> domain in the otic floor. Note that the amount and duration of delamination of <i>neurog1⁺</i> neuroblasts is strongly enhanced by knockdown of <i>fgf5</i> (F, G) or ablation of mature neurons (L). Activation of <i>hs:fgf8</i> reverses the effects of <i>fgf5-</i>MO (H). Scale bar, 25 µm. Transverse sections are shown with lateral to the left and dorsal up. Wholemount images show dorsolateral views with anterior to the left.