Fig. 2

Dynamic changes in hox expression in the embryonic hindbrain and neural crest. Whole-mount in situ hybridizations and transverse sections for (A?E) hoxa2 and (F?J) hoxb3. A, B, and F?H show double in situs with krox20 (red) as a marker of r3 and r5; all whole mounts are shown in dorsal view with anterior to the top. (A) At 12 h (5 somites) hoxa2 is expressed in r2 and r3 with diffuse limits (e.g., as marked by arrow at presumptive r1/r2 boundary). (B) At 14 h (10 somites) the anterior expression limit has sharpened (arrow) and low level expression is apparent in r4 and r5. (C) At 17 h (16 somites) expression is also apparent in the neural crest stream, migrating out to the second pharyngeal arch (pa2). (D) Transverse section through the r4 level at 17 h reveals that hoxa2 expression is limited to the ventral half of the keel; this sample is counterstained with eosin. (E) Transverse section through the r5 level at the 20 h shows reduced hoxa2 expression in the floor plate (fp). (F) At 12 h hoxb3 is expressed in r5 and r6 and more posteriorly; the anterior limit is diffuse at this stage. (G) At 14 h hoxb3 expression in r5 and r6 has upregulated and the anterior limit is beginning to sharpen. (H) At 17 h expression is apparent in the neural crest migrating out into the third pharyngeal arch. (I, J) Transverse sections through the r5 level at 17 and 20 h show reduced expression levels in the floor plate.