Fig. 2. Mesodermal origin of the Lbx1-expressing pre-otic mesenchyme. (A) DiI labellings at HH8-8⁺, before cranial neural crest cell migration. The cranial paraxial mesoderm was labelled on the right side at the axial levels indicated. To record the position of the injection, a further injection was made in the neural plate. (B-D) Lateral views on the trigeminal region of chick heads, analysed for Lbx1 expression (blue) 36-48 hours after DiI injection (red). (B) Mesoderm labelled at the level of the posterior midbrain (arrowhead) resides anterior to the Lbx1 domain (arrow). (C) Mesoderm labelled at the level of rhombomeres 3/4 is seen posterior to the Lbx1 domain (arrow), migrating into the hyoid arch, (hy, arrowheads). (D) The fluorescent signal coincides with Lbx1 expression when mesoderm was labelled at the level of rhombomere 2 (arrowhead and arrow). (E) Vibratome cross-section through the embryo shown in D, confirming co-localisation of Lbx1 and DiI signals. hy, hyoid arch; np/nt, neural plate/neural tube; ma, mandibular arch; mes, mesoderm; ov, otic vesicle; r, rhombomere. Scale bar in B: 200 µm in B-D; 100 µm in E.