Fig. 2. Rhombomere 2 and its derivatives are absent in Gbx2 hypomorphic mutants. (A-H) Whole-mount ISH using the probe and developmental stage indicated. (I,J) Whole-mount immunohistochemistry using an anti-neurofilament antibody (2H3). A,B,G-J are lateral views; C-F are dorsal views. Krox20 expression occurs normally in r3 and r5 of control (A) and mutant (B) embryos, but the unstained r2 region between the Krox20 and Fgf8 domains is absent in mutant embryos. Cyp26c1 is overstained to demonstrate that r2-specific expression is absent in mutants (D), but evident in r2 and r4 of control embryos (C). Lateral Cyp26c1 staining is due to mesenchymal expression. The anterior limit of Hoxa2 expression in mutants ends at r3 (F), demonstrating that the normal Hoxa2 r2 domain (E) is absent. Krox20-specific expression in cranial nerve (V) boundary cap cells is absent in mutants (H) compared with controls (G). (I) Visualization of cranial nerve and ganglia patterning in a control embryo. (J) In mutants, neural crest-derived sensory neurons connecting the ganglion of the cranial nerve (V) to the hindbrain are absent, and the mandibular branch (N5a) extending into the first branchial arch fails to develop. III, oculomoter; IV, trochlear; V, trigeminal; VII, facial; VIII, vestibulocochlear; d, distal; N5a, mandibular branch of cranial nerve (V); N5m, maxillary branch of cranial nerve (V); N5o, ophthalmic branch of cranial nerve (V); ov, otic vesicle; p, proximal.