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Fig. 1 Expression patterns of cxcr4b and cxcl12a during development of the olfactory placode in zebrafish. Whole-mount in situ hybridization analysis of cxcr4b (left panels) and cxcl12a (right panels) expression in the anterior head during the first 2 days of embryonic development. (A-D) Dorsal views with anterior to the bottom. (E-H) Frontodorsal views with anteroventral to the bottom. (I-L) Frontal views with ventral to the bottom. (M-R) Dorsal views with anterior to the top. (S,T) Ventral views with anterior to the top. From 12 to 16 hpf, cxcr4b is expressed bilaterally along the anterior-lateral edge of developing neural tube (A,C,E), whereas cxcl12a is expressed medially adjacent to the cxcr4b-expressing bilateral stripes (D,F). Progressive convergence of cxcr4b-expressing domains occurs between 16 and 20 hpf (E,G,I) to form the olfactory placodes. cxcr4b expression in the olfactory placode persists during initial phase of axon pathfinding (K,M,O,Q,S), although cells near the olfactory pit (arrowheads in O,Q,S) are devoid of cxcr4b expression. At the same period, cxcl12a expression is detected at the olfactory placode-telencephalon border (arrowheads in L,P,R), as well as in the anterior tip of telencephalon. Scale bar: 100 µm. a, anterior; d, dorsal; op, olfactory placode; p, posterior; tel, telencephalon; v, ventral.