Fig. 2. ast embryos show OSN axon pathfinding defects. OSN axons in wild-type and homozygous ast embryos were labeled by external application of DiI at 3.5 days postfertilization (dpf) and viewed by confocal microscopy. (A) Diagrams representing frontal and dorsal views of the 3.5-dpf zebrafish head. ob, olfactory bulb; oe, olfactory epithelium; tel, telencephalon; di, diencephalon; a, anterior; p, posterior; d, dorsal; v, ventral. Broken rectangles indicate the region observed in B-G. (B-G) One wild-type (B,C) and two ast (D,E; F,G) examples are shown in frontal (B,D,F) and dorsal (C,E,G) views, as composite images generated from the series of optical sections. (D-G) In ast mutants, many axons reach the OB, but some fibers misroute posteriorly and penetrate into the diencephalon without reaching the OB (arrowheads and arrows). The posteriorly projecting fibers occasionally cross the midline (arrows in F,G). Scale bar: 100 µm.