miR-7 expression in B. lanceolatum and B. floridae. Anterior to the left and dorsal up in all whole mounts. Cross-sections are viewed from the anterior end of animal. Scale bars are 50 μm for whole mounts and 25 μm for cross sections. A: Side view of 22-h neurula. Expression is restricted to the cerebral vesicle (cv) and anterior pharyngeal endoderm (arrow). B: Side view of a first gill slit larva. C: Higher magnification of the anterior end of larva in B. Expression is observed in cerebral vesicle and preoral pit (arrow). D, E: Cross sections through levels d and e in C show several ventrolateral labeled cells of the anterior cerebral vesicle (arrowheads). F, G: Cross sections through levels f and g in C show miR-7 expression in cells of preoral pit (arrows). H: Side view of 20-h neurula showing expression in the cerebral vesicle. I: Side view of a three gill slit larva. J: Anterior enlargement of the anterior end of the specimen in I. miR-7 is expressed in cells of the frontal eye complex and in further regions of cerebral vesicle (cv). A strong signal was also found in the preoral pit (arrow). Arrowhead in C and J points to pigment spot associated with frontal eye.