miR-137 expression in B. lanceolatum. 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 16-h neurula. Expression was found in the most anterior part of the neural tube (arrow). B: Lateral view of 24 hr larva. Transcripts are visible in a cluster of cells (arrow) of the posterior cerebral vesicle (cv). C: Side view of 48-h larva. D: Higher magnification of the specimen in C with detail of labeled cells of the cerebral vesicle just above the preoral pit (po). E, F: Consecutive cross sections trough levels e and f in D showing transcripts in a group of ventrolateral cells that forms the infundibular organ. G: Side view of 72 hr larva. Expression was found in the neural tube and in cells located around the mouth (arrow) and in the first gill slit (arrowhead). H: Anterior enlargement of specimen in G. Two groups of cells in the cerebral vesicle express miR-137. The most anterior is found in the middle (arrow), the more posterior (arrowhead) correspond to the cells of the infundibular organ and to the tegmental neuropile. I: Cross section through level indicated by an arrow in H. J: Cross section through level indicated by an arrowhead in H, K, L: Enlargements of different parts of the neural tube of the specimen in G showing miR-137 expression in segmentally arranged nerve cells. M, N: cross sections through levels m and n in K and L, respectively. miR-137 is expressed in ventrolateral nerve cells.