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Fig. 2 A-Y: Expression of pet1 (A-L) and tph2 (M-Y) revealed by in situ hybridization on whole-mount embryos/larvae (A-J and M-V) or brains (K-L and X-Y). Insets show higher magnifications of the boxed areas. The earliest pet1-positive cells are likely adrenal gland (Aa′ and Bb″) and blood (Aa″) precursors. B: The first pet1-expressing cells in the rhombencephalon are detected at 25 hours postfertilization (hpf). O: Expression of tph2 in this location is not detected until a few hours later. Afterward, pet1 and tph2 expression highlights two parallel stripes of precursors lining the hindbrain floor plate. E,G,I,S,U: These are organized in an anterior (arrowhead) and posterior (arrow) cluster separated by a gap (white arrow). H: An additional column of pet1-positive cells runs along this domain in a slightly more dorsal location (stars in h′). Starting from 60 hpf tph2 expression can also be seen in an additional cluster of cells corresponding to the pretectal complex (Kaslin and Panula,[2001]). In addition, tph2 was detected in the epiphysis at all stages examined. e, epiphysis; ptc, pretectal and thalamic complex.