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Fig. 2
Expression pattern of prdm1 in sea lamprey embryos. A–N: Whole-mount in situ hybridization with the full-length lamprey prdm1 probe. Orientation: anterior is at the top in dorsal views, or in the top right-hand corner in side views. L2,O: A 12-micron sections through the stained embryos. A: Dorsal view of an embryonic day (E) 4.0 embryo, showing prdm1 transcripts expressed uniformly throughout the ectoderm and the neural plate border, but absent from the neural plate. B: Side view of the same embryo. C: Dorsal view of the embryo in (A) with its ectoderm removed, demonstrating that prdm1 is also expressed in the paraxial mesoderm at this stage. D,E: Starting at E4.5 (D), high levels of prdm1 are expressed in the neural plate border (white arrows) and in the preplacodal domain (black arrowhead), and lower levels in the ectoderm. E: A similar pattern of prdm1 expression is seen 6 hours later at E4.75. F,G: Dorsal (F) and side (G) view of the same E5.0 embryo, showing strong expression in the premigratory neural crest (white arrow). H: Embryo at E5.5. prdm1 is turned off in the neural crest as it starts to migrate. I: Lateral view of an E6.5 embryo. prdm1 transcripts are no longer seen in the anterior dorsal neural tube. J,K: Dorsal (J) and side (K) view of an E7.5 embryo. prdm1 is expressed in the trunk dorsal neural tube, in the branchial basket and in somites. L: E8.5 embryo showing prdm1 expression in the branchial arches. M: Side view of the head of an E10.5 embryo, showing prdm1 expression in the lips and lens (lp) and otic placodes (op). N: Side view of E20 embryo, showing prdm1 expression in the cartilage of the branchial arches. (L2) Transverse section through the head of the E8.5 embryo in (L) showing prdm1 expression in the endoderm and the lateral region of the neural crest (black arrowhead, compare with panel O). O: Section through an E10.5 embryo stained for Col2a2, which at that stage is expressed in the neural crest components of the branchial arches. Black arrow marks the lateral component of the neural crest that corresponds to the prdm1 expressing cells in (L2). m, paraxial mesoderm; pnc, premigratory neural crest; op, otic placode; lp, lens placode. White arrows indicate neural plate border, black arrowheads, preplacodal domain.