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Fig. 2 - Supplemental 1 Sox2 expression shows the time-course of sterlet lateral line organ development. Sterlet embryos and yolk-sac larvae immunostained at selected stages for Sox2, which labels lateral line primordia/sensory ridges and supporting cells in lateral line organs. Black arrowheads indicate examples of developing neuromasts; white arrowheads indicate examples of developing ampullary organs. Non-lateral line expression is seen around the mouth and nares, in the eye, gill filaments and taste buds on the barbels. Scattered individual Sox2-positive skin cells are most likely Merkel cells. (A?D) At stage 32 (A,B) and stage 35 (C,D), Sox2 expression is seen in the otic/anterodorsal and anteroventral lateral line primordia, with a ring pattern around developing neuromast primordia. (E?H) At stage 37 (E,F) and stage 39 (G,H), Sox2 is expressed at the edges of neuromasts and in interneuromast cells. (I?L) At stage 42 (I,J) and stage 45 (K,L), Sox2 is expressed at the periphery of neuromasts and ampullary organs, with stronger expression in neuromasts. The images shown in K,L were previously published in Mina?ík et al., 2024a. (M?P) Sox2 expression on the trunk at stage 38 (M), stage 39 (N), stage 42 (O) and stage 45 (P) shows the progression of the migrating lateral line primordia (primI, primII and primD). Low-power insets show the location of these trunk regions. Sox2 is expressed only weakly in primI (e.g., M,N), but strongly in primI-deposited neuromasts and interneuromast cells (M?P). PrimII is located a little dorsal to the primI-deposited line (M?P), and primII-deposited neuromasts are slightly offset dorsally from the line of primI-deposited neuromasts (O,P). Sox2 expression only reveals neuromasts deposited by primII and primD, not interneuromast cells (O,P; compare with the primI-deposited line in M-P). Abbreviations: b, barbel; e, eye; g, gill filaments; m, mouth; n, naris; prim, migrating lateral line primordium (primI, primary; primII, secondary; primD, dorsal); S, stage. Scale bar: 250 ?m.