Fig. 2

The Bmp signalling pathway is active throughout the developing lateral line system in sterlet. Immunostaining in sterlet. Black arrowheads indicate examples of developing neuromasts; white arrowheads indicate examples of developing ampullary organs; yellow arrowheads indicate lateral line nerves. (A–P) Immunoreactivity on the head for phospho-SMAD1/5/9 (P-Smad) as a proxy for Bmp signalling activity. At stage 30 (A,B), weak immunoreactivity is seen in the region of the anteroventral lateral line primordium and by stage 34 (C,D) in lateral line primordia, with a ring pattern around developing neuromast primordia. At stages 36–40 (E–J), immunoreactivity is weak around developing neuromasts and prominent in lateral line nerves (yellow arrowheads). At stage 40 (I,J), diffuse immunoreactivity is also seen in regions flanking the nerves where ampullary organ primordia are forming. Non-lateral line immunoreactivity is present around the mouth and nares, in barbel primordia, gill filaments, and a patch that is most likely the developing muscle m. protractor hyomandibulae. Between stages 42 and 45 (K–P), immunoreactivity disappears in lateral line nerves and is increasingly detected at the periphery of ampullary organs and neuromasts (strongly in supraorbital and infraorbital neuromast lines). (Q–T) Immunostaining on the trunk (boxes on low-power insets indicate the location of the trunk regions shown). At stage 39 (Q,R), pSMAD1/5/9 immunoreactivity is seen in primI and a diffuse trail behind it, and in primII (Q). For comparison, Sox2 is expressed weakly in primI and strongly in primI-deposited neuromasts and interneuromast cells, plus primII (R). At stage 45 (S,T), pSMAD1/5/9 immunoreactivity is seen in primD and primII plus a weak trail behind it, with greater intensity at the periphery of primII-deposited neuromasts (S). For comparison, Sox2 expression is strong in primII, primD and all neuromasts; arrowheads indicate examples of primII-deposited neuromasts (T). Abbreviations: b, barbel; e, eye; f, fin; g, gill filaments; m, mouth; n, naris; ph, m. protractor hyomandibulae; prim, migrating lateral line primordium (primI, primary; primII, secondary; primD, dorsal); S, stage. Scale bar: 250 μm.