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. 2012 Dec 13;8(12):e1003121. doi: 10.1371/journal.pgen.1003121

Figure 6. BMP signaling is required for nodal expression in the left-right organizer.

Figure 6

A, nodal expression in the endomesoderm requires BMP signaling. Both treatments with BMP4 protein and injection of bmp2/4 or alk3/6 morpholinos abolish nodal expression in the endoderm at gastrula stage. B, nodal expression at pluteus stage in the bmp2/4 or alk3/6 morphants. Following inhibition of bmp2/4 or alk3/6 by morpholino injection into the egg, nodal expression occurs randomly on the left or right sides. The arrowheads point to the expansion of nodal to the dorsal part of the ciliary band. C,D, Alk3/6 function is required in the endomesoderm for nodal expression in the left-right organizer. Following injection of alk3/6 morpholino into the four vegetal blastomeres, nodal expression is not inititated in the endomesoderm and is randomized at pluteus stage. E–G, Effects of inhibiting BMP signaling on either the left or the right side on nodal expression. Embryos were injected with the bmp2/4 morpholino together with FLDX (green) into one blastomere at the two-cell stage, the position of the lineage tracer was revealed after in situ hybridization (red in pictures). F Injection of the bmp2/4 morpholino targeting the left side more strongly affected nodal expression in the organizer at gastrula stage than injections targeted to the right side. Injection of the bmp2/4 morpholino on the left but not on the right side randomizes nodal expression at pluteus stage. H, BMP signaling in the endomesoderm is biased towards the left side at late gastrula stage. Fluorescence microscope images (3 middle images) and confocal microscope image (lower picture) of embryos at the late gastrula stage immunostained with an anti phospho Smad1/5/8 reveal a domain of strong BMP signaling near the tip of the archenteron. This staining was asymmetric in about two third of the embryos, with stronger staining in the dorsal-left sector of the archenteron opposite to the nodal expressing territory. AV, animal view; DV, dorsal view; plut, pluteus larva; R, right side; L, left side.