Reciprocal Signaling between the Ectoderm and a Mesendodermal Left-Right Organizer Directs Left-Right Determination in the Sea Urchin Embryo
Figure 5
FGF/MAP kinase signaling is required for nodal expression in the left-right organizer.
A, The early asymmetrical expression of nodal in the endoderm is lost following inhibition of FGFR/MAPK signaling. In embryos treated with the FGF receptor inhibitor SU5402 or the MEK inhibitor U0126 from fertilization onwards, nodal expression is not initiated in the endomesoderm. DAPT treated embryos show bilateral expression of nodal in the endomesoderm but embryos treated with DAPT followed by U0126 treatment at mesenchyme blastula stage do not express nodal in the endomesoderm. B, Inhibition of ERK or FGF signaling randomizes nodal and univin expression in the ectoderm as well as pitx2 and sox9 expression in the coelomic pouch precursors at pluteus stage. C, Inhibition of MAPK signaling randomizes the positioning of the rudiment. D, Time course of U0126 treatments. Embryos were treated with U0126 starting at the indicated stages and the sidedness of nodal expression was scored at pluteus stage. The efficiency of U0126 treatment on left-right asymmetry drops after 22 hpf, which coincides with the onset of asymmetrical expression of nodal in the endoderm. The percentages and total number of embryos in each experiment are indicated at the bottom of pictures. E, The drop in the ability of U0126 to block nodal expression coincides with the onset of asymmetrical expression of nodal in the endoderm. AV, animal views; DV, dorsal views; L, left; R, right.