MAPK activation during early development in <i>Platynereis dumerilii</i>.

<p>Antibody staining against di-phosphorylated, activated MAPK/ERK (dpERK) in red or white, DNA-staining with Hoechst appears in blue, actin was marked by FITC-coupled Phalloidin (in green). <b>A, B</b> Embryos at the 38- and the 46-cell stage exhibit no dpERK staining in the mesentoblast (4d) and its descendants ML and MR. <b>C.</b> Initial dpERK staining was detected within the nephroblasts (n) in the animal hemisphere of a 7.5 hpf early blastula. <b>D.</b> MAPK activation is still visible during further head kidney development in the mid-blastula (10.5 hpf). <b>E–G″.</b> dpERK staining is visible during gastrulation in nuclei of small cells (arrowheads) and macromeres (M) in the region of the blastopore. <b>F–F′.</b> Micromeres with MAPK activity show an accumulation of filamentous actin at 15 hpf. <b>G′–G″.</b> MAPK positive cells in the region of the blastopore at 15 hpf. <b>G″′.</b> dpERK positive macromere nuclei in the same embryo as in G but different focal plane. <b>H, H′. </b><i>Pdu-twist in situ</i> hybridization in combination with dpERK staining in a 15 hpf embryo. Activated MAPK and <i>Pdu-twist</i> positive cells are in close proximity at the region of the blastopore (asterisk). Arrows point towards two dpERK-positive nuclei that are in the same focal plane as the nuclei (arrowheads) of two <i>Pdu-twist</i> (black) expressing cells. Scale bars are 50 µm and 10 µm in whole embryo views and close-ups, respectively.