Fig. 1. Effect of two MEK inhibitors on egg morphology. (A) Light-microscopy images of changes in the general morphology of eggs from two sea urchin species, P. lividus (top) and L. pictus (bottom), induced after a 3-hour treatment with U0126 or PD98059. Untreated eggs showed a clear nucleus and no apparent fertilization membrane (upper left inset), whereas figures of constrictions and blisters, NEB and a thin fertilization membrane for a few eggs (lower left and right insets) were induced in treated eggs of both species. Bars, 50 µm. (B) Confocal microscopy images of microtubule polymerization and chromatin condensation induced in unfertilized eggs of P. lividus after treatment with 1 µM U0126. Chromatin was labeled with Hoechst 123 (blue) and microtubules were labeled with an anti-tubulin antibody (red). Eggs were observed (a) before treatment with U0126 or (b) 30 minutes, (c) 1 hour, (d) 2 hours, (e) 3 hours or (f) 4 hours afterwards. Bar, 50 µm. Similar results were obtained in the presence of PD98059 (supplementary material Fig. S1, Table S1).