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Fig. 3. Activation of MAP kinase is essential for PMC ingression and
skeletogenesis. Embryos were incubated at the indicated times with 10 µM of
the MEK inhibitor and scored at the early gastrula stage for the presence of
micromeres, and at the pluteus stage (48-72 hours) for the presence of
spicules. Pictures of representative embryos obtained after the treatments are
shown on the right. When U0126 treatment was started before hatching and was
continued up to the pluteus stage, PMC ingression and spicule formation was
blocked (A,B). If the inhibitor was added after beginning of hatching, PMC
ingressed into the blastocoel but did not differentiate further (C,D). When
the inhibitor was added at the two-cell stage or prehatching blastula stage
PMCs formation was prevented (E), but if the inhibitor was removed soon after
the beginning of invagination of the archenteron, spicules nevertheless formed
(F). Spicules did not form when the inhibitor was removed after the late
gastrula stage (G,H). ERK activity is required first between the prehatching
blastula stage and the swimming blastula stage for ingression of the PMCs
(green), and then during gastrulation for differentiation of the PMCs into
spicules (yellow).
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