Fig. 9

Thyroid hormones bind and increase MAPK phosphorylation in the somatocoel and in presumptive skeletogenic mesenchyme in the larval arms in O. aculeata. Larvae were exposed for 90 min prior to fixation. A, B Maximum intensity projection of Ophiopholis aculeata larvae (ophiopluteus) after T4 exposure. Larva A is in the early stages of metamorphic development (Stage 0 per our scheme). Larva B is in Stage 2, possessing a five-lobed hydrocoel. C–C″’ Single focal slice revealing T4-binding locations and MAPK phosphorylation in a larval arm. Arrowheads (▶) indicate presumptive skeletogenic mesenchyme. D-D″’ Single focal slice showing 5-lobed hydrocoel. Arrowheads (▶) indicate hydrocoel lobes. Little to no T4 binding is detected in the hydrocoel and hydrocoel MAPK phosphorylation does not differ from control larvae. E-E″’ Single focal slice showing somatocoel adjacent to the gut. Regions of increased T4 binding and MAPK phosphorylation are indicated with arrowheads (▶). T4-binding cells, indicated with arrowheads (▶), do not present with greatly increased MAPK phosphorylation. hc: hydrocoel, gut: midgut, sc: somatocoel, la: larval arm. F T4 binds primarily to the midgut and presumptive skeletogenic mesenchyme cells in ophiuroid larvae (n = 4, p < 0.05, significant differences exist between cell types with no shared letter). We observed little binding in the ciliated bands and epithelial tissues, with some observed binding in the hydrocoel and posterior coelom/somatocoel. G T4 increases MAPK intensity in O. a. larvae (n = 4, p < 0.05). This effect may be partially inhibited by RGD peptide (n = 4, p > 0.05)