Transmission electron microscopy photographs of Stylophora pistillata with (a–c) or without (d) osmium post-fixation step. In (d), the sample was incubated with 3 nm gold nanoparticles. The apical side of the ectoderm (a,b,d) and the endoderm (a,c) show large vesicle formation, typical of macropinocytic processes. Note that on the oral ectoderm apical side, the forming macropinosomes appears loaded with mucus. Ext, external sea water medium; Coel, coelenteric cavity; mp, macropinosome; Sym, symbiont; MV, microvillis; nu, nucleus; mg, mesoglea; gc, gland cell; SJ, septate junction.