Fig. 1.

Cryo-SEM micrograph of a high-pressure frozen and freeze-fractured sea urchin embryo at the late gastrula stage. (A) The spicule cross-sections are marked with arrows. The archenteron (Ar) and epithelial cells (Ep) of the embryo are fractured, showing large numbers of intracellular vesicles of 0.5–1.5 μm. Note the multivesicular body adjacent to the PMC (arrowhead), enlarged in Fig. S1C. The blastocoel is marked with an asterisk. (B) The spicule cross-section (S; BSE image of the spicule area in Fig. S1B) is adjacent to a PMC that contains many vesicles, some empty and some packed with nanospheres (arrow). (C) Enlargement of the marked vesicle in B, showing nanospheres of 20–30 nm.