Movie S1.

A video showing cryo-FIB-SEM serial sections that were aligned to form a data cube of X = 20 µm, Y = 12 µm, Z = 11 µm, with a voxel size of 10 × 10 × 20 nm. The FIB milling was orthogonal to the spicule long axis, yielding a series of SEM images in which the cross-section of the spicule (S) is surrounded by PMCs that create the syncytium enveloping the spicule. N, nuclei. Pause 1: Spicule cross-section with its associated membrane and two PMCs on either side of the spicule. Note that the plasma membrane of the PMC on the left side surrounds the spicule, as expected for the syncytium. Organelles and vesicles of different sizes and morphologies are shown, including a vesicle that is open to the blastocoel (asterisk). Note the contact of this vesicle with other vesicles to its left. This vesicle thus has a branched structure. The small dark bodies designated as B are presumably lipid granules. The elongated bright haloes associated with these bodies are charging artifacts. Pause 2: A PMC that envelops the spicule is shown to the left of the spicule. An ellipsoid-shaped vesicle (asterisk) is in contact with the plasma membrane, and has an opening toward the blastocoel. Charging artifacts around small dark bodies (B) result in smearing of the area.