Fig. 3: TEM images showing sequential stages of valve formation and exocytosis in S. turris.

Left column shows a representative image of each stage, with higher magnification views of the boxed areas in the right columns. a Valve formation starts at the apex of the cell. a’ Growing silica inside the SDV that is located directly under the plasma membrane. a” The SDV ends at the growing silica edge. The inset shows the bilayers of the proximal (purple) and distal (pink) SDV and the plasma membrane (yellow) in higher magnification. b Later during valve formation, b’ the polygonal layer (arrow) is formed on top of the base layer, and b” the growing edge of the new valve reaches the rim of the parental valve. c Silicification of the new valve is completed while it is still fully enclosed in the SDV. Mature valves can be recognized by: c’ the fully formed and flattened polygonal layer, and c” a pronounced hook shape at the valve rim (arrow). d At the onset of exocytosis, d’ membranes still surround most parts of the new valve, d” but they no longer form a complete enclosure (arrowheads). e Shortly after exocytosis, e’ distal membranes lose their structural integrity (arrowheads), and e” the plasma membrane is now continuous (arrow) under the new valve. f–f” After completion of exocytosis (n = 75), there are no traces left of the distal membrane remnants. To the right of the images, a table summarizes the number of cells observed in each developmental stage and the number of cases when membrane invaginations were present (see details in Fig. S8 and main text). Scale bars represent 5 μm (a–f), 1 μm (d’, e’, f’), 500 nm (a’, c’, c”, d”, e”, f”), 200 nm (a”, b’, b”, c’ inset) and 50 nm (a” inset).