Movie 3.

Tridimensional rendering of the primordial germ line displaying the proto-rachis (white compartment), to which both PGCs (red, with nuclei in blue) and membrane lobes (magenta) are connected via intercellular bridges (yellow spheres). This rendering was made from reconstructed EM images. Each image was imported into Inkscape software (v1.1) and the membrane defining each compartment was manually traced. To make intercellular bridges more visible, each was defined as a yellow sphere with a diameter equal to that of the bridge. Each compartment was then filled with colour in separate channels (1 channel per colour) and saved as a tif file. The file was imported into Imaris software (v9.2.1, Bitplane) and the “surface” tool was used to generate a 3D rendering of each defined compartment in each slice. The “clipping plane” tool was used to generate a composite image of each slice and these composite images were assembled with ImageJ software into a movie that sequentially goes from the bottom to the top of the primordial germ line. The “animation” tool from Imaris software was used to rotate the entire reconstructed 3D image.