Extended Data Fig. 4: Immuno-electron microscopy labeling of ribosomes.

a, Raw images of neocortex coronal sections at E12.5 and E15.5 shown in Fig. 4c, immunolabeled with anti-ribosomal protein uS7 followed by 2.5 nm gold secondary (dark black spots), which were automatically detected and quantified in FIJI (magenta spots in Fig. 4c). Electron microscopy was performed in regions corresponding to the stem cell niches of the ventricular zone (VZ) and sub-ventricular zone (SVZ), in addition to regions of differentiating neurons in the cortical plate (CP), which includes both lower layers (LL) and upper layers (UL) at later stages. Quantification of nanogold secondary signal was performed per unit area of the cytoplasm, with nuclei excluded by tracing the nuclear membrane (black lines in Fig. 4c). b, Primary antibody leave-out controls were prepared in parallel. Cell images were captured from 2 independent brains, 2 sections/brain, at each developmental stage. Quantification of each image with n images quantified is reported in Fig. 4d.