Volume Electron Microscopy of Cortical Organoids: Methods for Region Identification, Connectome Reconstruction, and Organelle Segmentation

Volume electron microscopy (vEM) has become a powerful tool for 3D ultrastructural analysis of neural circuits, yet its application to human brain organoids remains limited, particularly for connectomic studies. Here, we established a comprehensive and scalable workflow for applying vEM to human cortical organoids, integrating correlative light and electron microscopy, large-area SEM mosaic imaging, focused ion beam-scanning electron microscopy (FIB-SEM), and transmission electron microscopy (TEM) validation. By systematically comparing two embedding protocols in use, we demonstrated that the DeFelipe and Fairen (1993)/Cano-Astorga et al. (2024) method provides optimal compatibility with toluidine blue-stained semithin sectioning and enables reliable synapse segmentation and neurite tracing. In contrast, the Deerinck et al. (2010) protocol offers enhanced membrane contrast but limits postsynaptic density visualization. Using FIB-SEM imaging of peripheral, neuropil-like regions of cortical organoids, we achieved accurate 3D reconstruction of synapses, neurites and intracellular organelles, enabling quantitative assessment of synaptic apposition surfaces, neurite trajectories, and organelle distribution across defined cellular compartments. Together, our results demonstrate for the first time the feasibility of micro-connectomic reconstruction in human cortical organoids at nanometer resolution. This methodological framework expands the applicability of vEM to organoid systems and provides a robust foundation for future studies of human brain development, disease modeling, and therapeutic evaluation at the synaptic and subcellular level.