An efficient protocol of cryo-correlative light and electron microscopy for the study of neuronal synapses

Cryo-electron tomography is an emerging electron microscopy technique for determining threedimensional structures of cellular architectures near their native state at nanometer resolution, with a shortcoming of lack of specific labels. Fluorescence light microscopy, on the other hand, specifically visualizes target cellular and molecular components with fluorescent labels, but is limited to a resolution of tens to hundreds nanometers. Combining the advantages of the two techniques, we have developed a cryocorrelative light and electron microscopy system. Our system consists a custom-designed cryo-chamber that allows for fluorescence imaging of frozen-hydrated samples, and an algorithm to achieve accurate correlation. With this system and our optimized protocol, high-quality tomograms of neuronal synapses labelled by specific fluorescent tags in cultured hippocampal neurons are obtained at high efficiency.