What is the fine-grained topic of the following text: Cryo-electron microscopy (cryo-EM) allows the three-dimensional (3D) visualization/ characterization of biological structures in a close-to-native state that range from individual macromolecular complexes and large molecular machines to the intricate macromolecular network within entire cells. Only return the topic.
3D visualization of biological structures using Cryo-electron microscopy