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Subcellular localization of Forssman glycolipid in epithelial MDCK cells by immuno-electron- microscopy after freeze-substitution. J. Cell Biol. 115, 1009–1019 (1991). 22. Parton, R. G. Ultrastructural localization of gangliosides; GM1 is concentrated in caveolae. J. Histochem. Cytochem. 42, 155–166 (1994). 23. Koster, A. J. et al. Perspectives of molecular and cellular electron tomography. J. Struct. Biol. 120, 276–308 (1997). The success of reductionistic approaches in biomedical research has yielded an unprece- dented knowledge of the components that are involved in biological processes, and researchers now face the challenge of inte- grating this knowledge into a more complete understanding of whole systems. For example, the revolution in molecular biology has given us important new insights into the function of genes and gene products that might guide embryonic development. Many of the signalling pathways that are involved in the specification of cell types and the pat- terning of tissues are being defined, as are the transcription factors that are involved in controlling cell-type-specific and region- specific gene expression. To answer the basic question of how an embryo develops, we must determine how these molecular processes are assembled into an organism. The classic publications in the field of experi- mental embryology 1–3 illustrate the power of describing cell behaviours (in terms of cell lineages and movements) and then perturb- ing the development of an embryo to test hypotheses regarding the underlying mecha- nisms. Advanced imaging techniques offer an important stepping stone to integrate these disparate approaches 4 , and allow questions about cellular and molecular signalling events to be posed in the most relevant setting of the intact embryo. Imaging thick specimens The focus of intravital imaging is to follow cellular and subcellular events in the context of an entire organism. This presents a signif- icant challenge to researchers, because all but the youngest embryos are significantly thicker than can be imaged easily with light. The image collected by a camera in a wide- field light microscope is typically blurred, because light from above and below the MRI: volumetric imaging for vital imaging and atlas construction Russell E. Jacobs, Cyrus Papan, Seth Ruffins, J. Michael Tyszka and Scott E. Fraser Magnetic resonance imaging (MRI) is well known for its ability to capture non-invasively the three-dimensional structure of complex tissues such as the human brain. The physics underlying this technique means that it can be refined to collect high-resolution images in settings that would scatter the radiation used in direct-imaging techniques. This makes microscopic MRI a powerful tool to observe events and structures deep inside otherwise opaque soft tissues. REVIEW