Alexandra Graff-Meyer, Laurent Gelman and Rainer Friedrich

As a technical platform for advanced imaging and microscopy at the Friedrich Miescher Institute for Biomedical Research (FMI) we provide cutting-edge service in light and electron microscopy. We support scientists and external collaborators with challenging research questions in epigenetics, quantitative biology, and neuroscience by providing expertise, high-end instrumentation, and technical support. In neuroscience, a major challenge addressed in the group of Rainer Friedrich is the quantitative analysis of synaptic connectivity among large numbers of neurons to understand how the structure of a neuronal circuit determines its function. The group addressed this question using the olfactory bulb of a zebrafish larva as a model system. This circuit is small, consists of approximately 1000 neurons, and performs complex transformations of distributed neuronal activity patterns that support odor discrimination. The group first used 2-photon calcium imaging to measure odor-evoked activity across approximately 30% of all neurons and subsequently reconstructed all neurons and their synaptic connections to obtain a “wiring diagram”. The main challenge in reconstructing the wiring diagram is the imaging of brain tissue at nanometer resolution throughout large volumes (1-4). Wanner et al (5-7) accomplished this using serial block-face EM (SBEM), an approach that is based on the operation of an automated ultramicrotome inside the vacuum chamber of a scanning electron microscope to acquire continuous stacks of images from the sample block face (1). Subsequently, improved staining for large samples was established (8) and software was developed to support complex data acquisition schemes, automatic error handling, remote microscope control, and other tasks (9). In this presentation, we will give you an insight into the full workflow used for the reconstruction of neurons and synapses in the olfactory bulb and show/discuss the different scientific and technological challenges to reconstruct the complete wiring diagram of an entire brain area in a vertebrate.

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2. Denk W., Briggman K. L. & Helmstaedter M. Structural neurobiology: missing link to a mechanistic understanding of neural computation. Nat. Rev. Neurosci. 13, 351–358 (2012).
3. Briggman K. L. & Bock D. D. Volume electron microscopy for neuronal circuit reconstruction. Curr. Opin. Neurobiol. 22, 154–161 (2012). 
4. Lichtman J. W. & Denk W. The big and the small: challenges of imaging the brain's circuits. Science 334, 618–623 (2011).
5. Wanner, A. A., Genoud, C., Masudi, T., Siksou, L., & Friedrich, R.W. (2016). Dense EM-based reconstruction of the interglomerular projectome in the zebrafish olfactory bulb. Nat. Neurosci 19, 816-825.
6. Wanner A. A., Genoud C., Friedrich R. W. 3-dimensional electron microscopic imaging of the zebrafish olfactory bulb and dense reconstruction of neurons. Sci. Data 3: 160100, (2016).
7. Wanner, A. A., & Friedrich, R. W. (2020). Whitening of odor representations by the wiring diagram of the olfactory bulb. Nat Neurosci 23, 433-442.
8. Genoud, C., Titze, B., Graff-Meyer, A., & Friedrich, R.W. (2018). Fast Homogeneous En Bloc Staining of Large Tissue Samples for Volume Electron Microscopy. Frontiers Neuroanatomy 12:76.
9. Titze, B., Genoud, C., and Friedrich, R.W. (2018). SBEMimage: Versatile Acquisition Control Software for Serial Block-Face Electron Microscopy. Frontiers in Neural Circuits 12:54.