To analyse neuronal networks of the brain, it would be very helpful if one could visualize these networks in whole brain preparations. Up to now this was impossible and sequential slicing was necessary for 3-D reconstructions with limited resolution. To overcome the problems of this mechanical approach, we developed ultramicrocopy, a new kind of microscopy based on darkfield illumination. This microscopy allows optical sectioning of whole mouse brains and was combined with a new approach to clear fixed neuronal tissue. By the detection of stray light in cleared whole brains myelinated fiber tracts could be visualized. Illumination of the brains with blue light allowed visualization of neurons labeled with GFP by fluorescence. This way we could detect single neurons in hippocampi inside whole brains. The dendritic trees of CA1 neurons could be visualized including dendritic spines. Using ultramicroscopy, it should be possible to investigate also the complex neuronal networks in the neocortex.

Supported by SFB 391