The challenge facing the scientists was to treat a large piece of tissue so that it is evenly fixed and stained right through to the inside.
In their latest study, the Heidelberg-based researchers demonstrated that the brain of a mouse can be prepared in a way that enables it to be analysed whole using “block-face” electron microscopy. The Max Planck research group would now like to image a whole brain with the “serial block-face” microscope so that they can study the neuronal connections in the entire mouse brain.
In a current study, Shawn Mikula from Denk’s department succeeded in preparing a mouse brain in such a way that he was able to analyse it using block-face microscopy and trace the axons. This works for small pieces of tissue, but up to now it was not possible for tissue the size of a mouse brain. To examine tissue using this method, it must be fixed, stained and embedded in synthetic material. In 2004, scientists working with Denk developed a new method that enabled them to do just this: “serial block-face” scanning electron microscopy. To obtain an overall picture of a brain, the researchers have to analyse large pieces of tissue. Despite their minute diameter, axons can become very long and extend from one end of the brain to the other. “The electron microscope is the only microscope with a high enough resolution to enable individual axons lying next to each other to be distinguished from each other,” says Winfried Denk. Most axons are less than one micrometre thick, some even smaller than 100 nanometres. Analysing this network under the microscope is one of the biggest challenges facing the neurosciences. Neurons transmit information through their extensions – the axons – and form a complex network of connections, which provides the basis for all information processing in the brain.