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The ExA-SPIM method - expansion-assisted selective microscopy with planar illumination
The Allen Institute has developed a special microscope based on a technique in which the whole brain is expanded. This makes it much easier to examine large pieces of tissue such as the brain of a mouse. It has a larger field of view and can look deeper into the tissue than previous microscopes. It enables centimeter-sized samples to be viewed in high detail with short exposure times without having to cut them. The almost isotropic resolution and low segmentation result in very little distortion of the image.  In combination with new methods for brightening and expanding the tissue, it has already been demonstrated on whole mouse brains and other large tissue samples.

Introduction
Biological tissue is organized in different structural sizes. To understand the organization of individual cells, it is necessary to view the architecture of the tissue at all these sizes simultaneously. However, standard microscopes have a limited field of view and cannot look very deeply into large pieces of tissue. Therefore, researchers have had to cut the tissue into small pieces in order to view it in high resolution. This can distort the tissue and make it more difficult to reassemble images later. 
 

Imaging of large tissue areas
The brains of mice were expanded and imaged with a microscope. This allows individual neurons to be tracked throughout the brain. This is important for understanding signal transmission in the brain and identifying different types of neurons. Large projects investigating the diversity of neuron types in the mouse and human brain could benefit from this method. The method allows much faster data acquisition.

The ExA SPIM microscope is ideal for large-scale tissue imaging. It is characterized by a very high resolution and offers the possibility to view large areas at once. The imaging performance is very uniform across the entire sensor and the microscope is fast enough to capture many images in a short time. The ExA-SPIM microscope uses special industrial lenses to capture a larger area at once and look deep into the tissue.

The development of new methods in tissue preparation and microscopy has an impact on the viewing of biological samples. By combining microscopy and the tissue expansion method, it is now possible to view tissue with high resolution and over large areas without having to cut it into small pieces. This technique could be important for many applications in biology and medicine. For example, the interaction of the immune system with tumors should be better understood. By advancing the technology and developing new software for image processing and analysis, we hope to gain more detailed insights into the complex structure of living organisms.