Transparent fruit flies

The nervous system of an animal can be studied by cutting it up into thin layers – however this inevitably leads to the destruction of the cellular structures in the tissue. Analyzing complex nerve connections is then hardly possible.

The far more elegant method is the so called optical "clearing" of the various tissues using chemical processes that make the animal transparent. Interesting structures in the tissue may then be selectively marked and analyzed.

At the Vienna University of Technology, a clearing method has now been developed that can be applied to insects, which is a particularly difficult task. With an improved light-sheet microscope (a so-called ultramicroscope), it is now possible to image large nerve tissue samples and take high-resolution pictures of complex neural networks that have been labeled with fluorescent molecules.

"We can learn a lot about the nervous systems of animals by using genetic engineering to insert special molecules into the nerve tissues, which can then be made to fluoresce," says Marko Pende, a Ph.D. student at TU Wien (Vienna). The big question is how these fluorescent molecules can be imaged without damaging the tissue.

One method that has been used with great success is ultramicroscopy. Transparent tissue is illuminated with a laser beam, which is widened by special optical elements, creating a two-dimensional flat surface of light. This surface penetrates the tissue and illuminates those fluorescent molecules that lie exactly in that plane. Layer by layer, the tissue can be analyzed with this light-sheet, creating a three-dimensional model from the two-dimensional frames on the computer.