The fly Drosophila melanogaster has become one of the prime model systems in brain research. Its brain consists of about 200.000 neurons. Given this relatively small brain, the fly shows a surprisingly complex repertoire of behaviors, e.g. orientation, courtship, learning and memory. The sophisticated genetic tools available for Drosophila allow for new kinds of manipulations of the brain. With these one can, for instance, correlate changes in behaviour with changes in the available neural circuitry and hence gain insight into brain function.
The anatomy of the fly brain was first described about 100 years ago (see citations in Hertweck, 1931, Zeitschr. F. wiss. Zool. 139, 559-663). Until today the description has remained at the level of neuropil regions, with the exception of the optic lobes, antennal lobes, mushroom bodies and central complex. On the other hand molecular genetics allows us to describe the activity of single genes. Spatial and temporal expression patterns are a crucial aspect of gene activity. Three-dimensional brain models help us to relate these gene activities to single brain structures and even neurons.
Until some years ago, the reconstruction of three-dimensional models was only possible from serial sections, a process that was very labor- and time-consuming. Today the confocal microscope allows one to generate such sections optically without destroying the specimen. Images acquired with the confocal microscope can reach resolutions of less than 1µm. The only preconditions for the optical sectioning are that the tissue is transparent and that the structures of interest can be labeled fluorescently. Today it is even possible to express and visualize fluorescing proteins in living animals.