Since its discovery in the 17th century, the light microscope has been the key to new biological and medical discoveries. Light, however, propagating as a wave, is subject to the phenomenon of diffraction, whose resolution-limiting effects were first described by Ernst Abbe in 1873. Abbe observed that structures which were closer to each other than ~200nm could not be visually separated when observed using visible light; when viewed through the optical microscope they are perceived as a blurred, single entity. Abbe's realization of the resolution limitation of the optical microscope was long thought to be a unalterable law of far-field light imaging. Achieving higher resolution required the use of an electron microscope.
Despite that fact that electron beams can be more tightly focused, it is often difficult to efficiently label the proteins of a cell to render them visible with an electron microscope. Moreover, electron beams are only able to penetrate the first several micrometers of a biological sample. For these reasons, among others, despite using electron microscopy for high-resolution cell imaging, many questions of nerve function remained unanswered. In contrast, using fluorescent molecules as markers, one can specifically label individual proteins with high efficiency, rendering them visible with the conventional fluorescent microscope. Unfortunately the high resolution required to separate nanoscale structures was lacking due to the diffraction barrier.
In recent years researchers in the department of NanoBiophotonics at the MPI for Biophysical Chemistry in Göttingen have been able to break the Abbe resolution limit of far-field optical microscopy, as applied to fluorescent imaging, using a technique known as Stimulated Emission Depletion (STED) microscopy. The STED microscope used to obtain data for both publications is able to attain a resolution of 50-70 nm; the original fluorescent spot, roughly 200 nm in diameter, is reduced in surface area within the imaging plane by roughly an order of magnitude using the STED technique.
This resolution was sufficient for researchers from the neurobiology department to visualize, for the first time, individual synaptic vesicles - more precisely, to visualize the protein synaptotagmin, which is embedded in the membranes of individual vesicles. Vesicles are membrane 'bubbles' roughly 40 nm in diameter filled with neurotransmitters, which transport chemical messenger molecules to synapses, the contact points between nerve cells, enabling nerve signals to pass between cells. Their contents are released at the synapse when the vesicle membranes fuse with the membrane of the nerve cell. Previously it was unclear whether the proteins sticking in the vesicle membrane (e.g. synaptotagmin) spread out over the cell membrane after the fusion event or they remained together, localized in the membrane patch which previously formed the vesicle. With the aid of STED microscopy the researchers in Göttingen were able to show that the synaptotagmin molecules of a single vesicle remain together after fusion. The membrane of the nerve cell thus behaves in an 'economical' fashion: the vesicle proteins released onto the membrane of the nerve cell can be collectively reabsorbed to form another vesicle.
Neural vesicles do not fuse with the cell membrane with equal probability at all locations along the synapse junction, but preferentially at so-called 'active zones.' The bruchpilot protein discovered in fruit flies plays 2 a decisive roll in the formation of these active zones. This is explained in the Science publication by Kittel et al. With STED imaging the scientists discovered that the bruchpilot protein is distributed in rings of ca. 150nm diameter, forming the active zones. In these areas it appears that bruchpilot establishes the proximity between the calcium channels and the vesicles enabling the efficient release of neurotransmitters.
These studies demonstrate for the first time that resolution below a half-wavelength of visible light is no longer reserved for the electron microscope when observing cells. As demonstrated by recently completed research (please see the press release from XXth of May, 2005), the resolution of STED microscopy can be further increased, in principle to reach molecular scales. The STED microscope has opened a new chapter in the story of light microscopy, one in which the fundamental questions of biological processes at the nanoscale can potentially be answered with focused light.
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