A clever “superresolution” microscope has allowed researchers to observe changes in a single neuron in the brain of a live mouse. The exceptional level of detail even showed protrusions called dendritic spines moving and changing shape.
The best way to learn about cells is to observe them in context in a living animal. To do so obviously requires a high resolution microscope, but even the best optical microscopes cannot discern features smaller than around 200-300 nanometres, half the wavelength of visible light. To go further and see smaller features involves using an electron microscope, for which the materials need to be prepared by freezing, staining or coating, so cannot be used with living tissue in vivo.
Sebastian Berning and colleagues at the Max Plank Institute for Biophysical Chemistry got around this optical limit by developing a type of Stimulated Emission Depletion, or STED microscopy. This relies on the cells containing a fluorescent dye that can be excited by absorbing certain frequencies of light, but also “de-excited” using other frequencies. By using lasers of different frequencies and varying the intensity across a sample, STEM microscopes are able to only excite a tiny portion of the field of view, effectively increasing the sensitivity from a minimum of 200nm down to less than 70nm across.
Berning and colleagues then pointed their STEM microscope at the brains of mice genetically engineered to express Enhanced Yellow Fluorescent Protein (EYFP) in their neurons. Employing a glass window in the skull, they were able to observe live, healthy neurons in situ. By taking images every few minutes, they were able to see dendritic spines moving and changing shape. As these are small processes that stick out of neurons, and are involved in receiving signals from other cells, understanding these could help us to get to grips with how the brain grows and develops, as well as changes over time.