RESEARCHERS have peered inside living nerve cells and seen the scaffolding that holds them up. The technique, which borrows a trick from quantum optics, will help scientists understand how the brain wires itself, and illuminate what happens to nerve cells in diseases such as Alzheimer鈥檚.
Most cells, including neurons, have an internal skeleton of polymers called microtubules that can lengthen and shorten as necessary. Microtubules provide structural support and also act as railroads along which proteins are delivered to various parts of the cell. Understanding the development and organisation of this cytoskeleton is crucial. But until now, to study microtubules biologists have had to kill cells and view them under an electron microscope.
Now physicist Watt Webb and doctoral student Daniel Dombeck of Cornell University in Ithaca, New York, and their colleagues have used lasers to light up the microtubules in living cells.
Advertisement
If a laser beam hits materials that have a certain asymmetrical structure, the materials radiate light at twice its original frequency 鈥� an effect called second harmonic generation. The technique is normally used to study the physical properties of materials, but Webb and Dombeck realised that microtubules also have the necessary asymmetry: a 鈥減ositive鈥� end that can grow further, and a 鈥渘egative鈥� end that inhibits growth.
When the researchers fired laser pulses onto ultra-thin slices of rat brain, they found that the neurons鈥� axons 鈥� long extensions that carry the cells鈥� outgoing signals 鈥� shone at twice the laser鈥檚 original frequency. Axons contain large numbers of microtubules that all point in the same direction, so the frequency-doubled light from them combines to create a strong signal, says Dombeck. But in other extensions of neurons called dendrites, which carry incoming signals, the microtubules are randomly oriented and the light cancels out.
Using this imaging technique, the team took a series of pictures to determine the time at which the extensions on a newborn neuron differentiate into axons and dendrites. In five-day-old rat neurons, the budding extensions were filled with well-aligned microtubules that showed up as thin, bright lines. But by the seventh day, one extension had become an axon while the others had become dendrites, no longer showing up under the laser light (Proceedings of the National Academy of Sciences, vol 100, p 7081).
鈥淭his new method is very provocative and powerful,鈥� says Peter Baas, a neurobiologist and expert on microtubules at Drexel University in Philadelphia. 鈥淔irstly, it enables scientists to obtain a huge amount of information without having to go through all the tedious electron microscopy. Secondly, it works on living tissue.鈥�
The method promises to reveal what goes wrong in neurological disorders. 鈥淭his method could be very powerful for looking at abnormalities in microtubule organisation in Alzheimer鈥檚 or in damaged spinal cords,鈥� says Baas.