Using optical projection tomography (OPT), researchers have produced 3-D images of fruit-fly brains in various stages of degeneration. These images could one day improve our understanding of a variety of neurodegenerative diseases in humans.
“We hope in the future to answer questions about genes and proteins that function in diseases like Alzheimer’s and Parkinson’s,” says Leeanne McGurk, a PhD student with the Medical Research Council’s Human Genetics Unit, in Edinburg, Scotland, who worked on the fruit-fly imaging project.
Fruit flies, or drosophila, are often used to study disease progression because their monthlong life span means that diseases progress quickly, and because the flies have many of the same genes as humans. Moreover, age-related defects in the fly brains produce tiny holes, or neurovacuoles, in a process that is similar to the one that turns healthy human brains into the tangled brains of people who suffer from Alzheimer’s disease. But dissecting fly brains, which are only about one millimeter across, was a laborious process that often damaged the specimen.
So the team decided to use OPT to create images of the intact brains of the flies. The technique had originally been designed to create images of tiny mouse embryos, and it has since been utilized to examine adult mouse tissue and some human embryos.
In this experiment, the researchers first had to bleach the insect, as it has a dark exoskeleton that prevents it from being examined with a normal microscope. Then they embedded the tiny fly in gel and slowly rotated it 360 degrees as a camera took 400 pictures.
Those pictures were transformed into 3-D images, using software also developed under the umbrella of the Medical Research Council, the United Kingdom’s version of the National Institutes of Health. The images showed clearly the neurovacuoles present in the fruit-fly brains: flies of different ages and genetic makeups appeared to have different stages of brain degeneration.
According to the researchers, the brain images they created would have been impossible with traditional technology. “OPT works just like an x-ray CT scanner, only using light instead of x-rays,” says James Sharpe, one of the study’s authors and a research professor at the Systems Biology Centre for Genomic Regulation, in Barcelona, Spain. This is in contrast to traditional techniques such as confocal microscopy, which scans one section of a specimen at a time.
“In confocal microscopy, you try and focus very sharply on one plane, trying to minimize informational noise coming from above or below,” says Sharpe. However, he says, OPT imaging takes visual information from as large a depth as possible, and then uses the rotation to figure out where the different parts of the image lie. This gives a much more detailed picture of the specimen. The study of the fruit-fly brains was published recently in the journal Public Library of Science ONE.
OPT also fills in the imaging gap: traditionally, specimens between 1 and 10 millimeters were too big to image with a confocal microscope and too small to put in an MRI scanner. OPT is also less costly and can sometimes offer higher-resolution images than an MRI scanner.
“While you can put a human in an MRI scanner, you just don’t have that option with something much smaller,” says McGurk.
There are, of course, limitations to the use of OPT because the resolution is dependent on the transparency of the sample. Some brain-imaging experts say that although this technology may help scientists understand the mechanisms of neurodegeneration, it won’t be used in adult human brains anytime soon.
“There is very little clinical application to this, since the light would have to go through the head,” says Michael Weiner, who works in MRI imaging of Alzheimer’s and other diseases at the University of California San Francisco Medical Center. “I think that it is a technique which needs to be further developed [before it can be used] for human applications.”
Sharpe says that the team is already working on ways to improve the resolution and contrast.