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New Process Expands Samples for Microscope Magnification

Ed Boyden

Ed Boyden (Mass. Institute of Technology)

16 January 2015. Researchers at Massachusetts Institute of Technology developed a process for expanding the size of tissue samples that makes possible high resolution images with ordinary laboratory materials and microscopes. The team led by MIT bioengineering professor Ed Boyden published its findings yesterday in the online journal Science Express (paid subscription required).

Microscopes have been used for centuries with lenses to magnify specimens in laboratories. But conventional microscopes have visual limits, determined in part by the wavelength of light used to illuminate the sample. As a result, many objects at nanoscale dimensions — 1 nanometer equals 1 billionth of a meter — which includes functions of cells interacting with their environment.

Boyden and his lab colleagues note that super-resolution techniques are available to get past these limits, but they require small, thin samples or can take extended periods of time, especially when imaging large samples. “If you want to map the brain, or understand how cancer cells are organized in a metastasizing tumor, or how immune cells are configured in an autoimmune attack,” says Boyden in a university statement, “you have to look at a large piece of tissue with nanoscale precision.”

The researchers addressed the problem through the properties of the sample rather than the imaging technology. Their solution, called expansion microscopy, involves expanding the size of the sample to make it easier to image at higher resolutions. The team starts with labeling cells or proteins with antibodies that bind to components of interest in the specimen, and linked to fluorescent dyes.

The specimen is then embedded in a polyacrylate compound, a highly absorbent material used in diapers to keep moisture away from the baby’s skin, but also expands when in contact with water. The antibodies binding to the targets also have chemical anchors that attach their corresponding fluorescent dyes to the polyacrylate material. The combination of polycrylate compound and specimen is then heated to form a gel that expands the size of the specimen.

As the gel forms, the specimen expands uniformly, expanding to 100 times its original size. Fluorescent antibody-labels applied to cell components at the start of the process are anchored to the polyacrylate and remain in their same relative position for imaging. The result says co-author Paul Tillberg is “a three-dimensional, fluorescent cast of the original material. And the cast itself is swollen, unimpeded by the original biological structure.”

The researchers tested this process with cultured cells and samples of mouse brain tissue returning images with a conventional confocal microscope equivalent to three-color super-resolution images. The brain tissue specimen, says the team, measured 500 by 200 by 100 microns, far exceeding the capabilities of conventional microscopes.

The authors applied for a patent on their technology, to be assigned to MIT. They tell more about expansion microscopy in the following video.

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