A closer look at cells

ILLUSTRATION: PROTEIN DATA BANK

Conventional and confocal optical microscopy have reached a resolution limit for measuring dimensions smaller thanvisible light wavelengths,which has prompted researchers to develop innovative and more powerful methods to observe cell structure and function.

NSOM, a cross between scanned probe imaging and conventional optical microscopy, uses an aperture that is smaller than the wavelength of light positioned in the near-field of the observed object—close enough to obtain subwavelength resolution. During visualization, light from the aperture interacts with the object and then is collected by a lens and recorded electronically.

Using NSOM to visualize living cells presents several difficulties. Unlike previously visualized solid structures, such as photonic crystals, cells are soft, present larger surface variability, and exhibit low optical contrast, which is further decreased by the liquid environment of cell culture media.

Andrei Sommer and Ralf-Peter Franke at Ulm have addressed these difficulties and described the use of NSOM to observe the morphology of living cells in a liquid environment.

They cultured human endothelial cells on polished titanium disks, which increased irradiation-induced contrast and eliminated the need for contrast-enhancing agents (J. Proteome Res. 2002, 1 (2), 111–114). The modified NSOM system, using a hydrophobically coated optical fiber, allowed researchers to visualize the topography and organelles of the cells. This was the first report of live cell imaging in an aqueous environment using NOA techniques.

Although the use of NSOM to view living cells is in its early stages, the German researchers suggest that their work indicates its potential for allowing scientists to look more closely at the functions of protein complexes and single molecules in a biologically active cell.