Biologists can take beautiful pictures of whole cells, and they can zoom all the way in to the molecular structure of single proteins. But they haven't had a good way to look at midsize organelles, like the actin filaments that cleave a cell in half or the protein-packaging Golgi apparatus. Now, researchers have opened a new window on such structures with a technique that creates three-dimensional images of intact organelles.
The technique, called cryoelectron tomography (cryo-ET), has been pioneered by Wolfgang Baumeister of the Max Planck Institute for Biochemistry in Martinsried, Germany. In the 8 November issue of Science, his team reports the use of cryo-ET to image a network of actin filaments in the amoeba Dictyostelium discoideum. These pictures provide the first structural measurements of actin's geometry in an intact cell.
The technique works something like a CT scan. Penetrating beams of radiation create two-dimensional image slices that a computer assembles into a 3D image. Although the concept has been around for more than 3 decades, Baumeister explains, only recently have technical advances enabled researchers to obtain data quickly enough to prevent the electron beam from damaging cell structures. Unlike traditional techniques, cryo-ET does not require staining, fixation, or disruption of the cell membrane--all of which can introduce artifacts.
Cryo-ET will make possible new analyses of cell structure. Baumeister's group, for example, hopes to create a 3D map of all the organelles in a eukaryotic cell and use it to create the first integrated link between atomic-level images (from nuclear magnetic resonance and x-ray crystallography) and cell-level images from light microscopy. The technique shows particular promise for imaging any organelle that can be found in the thin margins of the cell, says structural biologist Kenneth Taylor of Florida State University, Tallahassee, such as the Golgi apparatus, microtubule organizing center, cilia, and flagella.
The Baumeister lab has shown that cryo-ET is feasible, says Taylor, and, as the technique is further refined, it should become easier and cheaper to use. “There will be an explosion in cryo-ET in the next 10 years or so," predicts microscopist Timothy Baker of Purdue University in West Lafayette, Indiana. "It will be a major player in cell biology, no doubt about it.”