Ultrarapid freezing and cryomicroscopy of frozen hydrated material makes it possible to visualize samples that have never been exposed to chemical fixatives, dehydration, or stains. In principle, freezing and cryoimaging methods avoid artifacts associated with chemical fixation and processing and allow one to visualize the specimen in a condition that is close to its native state. Here we describe a way to use a high voltage electron microscope (HVEM) for the cryoimaging of frozen hydrated PTK1 cells.PTK1 cells were cultured on formvar-coated, carbon stabilized gold grids. After three days in culture, the grids were removed from the culture medium and blotted in a humidity chamber at 35° C. In some instances, the grids were rinsed briefly in 0.16 M ammonium acetate buffer (pH 7.2) prior to blotting. After blotting, the grids were transferred to a plunging apparatus and plunged into liquid ethane held directly above its freezing point. The plunging apparatus consists of a vertical slide rail that guides the fall of a mounted pair of forceps that clamp the specimen. The forceps are surrounded by a plexiglass humidity chamber mounted over a dewar of liquid nitrogen containing an ethane chamber. After freezing, the samples were transferred to liquid nitrogen and viewed in a JEOL JEM 1000 equipped with a top entry cold stage designed and built by Mr. George Wray (Univ. Colorado). The samples were routinely exposed to electron doses of 1 e/Å2/sec, and viewed at a temperature of −150° C. A GATAN video system was used to enhance contrast and to estimate the correct amount of underfocus needed to obtain phase contrast at various magnifications. Low dose micrographs were taken using two second exposures of Kodak 4463 film. The state of the solid water in the specimen was determined by diffraction using a 30/μm field limiting aperture and a camera length of 1 meter.
Low-dose, phase-contrast mammography with high signal-to-noise ratio