Preparing Fission Yeast for Electron Microscopy

Freezing samples while simultaneously subjecting them to a rapid increase in pressure, which inhibits ice crystal formation, is a reliable method for cryofixing fission yeast. The procedure consists simply of harvesting cells and loading them into a high-pressure freezer (HPF), and then operating the device. If equipment for high-pressure freezing is not available, fission yeast can be frozen by plunging a monolayer of cells into a liquid cryogen, usually ethane or propane. Unlike the HPF, where relatively large volumes of cells can be frozen in a single run, plunge freezing requires cells to be dispersed in a layer <20 µm thick. Unless frozen cells are to be imaged in the vitreous state, they must be fixed, dehydrated, and embedded for subsequent study by transmission electron microscopy; warming frozen cells without fixation badly damages cell structure. Fixation is best accomplished by freeze-substitution, a process in which frozen water is removed from samples by a water-miscible solvent that is liquid at a temperature low enough to prevent the cellular water from recrystallizing. Low concentrations of chemical fixatives and stains are generally added to this solvent such that they permeate the cells as the water is replaced. The activity of these additives is quite limited at the low temperatures required for minimizing ice crystal formation, but they are in the right place to react effectively as the cells warm up. Step-by-step protocols for HPF, plunge freezing, and freeze-substitution are provided here.