ABSTRACT
The freeze-fracture/freeze-etch technique, orginally introduced to examine the substructure of crystalline viruses, has been applied advantageously to the analysis of biological membranes. Two concepts from freeze fracture are central to current understanding of membrane structure and function: the location of the fracture plane and the composition and function of the small "particles" on the fracture faces of biological membranes. Initially, it was assumed that the fracture plane represented the natural interface between the hydrophobic membrane lipids and the aqueous cytoplasmic or extracytoplasmic compartments. In replica-label, unlabeled samples are frozen, fractured, and replicated as in conventional freeze fracture, then thawed and labeled. An advantage of replica-label over label-fracture occurs because labels are applied after fracturing and replication. Shadow-label is a new replica labeling technique that provides for high resolution labeling and potential for mapping intramembrane domains in larger intramembrane particles. Fixed and cryoprotected are shadowed with platinum, but are not coated with carbon.
