Interpretation of Freeze-Etch Replicas

Abstract

As briefly outlined above, the freeze-etching technique not only allows the observation of biological specimens without dehydration or embedding, but also reveals their ultrastructural organization in a tridimensional fashion. The tridimensional image is given essentially by a succession of fracture planes occurring in any given direction and separated one from another by identifiable steps or ridges. In a compact tissue (see Plate 1), the fracture planes are of two kinds: some occur at random without following natural boundary lines and expose for example the cell cytoplasm, the nucleoplasm, the extracellular space, whereas others follow preexistent boundaries within the cells or the tissue. Such boundaries are the membranes of the nucleus, the cytoplasmic organelles or the cell itself. The first kind of fracture is of relatively limited interest since, in most cases, it shows a more or less coarse granular substructure representing the frozen cytoplasmic matrix or the extracellular fluid and, in this respect, does not add much information as compared to conventional thin-section electron microscopy. It is therefore the fracture occurring along membranes which represents the capital advantage of freeze-etching over thin-sectioning, since it allows the visualization of the membrane inner structure, described now in more detail.