Scanning Electron Microscopy


A main controversial issue in cell biology concerns the molecular mechanism responsible for K+ accumulation in living cells and Na+ exclusion from them. The alternative theoretical descriptions of these phenomena are based on different assumptions about the physical state of cellular Na+, K+ and H2O. In this article it is shown with striated muscles how cryomethods and microanlytical electron microscopy may be used to test the opposing theories. It is concluded that these methods may yield more realistic informations about the physical state of cellular K+ and Na+ than measurements with ion sensitive microelectrodes or the reference phase method of Horowitz and coworkers. The results obtained with different cryomethods, especially with autoradiography of frozen hydrated preparations and electron microscopy of frozen hydrated cryosections support the view that most of cellular K+ is bound to macromolecules and hence osmotically inactive. These findings suggest that problems of cell volume changes either in the living state or during preparative procedures (e.g. during chemical fixation) can only be understood by a model which takes into account properties of the cytoplasmic matrix and its associated water. Further experimental evidence for such a model is provided by electron micrographs obtained after glutaraldehyde fixation of normal and swollen muscles which are compared with results obtained after freeze-substitution.

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