Our appreciation and understanding of living processes have been considerably enhanced by the development of probes and instrumentation that allow the localisation of proteins in living cells. Perhaps the most dramatic general finding that has emerged from the use of current imaging technology is the dynamism of structural assemblies in cells. Cell shape is formed by three primary polymer systems that make up the “cytoskeleton”: the actin filaments, intermediate filaments and microtubules.
Schematic representation of the cytoskeleton of a cell in tissue culture. The three main polymer systems are shown: (red) the actin filament arrays; (blue) the microtubules and (green) the intermediate filaments.
The three polymer systems of a fibroblast cytoskeleton, as seen in the fluorescence microscope after fixation and labelling with specific probes. (left) the actin cytoskeleton, labelled with fluorescent phalloidin; (middle) the microtubule cytoskeleton labelled with an antibody to tubulin; and (right) the intermediate filment cytoskeleton labelled with antibodies to the intermediate filament protein, vimentin. (Herzog et al., 1994).
The three cytoskeleton filaments in the electron microscope. The cytoskeleton can be visualised in the electron microscope after extracting cells with detergents to remove membranes, organelles and soluble components of the cytoplasm. In the examples shown here the cytoskeleton filaments have been contrasted by the negative staining method. Negative staining entails the drying of the preparation in a heavy metal salt (typically of tungsten or uranium) from an aqueous solution. The cell structures then appear light against the surrounding electron dense stain. (left) Electron micrograph of the peripheral region of the cytoskeleton of a B16 melanoma cell showing microtubules (25nm diameter) in a loose network of actin filaments (8nm diameter). The cell was extracted in a mixture of glutaraldehyde and detergent and negatively stained (Small, 1988; Small and Celis, 1978a). (right) Electron micrograph of intermediate filaments (10nm diameter) in an epithelial cell. To reveal the intermediate filaments the cell was first extracted with detergent and then in a concentrated salt solution to remove actin filaments and microtubules (Small and Celis, 1978b).
Far from being static, as the name would suggest, these polymer systems undergo continual turnover and rearrangement. This continual reorganisation is the essence of life itself as it underlies the changes in cell morphology and migration that shape the organism.
- Herzog, M., Draeger, A., Ehler, E. and Small, J.V. (1994). Immunofluorence Microscopy of the Cytoskeleton: Double and Triple Immunofluorescence. Cell Biology A Laboratory Handbook, Vol.2, 355–361.
- Small, J.V. (1988). The actin cytoskeleton. Electron Microsc. Rev. 1, 155–174
- Small, J.V., and Celis, J.E. (1978a): Filament arrangements in negatively stained cultured cells: the organization of actin. Cytobiologie 16, 308–325.
- Small, J.V., and Celis, J.E. (1978b): Direct visualization of the 10-nm (100 Å)-filament network in whole and enucleated cultured cells. J. Cell Sci. 31, 393–409.