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, 1988Small 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.

Related Pub­li­ca­tions

  • 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, 355361.
  • Small, J.V. (1988). The actin cytoskele­ton. Elec­tron Microsc. Rev. 1, 155174 PDF
  • Small, J.V., and Celis, J.E. (1978a): Fil­a­ment arrange­ments in neg­a­tively stained cul­tured cells: the orga­ni­za­tion of actin. Cyto­bi­olo­gie 16, 308325. PDF
  • 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, 393409. PDF

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