The crawl­ing move­ment of cells is dri­ven by the con­tin­u­ous reor­ga­ni­za­tion and turnover of the actin cytoskele­ton. Two abil­i­ties of actin fil­a­ments are exploited by the cell in order to move: the abil­ity to push by poly­mer­iza­tion and the abil­ity to con­tract by inter­act­ing with myosin. Actin poly­mer­iza­tion dri­ves the exten­sion of sheet-like and rod-like pro­tru­sions at the cell front, termed respec­tively lamel­lipo­dia and filopo­dia. Behind the pro­trud­ing front actin inter­acts with myosin to form con­trac­tile arrays that drive the translo­ca­tion of the trail­ing cell body (see Push­ing and Pulling).

Schematic rep­re­sen­ta­tion of the actin cytoskele­ton in a polarised fibrob­last. The dif­fer­ent organ­i­sa­tional forms of actin fil­a­ments are depicted: diag­o­nal actin fil­a­ment mesh­work in the lamel­lipodium, with asso­ci­ated radial bun­dles (pro­ject­ing filopodium and non-projecting microspike); con­trac­tile bun­dles of actin (stress fiber) in the cell body and at the cell edge; and a loose actin net­work through­out the cell. Arc-shaped bun­dles are some­times observed that move inwards under the dor­sal cell sur­face (arc). The dia­gram shows an ide­al­ized cell; in real­ity the actin arrays are inter­con­nected in var­i­ous com­bi­na­tions and geome­tries. Sites of adhe­sion of the cell with the sub­strate are also indi­cated, in red. The flat region behind the lamel­lipodium and in front of the nucleus (N) has been termed the lamella. At the cell front, in lamel­lipo­dia and filopo­dia, actin fil­a­ments are all polar­ized in one direc­tion, with their fast poly­mer­iz­ing ends for­wards (for push­ing); in the body of the cytoskele­ton, actin fil­a­ments form bipo­lar assem­blies with myosin to form con­trac­tile arrays (for retract­ing).

Flu­o­res­cence micro­scope images of the actin cytoskele­tons of dif­fer­ent cell types revealed by labelling with flu­o­res­cent phal­loidin. A, fish ker­a­to­cyte; B, chick embryo fibrob­last; C, mouse fibrob­last spread on polyly­sine with­out serum; D, fish fibrob­last imaged by Struc­tured Illu­mi­na­tion Microscopy (SIM).

The dif­fer­ent parts of the actin cytoskele­ton can be visu­alised in the elec­tron micro­scope after extract­ing cells with deter­gents to remove mem­branes, organelles and sol­u­ble com­po­nents of the cyto­plasm (Fig­ures 2-3A-D). In the exam­ples shown the cytoskele­ton fil­a­ments have been con­trasted by the neg­a­tive stain­ing method (Small and Celis, 1978; Small, 1981; Small et al., 1982).

Elec­tron micro­graph of a fibrob­last that was grown on a thin, plas­tic sup­port film and extracted with deter­gent to reveal the cytoskele­ton. Parts of the actin cytoskele­ton are marked: (Lp) lamel­lipodium; (Fp) filopodium; (Sf) stress fibre.

Details of dif­fer­ent parts of the actin cytoskele­ton fibrob­lasts pre­pared as above. The cytoskele­tons were dried in a heavy metal stain to reveal fil­a­ments in neg­a­tive con­trast.

Exam­ples of the actin fil­a­ment net­work in lamel­lipo­dia, with (top) some fil­a­ments merg­ing into a bun­dle. (Bot­tom) The image was obtained using elec­tron tomog­ra­phy and includes 40 slices of the tomo­gram (see sec­tion Elec­tron Tomog­ra­phy)

Bun­dles of actin form­ing filopo­dia. (Bot­tom) was imaged using elec­tron tomog­ra­phy.

A region deeper in the cyto­plasm that includes stress fibre bun­dles of actin, a back­ground net­work of actin fil­a­ments, micro­tubules, and inter­me­di­ate fil­a­ments.

Related Pub­li­ca­tions

  • Small, J.V., and Celis, J.E. (1978): Fil­a­ment arrange­ments in neg­a­tively stained cul­tured cells: the orga­ni­za­tion of actin. Cyto­bi­olo­gie 16, 308325. PDF
  • Small, J.V. (1981). Orga­ni­za­tion of actin in the lead­ing edge of cul­tured cells: influ­ence of osmium tetrox­ide and dehy­dra­tion on the ultra­struc­ture of actin mesh­works. J. Cell Biol. 91, 695705. PDF
  • Small, J.V., Rin­nerthaler, G, Hinssen, H. (1982). Orga­ni­za­tion of actin mesh­works in cul­tured cells: the lead­ing edge. Cold Spring Harb Symp Quant Biol. 46, 599611. PDF