Filopodia of macrophages.

The filopodia are slender cytoplasmic projections, similar to lamellipodia, which extend from the leading edge of migrating cells. They contain actin filaments cross-linked into bundles by actin-binding proteins, e.g. fimbrin (Hanein et al, 1997). Filopodia form focal adhesions with the substratum, linking it to the cell surface (Lodish et al, 2004). A cell migrates along a surface by extending filopodia at the leading edge. The filopodia attach to the substratum further down the migratory pathway, then contraction of stress fibres retracts the rear of the cell to move the cell forwards. Cells in culture, stained for keratin (red) and DNA (green) The cell is the structural and functional unit of all living organisms, and are sometimes called the building blocks of life. ...

The activation Rho family of small Ras-related GTPases and their downstream intermediates results in the construction of actin fibers (Ohta et al, 1999). Growth factors bind to receptor tyrosine kinases resulting in the polymerization of actin filaments, which cross-linked, make up the supporting cytoskeletal elements of filopodia. Rho activity also results in the activation of the phosphorylation of the ezrin-moesin-radixin group promoting the binding of actin filaments to the filopodia membrane (Ohta et al, 1999). GTPases are a large family of enzymes that can bind and hydrolyze GTP. The GTP binding and hydrolysis takes place in the highly conserved G domain common to all GTPases. ... Polymerization is a process of reacting monomer molecules together in a chemical reaction to form linear chains or a three-dimensional network of polymer chains [1]. There are many forms of polymerization and different systems exist to categorize them. ... Actin (red) profilin (blue) complex. ... The cytoskeleton is a cellular scaffolding or skeleton contained, as all other organelles, within the cytoplasm. ... Actin (red) profilin (blue) complex. ...

To close a wound in vertebrates, growth factors stimulate the formation of filopodia in fibroblasts to direct fibroblast division and close the wound (Bentley et al, 1986). In developing neurons, filopodia extend from the growth cone at the leading edge. In neurons deprived of filopodia by the removal of actin filaments, growth cone extension continues as normal but direction of growth is disrupted and highly irregular (Bentley et al, 1986). A fibroblast is a cell that makes the structural fibers and ground substance of connective tissue. ... A fibroblast is a cell that makes the structural fibers and ground substance of connective tissue. ... Neurons (also called nerve cells) are the primary cells of the nervous system. ... Actin (red) profilin (blue) complex. ...

References

• Molecular Cell Biology Fifth Edition Lodish, Berk, Matsudaira, Kaiser, Krieger, Scott, Zipursky, Darnell. pg. 821, 823 2004 by W.H. Freeman and Company.
• Bentley, D. and Toroian-Raymond, A. 1986.
  Disoriented pathfinding by pioneer neurone growth cones deprived of filopodia by cytochalasin treatment Nature. 323: 712-715
• Ohta Y., Suzuki N., Nakamura S., Hartwig J.H., Stossel, T.P. 1999.
  The small GTPase RalA targets filamin to induce filopodia Proc Natl Acad Sci. 96: 2122-2128.
• Hanein, D., Matsudaira, P., and DeRosier, D.J. 1997.
  Evidence for a Conformational Change in Actin Induced by Fimbrin (N375) Binding J. Cell Biol. 139: 387-396