Cell migration is a central process in the development and maintenance of multicellular organisms. Tissue formation during embryonic development, wound healing and immune responses all require the orchestrated movement of cells in particular directions to specific locations. Errors during this process have serious consequences, including mental retardation, vascular disease, tumor formation and metastasis. An understanding of the mechanism by which cells migrate may lead to the development of novel therapeutic strategies for controlling, for example, invasive tumour cells. Cells often migrate in response to specific external signals, including chemical signals and mechanical signals. Due to the highly viscous environment (low Reynolds number), cells need to permanently produce forces in order to move. Cells achieve active movement by very different mechanisms. Many less complex prokaryotic organisms (and sperm cells) use flagella or cilia to propel themselves. Eukaryotic cell migration typically is far more complex and can consist of combinations of different migration mechanisms. It generally involves drastic changes in cell shape which are driven by the cytoskeleton. Two very distinct migration scenarios are crawling motion (most commonly studied) and blebbing motility.
Studying cell migration (crawling cell migration) 
The migration of single mammalian cells is usually viewed in the microscope as the cells move randomly on a glass slide. As the actual movement is very slow — usually a few micrometers/minute — time-lapse films are taken so that a sped up movie can be viewed. This shows that, although the shape of a moving cell varies considerably, its leading front has a characteristic behaviour. This region of the cell is highly active, sometimes spreading forward quickly, sometimes retracting, sometimes ruffling or bubbling. It is generally accepted that the leading front is the main motor that pulls the cell forward.
Common features 
- cytoplasmic displacement at leading front
- laminar removal of dorsally-accumulated debris toward trailing end
The latter feature is most easily observed when aggregates of a surface molecule are cross-linked with a fluorescent antibody (see cap formation) or when small beads become artificially bound to the front of the cell.
Other eukaryotic cells are observed to migrate similarly. The amoeba Dictyostelium discoideum is useful to researchers because they consistently exhibit chemotaxis in response to cyclic AMP; they move more quickly than cultured mammalian cells; and they have a haploid genome that simplifies the process of connecting a particular gene product with its effect on cellular behaviour.
Molecular processes at the front 
There are two main theories for how the cell advances its front edge: the cytoskeletal model and membrane flow model. It is possible that both underlying processes contribute to cell extension.
Cytoskeletal model (A) 
Through experiment, it is found that the cell's front is a site of rapid actin polymerisation: soluble actin monomers polymerise there to form filaments. This has led to the view that it is the formation of these actin filaments, which pushes the leading front forward and is the main motile force for advancing the cell’s front. In addition, cytoskeletal elements are able to interact extensively and intimately with a cell's plasma membrane.
Membrane flow model (B) 
Studies have also shown that the front is the site at which membrane is returned to the cell surface from internal membrane pools at the end of the endocytic cycle. This has led to the view that extension of the leading edge occurs primarily by addition of membrane at the front of the cell. If so, the actin filaments that form at the front might stabilize the added membrane so that a structured extension, or lamella, is formed rather than the cell's blowing bubbles (or "blebs") at its front. For a cell to move, it is necessary to bring a fresh supply of "feet" (those molecules called integrins, which attach a cell to the surface on which it is crawling) to the front. It is likely that these feet are endocytosed toward the rear of the cell and brought to the cell's front by exocytosis, to be reused to form new attachments to the substrate.
Polarity in migrating cells 
Migrating cells have a polarity—a front and a back. Without it, they would move in all directions at once, i.e. spread. How this arrow is formulated at a molecular level inside a cell is unknown. In a cell that is meandering in a random way, the front can easily give way to become passive as some other region, or regions, of the cell form(s) a new front. In chemotaxing cells, the stability of the front appears enhanced as the cell advances toward a higher concentration of the stimulating chemical. This polarity is reflected at a molecular level by a restriction of certain molecules to particular regions of the inner cell surface. Thus, the phospholipid PIP3 and activated Rac and CDC42 are found at the front of the cell, whereas Rho GTPase and PTEN are found toward the rear.
It is believed that filamentous actins and microtubules are important for establishing and maintaining a cell’s polarity. Drugs that destroy actin filaments have multiple and complex effects, reflecting the wide role that these filaments play in many cell processes. It may be that, as part of the locomotory process, membrane vesicles are transported along these filaments to the cell’s front. In chemotaxing cells, the increased persistence of migration toward the target may result from an increased stability of the arrangement of the filamentous structures inside the cell and determine its polarity. In turn, these filamentous structures may be arranged inside the cell according to how molecules like PIP3 and PTEN are arranged on the inner cell surface. And where these are located appears in turn to be determined by the chemoattractant signals as these impinge on specific receptors on the cell’s outer surface.
Although microtubules have been known to influence cell migration for many years, the mechanism by which they do so has remained controversial. On a planar surface, microtubules are not needed for the movement, but they are required to provide directionality to cell movement and efficient protrusion of the leading edge. When present, microtubules retard cell movement when their dynamics are suppressed by drug treatment or by tubulin mutations. However, within extracellular matrix, microtubules are required for both directional and random movement.
See also 
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- http://www.microscopyu.com/moviegallery/livecellimaging/mdbk/index.html/ movie of MDBK cells moving over a glass slide
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- Cell Migration Gateway The Cell Migration Gateway is a comprehensive and regularly updated resource on cell migration
- Video tour of cell motility
- CellMotus Cell motility game
- Time-lapse cytometry A collection of videos showing proliferating cell cultures