Cell locomotion is undoubtedly complex, requiring coordinated activity of cytoskeletal, membrane and adhesion systems. Actin filaments themselves are likely to be involved in multiple force generating mechanisms. To begin a molecular analysis we need to dissect migration into sub-types of motility. Sub-Types of Motility in a Migrating Cell Forward motility of the membrane at the front of the cell is called protrusion. It has been intensively studied over the past few years, and is probably the aspect of migration for which we are closest to uncovering the molecular basis of force generation.
Adhesion is required for protrusion to be converted into movement along the substrate.
The process leading to forward movement of the nucleus and cell body we have termed traction . This is arguably the most important type of motility for generating overall migration, and also the least well understood in either mechanical or molecular terms. Whether this step is actively motile depends on the cell type. Strongly adhesive cells such as cultured fibroblasts tend to have a strongly adherent, extended tail and leave behind a trail of cytoplasmic fragments as they move. In weakly adhesive, fast moving cells such as amoebae and white blood cells, the tail is more rounded and this step is more efficient.