Consider the leading edge of a cell. What specifically is occurring at the leading edge to push the membrane forward? Why is the band of F-actin seen at the leading edge discrete; in other words, why is it only ~1 µm in width?
I know that branched microfilaments assemble at the leading edge with the (+) end facing the plasma membrane. The actin cytoskeleton is fixed with respect to the surface that the cell resides on, I believe this is accomplished by integrins at focal adhesions, and that the growing filament therefore pushes on the membrane, extending it forward. What I'm not sure about is how and why it maintains a constant width.
Can cell crawling occur in solution or is a substrate required?
I think the answer is that substrate is required, but I want to double check. I'm aware of other mechanisms of motility, for example using flagella. Am I correct in assuming that 'cell crawling' refers specifically to this type of movement, which involves lamellipodia and requires adhesion to a surface?