Diagram of regulation of PMN motility on fibronectin.

In panel A, α5β1 integrins (cup shapes on stems) at the leading edge form tight attachments to the fibronectin substrate (filled ovals on stems). During migration, PMNs move forward over the attachment sites while forming new attachments toward the front and releasing old ones toward the rear. The released integrins near the rear are internalized into endocytic vesicles (open circles inside cell) then delivered via the ERC (gray freeform inside cell) toward the front of the cell. Eventual delivery of integrins to attachment sites occurs by an unknown mechanism. When intracellular Ca⁺⁺ is buffered (panel B), old attachments cannot be released causing α5β1 integrins to accumulate at the back of the cell and become depleted from endocytic compartments.