Working model for adhesion assembly, turnover and maturation (a) During protrusion, adhesions initially assemble as punta (blue circle) in the lamellipodium (gray band); their formation is driven by or linked to actin polymerization. Following assembly, these nascent adhesions remain small and stable within the lamellipodium. The nascent adhesions turnover (clear circle) when the depolymerizing dendritic at the rear of the lamellipodium passes by them; this links the stability of these adhesions to the integrity of the dendritic actin. The formation and turnover of nascent adhesions do not require myosin II activity. In addition, nascent adhesions can grow along an actin template (maturing adhesions), which elongates centripetally at the lamellipodium-lamellum interface. The cross-linking activities of both myosin II and α-actinin, possibly in conjunction with contraction, are critical for the initial elongation of adhesions, and α-actinin is also required for the proper positioning of adhesions on actin filaments. Working synergistically with contraction, the cross-linking of actin by myosin II and α-actinin mediate further development and maturation of the adhesions.(b) Quantitative analysis of the mathematical model of actin and adhesion assembly in the lamellipodium predicts accurately the dynamic and exclusive nature of nascent adhesions in the lamellipodium in migrating cells. Top plot, mathematical model; Bottom plot, representative experimental result. V = protrusion rate; L = width of the actin branching zone; X = distance from the front to the rear; τ = time lag. See the , Materials section for a detailed description of the model, including its assumptions, governing equations, and solutions.