FIGURE 17-39 Steps in cell locomotion. Movement begins with the extension of one or more lamellipodia from the leading edge of the cell 1; some lamellipodia adhere to the substratum by focal adhesions 2. Then the bulk of the cytoplasm in the cell body flows forward due to contraction at the rear of the cell 3. The trailing edge of the cell remains attached to the substratum until the tail eventually detaches and retracts into the cell body. During this cytoskeleton-based cycle, the endocytic cycle internalizes membrane and integrins at the rear of the cell and transports them to the front of the cell (arrows) for reuse in making new adhesions 4.