Cell Movement

Other Mechanisms of Cell Movement

Actin filaments, the cytoskeletal protein of cells, found inside lamellipodia and filopodia, the cellular appendages composed of actin filaments, enable cells to move.

Instead of using the direct, powered motion provided by cilia or flagella, some cells appear to slowly crawl over flat surfaces. Examples of this behavior include skin cells moving to the site of damaged skin to repair the wound, cancer cells breaking off from the primary tumor to crawl to the bloodstream in order to affect other parts of the body, and immune cells penetrating blood vessel walls to crawl toward the site of an infection. When these cells determine they need to migrate, they channel the cytoplasm inside the cell in that direction and create a pseudopod. A pseudopod is a cytoplasmic extension that is used for location in certain cells and is called a "false foot." Phagocytes, cells that surround and digest pathogens in the blood, use pseudopodia to trap their prey. A phagocyte extends two pseudopodia in opposite directions so they surround the invader. The pseudopodia fuse when on the other side, creating a bubble called a vesicle that traps the prey within the phagocyte. This maintains the internal continuity of the phagocytic membrane. Single-celled organisms, such as species of Amoeba, also use pseudopodia to move and capture prey.

Found on the leading edge, the edge of the cell that is at the front of the motion of the moving pseudopods, are structures called lamellipodia. Each lamellipodium (plural, lamellipodia) is a dense network of actin filaments that face out toward the cell membrane. These are common in cells that move, such as phagocytes surrounding and digesting bacterial cells. The actin filaments provide the force that pushes the cell membrane out from the leading edge, providing the mechanism needed to move the cell. Studies have shown that lamellipodia can form and move on their own, without direct input from the cell's nucleus.

Another structure that is important to the movements and migration of cells is called a filopodium. A filopodium (plural, filopodia) is a thin extension of the lamellipodia that contains actin and serves as an antenna to collect information as the cell moves through its environment. In addition to cell migration and movement, filopodia are also involved with embryonic development and the healing of wounds. Filopodia also play a role in the binding of the cell membrane to the extracellular matrix, the microtubules and microfilaments that are found outside of the cell and help to hold the cell in place. When filopodia come into contact with a solid object, such as the cell wall of a plant or bacterial cell or a piece of debris, the rapidly forming actin filaments in a filopodium enable the filopodium to produce a backward motion. Once clear of the obstruction, the cell is then propelled forward.

Lamellipodia and Filopodia

Lamellipodia are extensions of the cell membrane that use the contraction of actin filaments to move the cell. Filopodia are sensory extensions of the lamellipodia that are also made from actin.