How and Why Cells Move

Why Do Cells Move?

Cell movement is required for a number of activities to occur within the body. White blood cells, such as neutrophils and macrophages must quickly migrate to sites of infection or injury to fight bacteria and other germs. Cell motility is a fundamental aspect of form generation (morphogenesis) in the construction of tissues, organs and the determination of cell shape. In cases involving wound injury and repair, connective tissue cells must travel to an injury site to repair damaged tissue. Cancer cells also have the ability to metastasize or spread from one location to another by moving through blood vessels and lymphatic vessels. In the cell cycle, movement is required for the cell dividing process of cytokinesis to occur in the formation of two daughter cells.

Steps of Cell Movement

Cell motility is accomplished through the activity of cytoskeleton fibers. These fibers include microtubules, microfilaments or actin filaments and intermediate filaments. Microtubules are hollow rod-shaped fibers that help support and shape cells. Actin filaments are solid rods that are essential for movement and muscle contraction. Intermediate filaments help stabilize microtubules and microfilaments by keeping them in place. During cell movement, the cytoskeleton disassembles and re-assembles actin filaments and microtubules. The energy required to produce movement comes from adenosine triphosphate (ATP). ATP is a high energy molecule produced in cellular respiration.

Steps of Cell Movement

Cell adhesion molecules on cell surfaces hold cells in place to prevent undirected migration. Adhesion molecules hold cells to other cells, cells to the extracellular matrix (ECM) and the ECM to the cytoskeleton. The extracellular matrix is a network of proteins, carbohydrates and fluids that surround cells. The ECM helps to position cells in tissues, transport communication signals between cells and reposition cells during cell migration. Cell movement is prompted by chemical or physical signals that are detected by proteins found on cell membranes. Once these signals are detected and received, the cell begins to move. There are three phases to cell movement.

• In the first phase, the cell detaches from the extracellular matrix at its foremost position and extends forward.
• In the second phase, the detached portion of the cell moves forward and re-attaches at a new forward position. The rear portion of the cell also detaches from the extracellular matrix.
• In the third phase, the cell is pulled forward to a new position by the motor protein myosin. Myosin utilizes the energy derived from ATP to move along actin filaments, causing cytoskeleton fibers to slide along one another. This action causes the entire cell to move forward.

The cell moves in the direction of the detected signal. If the cell is responding to a chemical signal, it will move in the direction of the highest concentration of signal molecules. This type of movement is known as chemotaxis.

Movement Within Cells

Not all cell movement involves the repositioning of a cell from one place to another. Movement also occurs within cells. Vesicle transportation, organelle migration, and chromosome movement during mitosis are examples of types of internal cell movement.

Vesicle transportation involves the movement of molecules and other substances into and out of a cell. These substances are enclosed within vesicles for transportation. Endocytosis, pinocytosis, and exocytosis are examples of vesicle transportation processes. In phagocytosis, a type of endocytosis, foreign substances and unwanted material are engulfed and destroyed by white blood cells. The targeted matter, such as a bacterium, is internalized, enclosed within a vesicle, and degraded by enzymes.

Organelle migration and chromosome movement occur during cell division. This movement ensures that each replicated cell receives the appropriate complement of chromosomes and organelles. Intracellular movement is made possible by motor proteins, which travel along cytoskeleton fibers. As the motor proteins move along microtubules, they carry organelles and vesicles with them.

Cilia and Flagella

Some cells possess cellular appendage-like protrusions called cilia and flagella. These cell structures are formed from specialized groupings of microtubules that slide against one another allowing them to move and bend. Compared to flagella, cilia are much shorter and more numerous. Cilia move in a wave-like motion. Flagella are longer and have more of a whip-like movement. Cilia and flagella are found in both plant cells and animal cells.

Sperm cells are examples of body cells with a single flagellum. The flagellum propels the sperm cell toward the female oocyte for fertilization. Cilia are found within areas of the body such as the lungs and respiratory system, parts of the digestive tract, as well as in the female reproductive tract. Cilia extend from the epithelium lining the lumen of these body system tracts. These hair-like threads move in a sweeping motion to direct the flow of cells or debris. For example, cilia in the respiratory tract help to propel mucus, pollen, dust, and other substances away from the lungs.

_Sources:

• _{Lodish H, Berk A, Zipursky SL, et al. Molecular Cell Biology. 4th edition. New York: W. H. Freeman; 2000. Chapter 18, Cell Motility and Shape I: Microfilaments. Available from: http://www.ncbi.nlm.nih.gov/books/NBK21530/}
• _{Ananthakrishnan R, Ehrlicher A. The Forces Behind Cell Movement. Int J Biol Sci 2007; 3(5):303-317. doi:10.7150/ijbs.3.303. Available from http://www.ijbs.com/v03p0303.htm}