Attracting Abundance

- 102 -

The Law

Belief - XIV

Our Cellular Biology - XIII

We will now learn how the cell moves.

Cytoskeletal filaments form the basis for movement of cells. Cell movement is a complex phenomenon primarily driven by the actin network beneath the cell membrane, and can be divided into three general components: protrusion of the leading edge of the cell, adhesion of the leading edge and deadhesion at the cell body and rear, and cytoskeletal contraction to pull the cell forward. Each of these steps is driven by physical forces generated by unique segments of the cytoskeleton. This review examines the specific physics underlying these phases of cell movement and the origins of the forces that drive locomotion. 

Cell movement or motility is a highly dynamic phenomenon that is essential to a variety of biological processes such as morphogenesis, wound healing, cancer metastasis (spread of a cancer or other disease from one organ or part of the body to another not directly connected with it) and immune response. For example, during morphogenesis there is a targeted movement of dividing cells to specific sites to form tissues and organs. For wound healing to occur, cells such as neutrophils (white blood cells) and macrophages (cells that ingest bacteria) move to the wound site to kill the microorganisms that cause infection, and fibroblasts (connective tissue cells) move there to remodel damaged structures. In all these examples, cells reach their target by crawling, which is the most common method of cell motion. 

As a cell moves on a substrate (the extracellular matrix if the cell moves inside an organism or a cover slide if it moves outside an organism), it experiences external forces, which include the viscous force or resistance from the surrounding medium and cell-substrate interaction forces, and internal forces that are generated by the cytoskeleton. In most animal cells, the cytoskeleton is the essential component in creating these motility-driving forces.

A cell begins to move in response to an external signal in its surrounding environment. This can be a physical, chemical, diffusible or non-diffusible signal that is detected by receptor proteins located on the cell membrane, and transmitted by them to the cell interior. 

Once cell movement begins, the process, which involves the constant restructuring of the cytoskeleton, can be divided into three stages in most cells. First, a cell propels the membrane forward by orienting and reorganizing (growing) the actin network at its leading edge. Second, it adheres to the substrate at the leading edge and de-adheres (releases) at the cell body and rear of the cell. Finally, contractile forces, generated pull the cell forward. As the extending edge moves forward, the cell constantly monitors the signal direction and tailors its direction of motion accordingly. 

Soon after the leading edge begins to protrude, adhesion molecules gathered in the extending region help attach the leading edge to the substrate. Cell-substrate attachments are created at the leading edge when actin bundles link the cytoskeleton to the substrate at certain sites via adhesion molecules. These attachments prevent the protruding leading edge from retracting. As the cell continues to adhere at the leading edge, it de-adheres at the cell body and rear of the cell.

Finally, the rest of the cell is pulled forward, mainly by contractile forces that are produced by myosin motors sliding on actin filaments, which are in the cell body and at the rear.

Note: Myosins comprise a family of ATP-dependent motor proteins and are best known for their role in muscle contraction and their involvement in a wide range of other motility processes in eukaryote cells. They are responsible for actin-based motility.

All the stages or processes described above are continuously running as the cell moves on the substrate.

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