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Locomotion does not only occur in single-celled organisms. Wound healing, blood clots and the elimination of intruders by the immune system are processes in the human organism in which mobile cells are actively involved - as well as disease processes such as the arteriosclerotic occlusion of blood vessels or the formation of metastases from tumor cells. Surface structures of microorganisms, injured and infected tissues, but also growth factors or thrombin serve as signals for cell migration. While cancer cells migrate relatively slowly (0.1 to 1 per hour), other cells such as the neutrophils of the immune system move at up to 30 relatively fast per minute.
Cells have a solar-like inner area, which is surrounded by a gel-like cortex. When moving, the cortex appears to flow out, and a transparent extension of the cell, the lamello podium, is created. Membrane or cell adhesion molecules attach it to the pad so that the cell has enough pull to move forward. Then the appendix detaches from the pad and the process begins again.
The formation of protrusions of the cell is based on the fact that the cortex becomes thinner in the direction of movement and the hydrostatic pressure creates a kind of "bulge" of the cell. The contents of this bulge immediately turn into a gel state and become transparent. The proteins actin and myosin are primarily involved in locomotion. Actin makes up 10% of the total cell protein in neutrophils, and even 20% in blood platelets, and occurs predominantly in the cortex and lamellopodia. It is significantly involved in the transition from the sol to the gel state in that the filaments form three-dimensional structures. The actin-binding protein (ABP), which links actin strands at right angles, ensures the three-dimensional crosslinking of the actin in the gel.
Actin-crosslinking proteins have so far been detected in many cell types, including the proteins fimbrin, α-actinin and filamine. The reverse process, the transition from the gel to the sol state, is carried out by Ca2+-dependent actin fragmentation proteins, to which e.g. the proteins gelsolin or villin belong.