Definitions of locomotor reactions

Random and directional locomotion

The paths of moving cells may be directional or random. In random locomotion, the different cells of a population move in directions that are random in relation to one another and to the environment, but the individual cells may persist in their initial direction of locomotion for several minutes. This is called a 'persistent random walk'. When observed over long periods, the cells turn so that, over long periods (>5 min), mean square displacement is proportional to time. In directional locomotion the cells show preference for a particular direction, and their morphological axis is oriented in that direction.

Locomotor responses to physical or chemical cues

The environment of the cell may determine both directional and random locomotion. Chemical cues determine reactions as follows.

Chemokinesis Chemokinesis has two forms: orthok-inesis and klinokinesis. In orthokinesis, the speed or frequency of locomotion is determined by the magnitude of the stimulus, i.e. the attractant concentration.

In klinokinesis the amount of turning the cell does is determined by the magnitude of the stimulus. Orthokinesis is thus a straightforward acceleration or deceleration of cells in response to stimuli. It is certainly important in leukocytes. Klinokinesis is of uncertain importance in leukocytes, though, in bacteria such as Escherichia coli, chemicals determine whether the organisms 'tumble' or show straight runs. In the rest of this article, the term 'chemokinesis' will be used synonymously with 'orthokinesis' as defined above.

Chemotaxis Chemotaxis is a reaction by which the direction of locomotion is determined by the direction and magnitude of a chemical stimulus. The stimulus is usually in the form of a concentration gradient. In leukocytes, chemotaxis is always positive, i.e. towards a gradient source. Negative chemotaxis away from a source may exist in other organisms.

Contact guidance Chemotaxis is not the only cue that causes cells to move directionally. Contact guidance is a reaction by which the direction of locomotion is determined by the shape or curvature of the substratum on which the cell moves, i.e. by physical rather than chemical properties of the environment. For example, in aligned matrices of collagen or fibrin, neutrophils and lymphocytes prefer to move in the axis of the aligned fibers rather than across them. Cells are equally free to move in both directions in that axis, i.e. this is not a unidirectional locomotion, like chemotaxis. It is likely to be an important determinant of the ability of leukocytes to accumulate, particularly in tissues with complex patterning.

Locomotor capacity Leukocytes only show the capacity for locomotion at certain stages of their development. Thus myeloid precursors in bone marrow are nonmotile, and lack chemotaxis receptors, but they acquire these as they develop, so that virtually all blood neutrophils and monocytes have a capacity for locomotion and chemotaxis. T lymphocyte precursors may be attracted chemotactically into the thymus, though as they mature, they may lose locomotor capacity. Many lymphocytes in blood and peripheral lymphoid tissues are not motile in the G0 stage of growth, but develop locomotor capacity when the enter the G, phase of cell cycle if cultured in the presence of mitogen or antigen. Obviously locomotor capacity is an essential prerequisite for chemotaxis, chemokinesis, etc.

Haptotaxis The term haptotaxis was introduced by Carter in 1967 to describe directional locomotion on a surface bearing a gradient of changing adhesiveness. It is now used in a different sense to describe chemotaxis on gradients of surface-bound attractants. This is postulated to be important for transmigration of leukocytes across vascular endothelium and through connective tissue matrices towards sites of inflammation. Leukocyte migration requires contact with a surface but is not always dependent on adhesion since all the morphological changes described above occur when chemoattrac-tants are added to cells in suspension. Moreover cells are capable of migrating through three-dimensional matrices without adhering since they can gain purchase by using the meshwork of fibres like a climbing frame. Nevertheless on two-dimensional surfaces, migration must be dependent on adhesion and there is now much evidence that, in sites of inflammation, the adhesion of both leukocytes and vascular endothelial cells is greatly increased.

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