Morphological events of leukocyte locomotion

Leukocytes which are unstimulated are spherical and immotile. This is their morphology both in the bloodstream and after careful in vitro preparation. Following stimulation with a chemoattractant, the cells change shape within a minute or so by extending a pseudopod, or lamellipodium, at one-pole. Slow contraction waves may then pass anteroposterior^ down the cell from this point. The cell thus adopts a typical, tapered, locomotor morphology (Figure 1). The ruffled anterior lamellipodium is initially hyaline and is rich in actin and actin-associated proteins but lacking in organelles. Behind it is the organelle-rich cell body with a posterior tail (uropod) which forms after the complete passage of a contraction from front to back. All these changes occur whether the cell is in suspension or on a surface. Surfaces, either two-dimensional or three-

Figure 1 A neutrophil migrating on a surface. Note the broad, ruffled anterior lamelli-podium which is free of organelles. Behind this is the cell body containing the nucleus and cytoplasmic organelles. The cell is 'wai-sted' about a third of the way down. Observation of moving cells shows that this is due to a constriction which passes from front to back as the cell moves forward. A posterior tail is also seen. Special techniques reveal that the anterior lamellipodium is rich in actin and actin-associated protein and that cell surface receptors are frequently concentrated at this pole (not shown in this picture). (Photography by courtesy of Dr Wendy Haston).

dimensional (such as collagen or fibrin gels) give purchase for locomotion. Similar changes are seen in neutrophils, macrophages, lymphocytes and eosinophils. They are initiated by binding of 'chemotactic' factors to cell surface receptors, and all of these cells show chemotaxis, though the neutrophil is the best studied. The term chemotactic factor is imprecise since such factors also activate chemokinesis (below) and other functions such as adhesion, oxidative metabolism, secretion, etc.

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