The cytoskeleton of human polymorphonuclear leukocytes: phagocytosis and degranulation

Current evidence indicates that polymorphonuclear leukocyte (PMN) chemotaxis and phagocytosis are effected by an actin-myosin contractile system. However, the structural relationship of the contractile cytoskeleton to cell motility is still in question. In addition, while evidence suggests that microtubules are responsible for orientation during chemotaxis, the role of microtubules in degranulation is unresolved. To determine the organizational relationship between these cytoskeletal elements and phagocytosis, we examined whole-mount preparations of PMNs engulfing bacteria. These preparations were examined in the transmission electron microscope (EM) and photographed as stereo pairs. Two important observations were made. First, there was an increased density of cytoskeletal elements in the pseudopod surrounding bacteria. Second, microtubule elements were intimately associated with lysosomal granules, vesicles, and phagosomes. Lysosomal granules and vesicles aligned along microtubules and clustered around phagosomes. This suggests that the microtubules may provide a tracking mechanism whereby lysosomes are specifically parceled out to phagocytic vacuoles. These results also suggest that phagocytosis and degranulation may involve different effector mechanisms.