MECHANICAL MODELS OF PSEUDOPOD FORMATION

The active locomotion of polymorphonuclear leukocytes into a glass pip ette has been recently reported using formyl-methionyl-leucyl-phenylal anine (fMLP) as a chemoattractant. The frontal portion of the leukocyt e appears clear and free of granules as observed in pseudopod formatio n. Three possible mechanisms for pseudopod formation are considered: ( 1) pressure-flow generated by actin-myosin contraction at the rear of the cell or at the base of the pseudopod; (2) osmotic pressure generat ed at the cell membrane, interior to the cell; and (3) actin polymeriz ation of the cell membrane at the leading edge of the pseudopod. Exper imental data on the movement of F-actin toward the rear of the cell, a way from the front, favors polymerization of G-actin to F-actin at the leading edge. The active role of osmotic pressure and contraction at the base of the pseudopod are possible but not yet proven.