Abstract Computer‐assisted methods have been employed to obtain a high resolution description of pseudopod expansion, cellular translocation, and the subcellular dynamics of MSP fiber complexes in the motile sperm of the nematode Ascaris suum . Although Ascaris sperm translocating in a straight line or along a curved path do not retract their pseudopod or significantly alter pseudopod shape, they move in a cyclic fashion, with an average period between velocity peaks of 0.35 × 0.05 min, which is independent of the forward velocity of sperm translocation. Expansion is confined to a central zone at the distal edge of the pseudopod for sperm translocating in a straight line and to a left‐handed or right‐handed lateral zone in the direction of turning, for sperm translocating along a curved path. For cells translocating in a straight line, the branch points and kinks of MSP fiber complexes move in a retrograde direction in relation to the substratum at an average velocity of 11 μm per min which is independent of the forward velocity of sperm translocation. The distal (anterior) end of a fiber complex, however, moves distally at the speed of sperm translocation when it emanates from the expansion zone, but when it is displaced to a nonexpanding surface of the pseudopod, it stops moving distally. When a cell is anchored to the substratum and is, therefore, nonmotile, the velocity of fiber complexes moving in a retrograde direction doubles. The unique aspects of pseudopod and MSP fiber complex dynamics in Ascaris are compared to the dynamics of pseudopod formation and actin filament dynamics in traditional actin‐based amoeboid cells, and the treadmill model for MSP polymerization is reassessed in light of the discovery that fiber complex branch points move proximally (posteriorly) at a fixed rate.
Sperm of the nematode, Ascaris suum, crawl using lamellipodial protrusion, adhesion and retraction, a process analogous to the amoeboid motility of other eukaryotic cells. However, rather than employing an actin cytoskeleton to generate locomotion, nematode sperm use the major sperm protein (MSP). Moreover, nematode sperm lack detectable molecular motors or the battery of actin-binding proteins that characterize actin-based motility. The Ascaris system provides a simple ‘stripped down’ version of a crawling cell in which to examine the basic mechanism of cell locomotion independently of other cellular functions that involve the cytoskeleton. Here we present a mechanochemical analysis of crawling in Ascaris sperm. We construct a finite element model wherein (a) localized filament polymerization and bundling generate the force for lamellipodial extension and (b) energy stored in the gel formed from the filament bundles at the leading edge is subsequently used to produce the contraction that pulls the rear of the cell forward. The model reproduces the major features of crawling sperm and provides a framework in which amoeboid cell motility can be analyzed. Although the model refers primarily to the locomotion of nematode sperm, it has important implications for the mechanics of actin-based cell motility. Movies available on-line.
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