Unlike most sperm cells, which swim with a flagellum, nematode sperm crawl with a pseudopod. This sperm cell's "body" (lower right) is stuck down on a glass slide, so it can't move forward, but the pseudopod is trying to crawl anyway. Membrane continuously flows from the tip of the pseudopod rearward to the base where it joins with the cell body; as the membrane flows rearward, it would normally push the cell forward (see crawling sperm), in the direction of the pseudopod. Within the pseudopod you can see a branched cytoskeletal network. Unlike almost all other cytoskeletons found in nature, this one does not contain the proteins actin or tubulin. Instead, it is a polymer of the Major Sperm Protein (MSP). MSP filaments assemble at the leading edge of the pseudopod and disassemble at the base. This "treadmilling" of the cytoskeleton is thought to generate the force that makes the pseudopod membrane flow.
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