For sea urchin sperm, finding an egg to fertilize in a vast ocean might seem like looking for a needle in a haystack. But the prickly creatures have devised a way to shorten their search: The eggs release a chemical homing beacon to help guide the sperm. Now, scientists from the Center of Advanced European Studies and Research in Bonn, Germany, have homed in on just how the sperm use this "chemotaxis" to navigate, the team reports this week in The Journal of Cell Biology. Scientists knew that the eggs of the sea urchin (Arbacia punctulata) release a small molecule called resact, which binds to receptor proteins on a sperm's tail, or flagellum. That allows calcium ions to enter the cell, and the increase in calcium controls how the sperm's flagellum beats, causing sperm to make either sharp turns or slow bends. To better understand the navigation mechanism, the team placed sea urchin sperm in tiny chambers, added a modified version of resact at precise time intervals, and recorded videos of their flagellum movements and of the calcium influx simultaneously. Rather than setting off immediately, the team found, the sperm first test the waters—sampling the resact for 0.2 to 0.6 seconds before determining the right way to go—in the direction of highest concentration. This sperm navigation system might be used by other species, the authors suggest, and their experimental tool provides a template for future studies of chemotaxis in other species, including humans.
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