How Hormone Puts a Kick in the Sperm’s Tail

It’s exhausting being a sperm. Having made the long-distance swim up the fallopian tube, a sperm must then rev up its tail to propel itself through the thick jelly-like coating of the egg. The female hormone progesterone, released by the egg, prompts the tail to switch from a smooth swimming motion to a frantic flicking, but exactly how has been puzzling. Researchers have now shown that the hormone acts directly on a sperm surface protein, a discovery that may suggest new nonhormonal contraceptives.

For 10 years, researchers have suspected that progesterone, which the egg releases in huge quantities, is responsible for the asymmetrical, whiplike tail movements that give sperm enough torque to penetrate the ovum. Because sperm respond to progesterone within seconds, scientists reasoned that the hormone must bind to a surface protein and not one within the cells, which would take longer for the progesterone to reach.

In 2001, researchers hoped they had found the progesterone receptor when they discovered that infertile men and mice sometimes had mutations that disrupted a protein, called CatSper, which ferries calcium ions in and out of sperm. This so-called calcium channel is found exclusively within sperms’ tails, but working out whether it responds to progesterone proved a thornier exercise than expected. Sperm are not easy cells to work with—for one thing, they don’t stay still.

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