Sperm are a lively bunch to begin with, but they become even more excited when they zero in on an egg. They swim faster and their tail movements become more forceful and erratic.
Now scientists have linked this behavior, called "hyperactivity," to selective protein channels in the sperm's tail.
Sperm hyperactivity is necessary for breaking through two physical barriers that protect the egg from fertilization.
The current study links two key findings made in recent years about sperm hyperactivity. The first was that a sperm's whip-like tail is studded with selective channels made from a protein called CatSper1 that allows only calcium ions through. Scientists also knew that hyperactivity was associated with sudden influxes of calcium into the tails.
A sperm's tail is called a flagellum, and it spins like a boat propeller to move the sperm forward.
Using a cellular recording technique called patch-clamping, the researchers established
definitively that the opening of CatSper1 is responsible for the influx of calcium into the sperm.
The calcium surge causes the sperm's tail to spin faster and more unevenly and the sperm is propelled through its viscous environment more forcefully.
In experiments involving mice, the researchers showed that sperm lacking CatSper1 can't achieve the hyperactive state they need for egg fertilization (Than,2006).
Hyperactivity has its place. In the last seconds before fertilization, a sperm, somehow spurred by a spike of intracellular calcium, launches into high gear. Without this final, frenzied burst of energy, the sperm cannot penetrate the egg’s outer layers and will never merge its DNA with that cloistered within the egg’s protective membrane (Garbers, 2003).
Garbers, D. et al. Hyperactivated Sperm Motility Driven by CatSper2 Is Required for Fertilization. Proc Natl Acad Sci USA 100, 14869-14874 (December 9, 2003).
Than, K. 2006. How Sperm Get Hyperactive, LiveScience Staff Writer
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