Sperm are the original guided missiles. Alvarez et al. show that changing calcium concentrations within the flagellum help direct a sperm to its target.

A sperm's path to the egg typically includes straight-ahead runs alternating with curves and even loops. The sperm's flagellum guides the journey by sensing the egg's come-hither molecules and adjusting its beating pattern to steer in the right direction. The attractants released by the egg cause calcium spikes in the flagellum. Early studies suggested that the concentration of calcium ions determines the flagellum's beating pattern, with high calcium levels spurring the sperm to turn. But recent research has called these findings into question.

Alvarez et al. provided sea urchin sperm with a caged version of cyclic GMP, which they could liberate with a pulse of UV light in order to trigger a calcium surge in the flagellum. The researchers found that sperm continued on a straight course even when calcium was abundant. Instead, sperm respond to a change in calcium level—in mathematical terms, its time derivative. The rate of calcium increase dictates how sharply the sperm turns, whereas the path of the subsequent run depends on the steepness of the calcium decline.

The researchers found that sperm from three other kinds of invertebrates also detect changing calcium concentrations, suggesting this mechanism might be widespread, even perhaps in human sperm. A key question, the researchers say, is whether the other main type of cell-propelling filament, the cilium, also relies on the same mechanism.

et al
J. Cell Biol.