Floating Fertility

It's actually astonishing that something as tiny as sperm could be affected by gravity. Physicists, says Tash, "might argue that the size of molecules critical to sperm movement are not big enough to be sensitive to gravity." But, he points out, the head of a sperm is about the same size as statoliths in plants - small floating granules that help plants tell up from down. Gravity may in fact affect things that are even smaller. Researchers, says Tash, are now beginning to find evidence that even the individual proteins that form the structures of the sperm tail may be sensitive to gravity changes.

Credit: Chris Patton, Stanford University. [More] The sperm moves by rotating its tail in a spiral motion through the water. This induces waves of force backward propelling the sperm forward. If the sperm hits a hard surface, like an egg, the spiral motion will cause the entire sperm to rotate.

No one knows exactly how gravity affects cells. It may have to do with the cytoskeleton: the structure that gives a cell its shape. Proteins that send signals are often physically connected with the cytoskeleton, says Tash. Perhaps, he says, there is a mechanism in which the gravitational forces on the sperm head are somehow transmitted into the cytoskeleton, which then affects the signalling pathways that alter movement.

This is a puzzle humans need to solve if we plan to spend much time in space. Ultimately, our exploration of space may rely on the ability of many species to reproduce in microgravity: not only humans, but also animals and greenhouse plants.

"For NASA," says Tash, "the basic underlying question is: Do changes in gravitational force affect the ability of species to reproduce?" Increasingly, the answer seems to be yes. "It's an area," he says, "that requires a lot more attention."