The fertilization process begins with a chemical reaction called phosphorylation, when an enzyme changes the functioning of a protein within a cell and allows activity to happen. In this case, the tails of sperm move to propel it forward.
On Earth, the tail movement stops when a second enzyme called phosphatase kicks in, but in microgravity, the second enzymes don’t work the same. In previous experiments with sea urchin and bull sperm, NASA says this activation happens more quickly in microgravity, while “the steps leading up to fusion happen more slowly, or not at all.” These delays or issues could prevent fertilization from happening in eggs.
Because gravity causes every object to pull every other object toward it, previous research on microgravity – sometimes called weightlessness or zero gravity – has shown that too much or too little gravity can change how a sperm behaves.
This time around, frozen samples of bull and human sperm will make their way to the ISS. Once aboard, the samples will be thawed and added to a chemical mixture that will trigger activation. Because bull sperm is more consistent in motion and appearance than human sperm, astronauts can deduce whether any recorded strange behavior is a result of something unusual about the sperm sample, or if it is indeed an effect from microgravity.
The samples will then be mixed with preservatives and sent back to Earth. Here, scientists will see whether the “steps necessary for fusion occurred and whether the samples from space differ from those activated on the ground.”