A sperm’s path to an egg is more a deadly obstacle course than a track sprint. The one ejaculated sperm cell in a million that is lucky enough to reach the fallopian tubes, where eggs await fertilization, must conquer thick, gelatinous layers of mucus and cells surrounding the egg to reach its prize.
Fortunately for the sperm, there is help. Two studies published today in Nature show how sperm sense progesterone, a female sex hormone, that has been released by cells surrounding the egg. The hormone may guide the sperm towards the egg as well as giving it a final push to get there, the research suggests. The findings could be used to design a new class of contraceptive drug.
“It really is a significant step forward in terms of how we understand what regulates sperm,” says Steven Publicover, a reproductive biologist at the University of Birmingham, UK, who was not involved in either study.
In some previous experiments, ejaculated human sperm have been shown to swim towards areas with high levels of progesterone. The hormone also causes the cells to beat their whip-like tails more powerfully to make it through to the egg, a condition called hyperactivity. “We’ve got good reason to think that the response to progesterone matters, but it’s bloody difficult to pin it down,” says Publicover.