Once sperm reach the relative safety of the Fallopian tubes, their journey to the egg is nearly over. However, success at this stage will rely on precision timing and the abilities of sperm to detect subtle cues and signals from the woman’s body and the egg. Their job is not quite done.
It is interesting to think about how many sperm actually get this far. Of the many millions that a man might ejaculate into his partner’s vagina, perhaps only 50 sperm will actually cross the threshold into the Fallopian tubes. This may seem very wasteful, but those sperm that do make it this far are his very best.
But what if they arrive too soon? If a couple were to have intercourse several days before the egg is released, sperm may die before the egg appeared. Or if they kept on swimming, they might swim out of the end of the Fallopian tube and past the ovary altogether. Both would be a disaster. Therefore, as you might now expect, the woman’s body has the answer.
Several research studies have now suggested that once the sperm enter the Fallopian tube, they tend to stick to its walls by the tip of their heads. Sperm that stick tend to live longer, remain in better shape and also may even use this as an opportunity to refuel. See it as a pit stop for sperm!
But sperm that are sleeping on the sidewalls could miss their chance if they don’t wake up again at the right moment, just before the egg appears.
How do they do this?
Scientists are not completely clear about those final moments of a sperm’s journey, as studying it at close quarters is really hard. But we think there are two important mechanisms that help sperm pull away from the sidewalls and find an egg to fertilise.
First, there is probably a subtle change of temperature along the length of the Fallopian tube, which occurs around the time of ovulation. We think that sperm are able to sense this gradient and are drawn along it to where it is warmer in the middle of the Fallopian tube (where the egg will eventually arrive).
Second, we also believe that sperm are able to detect subtle chemical signals, either given off by the egg once it is released from the ovary or that are released from the ovary at the same time the egg is (perhaps in the fluid that bathes the growing egg).
If either or both of these suggestions are true, then the success of this final part of the sperm’s journey will come down to how best it can respond to these signals in comparison to any other sperm nearby. However, there is an important aspect of sperm behavior that is really critical at this point.
So far in their journey, in order to be successful (such as getting through the narrow doorway into the Fallopian tubes), sperm have had to swim in quite a sober way. But now sperm can, and should, go crazy! In those final few minutes before reaching the egg, sperm change their style of swimming to become more erratic and frenetic. We call it “sperm hyperactivation” and it’s really very important.
We think that hyperactivated sperm achieve two things. First, they are able to search a greater volume of the Fallopian tube more efficiently and therefore considerably increase their chances of finding an egg. Second, once they make contact with an egg, the power generated from the frenetic beating of their tail gives them a greater energy to push the sperm head through to the inside.
But it doesn’t end there. Above the head of the sperm is a small bag of enzymes called an acrosome. Once the sperm touches the egg, special molecules on the egg surface trigger the acrosome to explode and the enzymes that are released help dissolve the egg coat and assist the winning sperm.
Once inside the egg, there is an immediate chemical reaction that suddenly hardens the egg membrane and prevents other sperm from pushing through. In a matter of hours the egg is fertilized, an embryo begins to develop and a new life is started.
Author: Allan Pacey BSc, PhD, FRCOG, Professor of Andrology at the Department of Human Metabolism, Academic Unit of Reproductive and Developmental Medicine, The University Of Sheffield