Have you ever wondered why men need to produce so many sperm? Women typically release only one egg per month, but when men ejaculate many millions of sperm are unleashed. If a man produces too few sperm, then his risk of infertility increases markedly. To understand why, we need to take a closer look at the journey sperm have to make inside a woman’s body and the many pitfalls and hazards they will encounter along the way.
The journey begins with ejaculation. At the climax of intercourse, sperm are deposited high in the vagina, close to the cervix and the opening into the woman’s womb. From a sperm’s perspectives the vagina is a very hostile place. It is designed to help protect her body from infection and vaginal acids quickly kill any unwelcome bacteria or viruses that enter from the outside world. But the downside is that these acids kill sperm too. Thankfully, sperm are ejaculated from the penis in a complex fluid called seminal plasma. This fluid acts as a buffer from these acids and helps to protect them for a short time. This gives the sperm a chance to enter the woman’s cervix; but only if the timing is right.
For most of the month, the channel of mucus running through the middle of a woman’s cervix is too viscous for sperm to enter. Again, this is deliberate to protect the woman’s body from attack should the vaginal acid fail to do their job. But it means certain death for any sperm that remain in the vagina for more than about 30 minutes. Dead sperm are broken down by the acids and are quietly disposed of by white blood cells. It’s journey’s end for these sperm. Hopefully others will fare better. What happens to allow sperm into the woman’s cervix is remarkable; and it’s an egg growing in the ovary that controls the process.
The release of an egg from the ovary is called ovulation. Women have an efficient process to select an egg for release each month and as it gets closer to the time of ovulation the egg grows in a small bag of fluid called a follicle. The follicle releases female hormones such as oestrogen, which softens the mucus in the cervix and makes it more watery and sperm-friendly. Therefore if intercourse happens in the few days run up to ovulation, the sperm that arrive in the top of the vagina during intercourse stand a fighting chance of burrowing into the mucus and making it on their way toward the egg.
Even when cervical mucus is at its most sperm friendly, not all sperm will manage enter. Cervical mucus is an amazing filter and sperm that are deformed or two-headed just won’t fit into the tiny channels that run through the mucus. Also, for sperm that swim too weakly, they simply don’t have the power to get inside. It sounds quite brutal, but it’s nature’s way of making sure that only the best sperm ever stand a chance of reaching the egg. But it means if a man produces only a few sperm, or a large number of abnormal sperm, then the chances of him getting enough sperm past the cervix filter are reduced. And that can mean sub-fertility.
For sperm that get through the cervix, the journey to the egg has only just begun. Unfortunately, there are further hazards and hurdles to overcome as they travel through the womb and into the Fallopian tubes. But those are another story.
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
Big photo credit: Menno van der Horst / Foter / CC BY