Why do we need so many sperm?
Happy 2019! Looking back at my blogs, it seems the most popular ones are the simplest. The ones where I attempt to explain the every day questions of infertility. Sure, chatting about fancy lab treatments like PGT-A is exciting, but most people are hoping they will never need a "fancy lab treatment" to get pregnant. They have good questions about the basics of getting pregnant, or not. So, let me start with something a lot of people wonder when we are reviewing sperm reports: Why do we need so many sperm to make a baby?
A man needs to ejaculate at least 30 million sperm into the vagina to get a woman pregnant. If he ejaculates between 10-30 million sperm it's fairly unlikely the woman will not get pregnant. A treatment like intrauterine insemination (IUI) is often needed and if < 10 million are ejaculated most will need IVF, often with ICSI to conceive. These are really rough guidelines with some exceptions.
This truly makes no sense. Why do we need millions of sperm when ultimately only one sperm cell will fertilize one egg to make one embryo?
When ejaculation occurs in the vagina, the sperm is exposed to acidic vaginal fluid (pH about 5). This acidic fluid will kill some sperm even as the more basic pH of the semen tries to neutralize the vaginal acid. Many sperm will quickly leave the vagina and it's acidic fluid behind, entering the cervix within minutes of ejaculation.
In the cervix sperm reach another challenge. The cervical mucous, particular at the borders, is compact with a microarchitecture that is hard for abnormal sperm, or sperm that cannot swim properly, to penetrate. Yet another challenge in the cervix is the female's immune response to sperm. White blood cells migrate to the cervix to attack foreign cells (usually bacteria coming up from the vagina, but also some sperm cells). The sperm that can get through the cervix and avoid attack by white blood cells, need to find the proper path into the uterus, which isn't easy as they need to travel in the grooves of the cervix. So many challenges to get through the cervix!
Those that make it through the cervix enter the uterus. The uterus has its own challenges for the sperm as white blood cells also migrate to the uterus to attack them. The uterus also needs to have proper muscle contractions to promote sperm travel through the uterus and out the Fallopian tubes. Women with uterine abnormalities like large fibroids or adenomyosis may not have the correct contraction pattern, making it hard or impossible for sperm to make it through the uterus.
Now the Fallopian tubes are much kinder to sperm - no white blood cells attacking them and the tubes have crypts and binding cells (endosalinpingeal epithelium) that let the sperm "take a break" and be stored temporarily on their journey to the egg.
Once they get close to the egg, in the tube, the sperm then need to go through capacitation (changing the sperm's surface) and hyperactivitation (changing the sperm's tail motion) to get ready for fertilization. Only the best of the best can do these preparatory steps.
Additionally the sperm have to be chromosomally normal (many are not - no matter the man's age) and undergo several complex steps to get to the egg and fertilize it. Once the lucky sperm does enter the egg to start the fertilization process the egg seals itself from other sperm entering. Ta-da: an embryo is made.
So, we start with millions in the vagina and end with just a few sperm at the egg in the Fallopian tube. Survival of the fittest with each ejaculation!
PGT-A is exciting, but most people are hoping they will never need a "fancy lab treatment" to get pregnant.