My house sits up on a bluff across the creek from a popular “put in” spot for kayakers. A picture with my house incidentally in the background once graced the cover of Kayaking Magazine! From my vantage point, I can watch the boaters as they navigate the rocks and the bridge pylons where the creek water rushes through. I was recently reminded of the kayakers while reading some new research on sperm movement through the female reproductive tract.
Turns out sperm don’t swim like we thought they did. If you’ve ever had the pleasure of seeing sperm under the microscope (so fun to watch! I used to occasionally spot them when I was evaluating vaginal fluid for STIs), or in videos, they appear to be whipping their tails from side to side like an eel as they swim along. But, not so. Turns out our little swimmers are more like one-armed kayakers paddling only on one side. You know what happens to them – they go around in circles. What keeps the sperm going forward? Like a kayaker doing a roll, the sperm moves in rotation. The sperm is actually rotating and stroking, actually drilling its way along the waterway towards the egg. I expect it’s the same movement the lucky winner uses to bore its way into the ovum once it gets there. (I encourage you to watch the sperm swim to the “Flight of the Bumblebee” here.)
Corkscrewing forward is not their only feat. Sperm are challenged when they reach narrow structures like fallopian tubes. The sperm are essentially swimming like spawning salmon upstream against the flow of fluid through the tubes. As the route narrows, the sperm are pushed back by the increased rate of flow. The weaker swimmers can’t fight against the tide, but the stronger (and smarter?) sperm swim to the walls where the flow is weaker and approach the pinch point from the side before making another attempt to sprint through the channel. Just like the kayakers do. The result is that only the strongest, healthiest sperm make it through the female obstacle course to the holy grail: the egg! But there’s even more to it than that.
We’ve known that the female reproductive tract is a sperm survival course where the swimmers are tested for their strength, geotracking ability, stamina, and their ‘major histocompatibility complex (MHC).” It’s more like an Iron Man competition than a swim meet. They must survive the acidic vaginal fluid, get through the molasses-like cervical mucus, fight off the antibodies that think they are foreign invaders (which they are), pick the right fallopian tube, and swim its entire 7-inch length (at the rate of about 1 inch every 15 minutes). Of the tens of millions of sperm in the ¾ teaspoon of semen that is released in a typical ejaculation, only about 250 reach the egg and only about ten percent of them are even capable of breaching the egg walls.
Now we find out that the egg is not just an innocent bystander in this whole process, like a princess at the end of a joust. Rather, she is actively reviewing the sperm CVs to decide which one she wants to recruit – another recent finding in human reproduction science. That sperm MCH I mentioned? It’s a bundle of genes that are programmed to fight infections. The egg wants to attract the sperm that has an MCH that contains genes that are different from the genes she herself has. That way, the offspring they create will be protected from a wider range of microbes and diseases. Ain’t she smart?!
So, Ms. egg has been secreting a secret sauce that attracts some sperm and repels others. The right Mr. sperm, who has an odor receptor on his head, is hopelessly attracted to her chemical perfume. He swims faster and straighter through the last two centimeters of follicular fluid to penetrate the egg and join his 23 chromosomes with hers. Tada! – we have a unique new embryo.
Other squirmy facts:
While researching this phenomenon, I gained some other interesting sperm expertise. First of all, I was unaware of the great interest in mammalian sperm. While looking for new discoveries in human sperm science, I was overwhelmed by the number of research papers dedicated to the sperm of animals including: mice, rabbits, hamsters, carp, bulls, stallions, boars, pigs, buffalo, Rhesus macaques, and zebrafish. Animal husbandry must be big business.
According to the World Health Organization, a normal human sperm count should:
- have a concentration of at least 20 million sperm per ¼ tsp
- be at least 1/2 tsp in volume
- contain at least 40 million sperm in the ejaculate
- contain at least 75% live sperm (it’s normal for up to 25 per cent to be dead); of these at least 30% should be of normal shape and form
- be swimming with rapid forward movement (at least 25%)
- be swimming forward, even if only sluggishly (at least 50%)
Healthy sperm are important to the survival of the species, right? You are likely aware that men today are making fewer (a decrease of over about 60% since the 1970s and still falling) and less virile sperm than they have in the past. This is occurring mainly in developed countries in North America and Europe and in Australia and New Zealand. Scientists’ best guess at the reason behind this continuing phenomenon is the “Western Lifestyle.”
What can be done to reverse this trend? Since we don’t now its exact cause, we don’t know its exact fix, but there are some well-researched recommendations. You can make your sperm more likely to win an Iron Man competition or survive a Class IV whitewater kayaking run by:
Eat less pizza, burgers, chips, processed meats (cold cuts), red meat, and sugary drinks
Eat more fruit, fish, walnuts, edamame and other beans, avocado, asparagus, lean beef, dark chocolate, garlic, and pomegranate. A vegetarian diet is also recommended.
Cut down or out: tobacco, alcohol, and weed
Get more sleep (at least 7 hours) and go to bed early (before 10:30 pm)
Wear boxers, not briefs – no more tighty whities
Supplement your diet with lycopene, vitamin C, and vitamin D
Exercise daily and keep your weight down
Decrease stress by utilizing relaxation techniques like meditation, yoga, breathing exercises, anger management programs, and counseling
My parting gift from sperm science: if you want sweet-tasting semen you should eat a diet rich in yellowish fruits like mango and pineapple. Furthermore, smoking (cigarettes or weed), alcohol, and coffee adversely affect its taste. So, there you go.
Addendum: Please check out Nicholas Kristof’s essay, “What Are Sperm Telling Us?” in the New York Times on 20Feb2021 – he talks about the endocrine disrupters that are disrupting our sperm – a really important issue that I missed in my blog. Worth a read!!
Thanks Kris, for making the world of reproduction so easily understood.