They Don't Swim the Way We Thought

[Recusant]

Spermatozoa, that is. The article tells a decent story even if there are some imprecise statements, so if you'd prefer to learn about this there, skip the--

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"Sperm fooled scientists for 350 years – they spin not swim" | The Conversation

Sperm is critical for the fertilisation of almost every living organism on our planet, including humans. To reproduce, human sperm have to swim a distance equivalent to climbing Mount Everest to find the egg. They complete this epic journey simply by wiggling their tail, moving fluid to swim forwards. Though over 50 million sperm will fail to reach the egg – the equivalent to more than six times the entire population of London or New York – it only takes one single sperm in order to fertilise an egg that will eventually become a human being.

Sperm was first discovered in 1677 – but it took roughly 200 years before scientists agreed on how humans are actually formed. The "preformationists" believed that each spermatozoa contained a tiny, miniaturised human – the homunculus. They believed that the egg simply provided a place for the sperm to grow.

On the other hand, the "epigenesists" argued that both males and females contributed to form a new being, and discoveries in the 1700s showed more evidence for this theory. Though scientists now better understand the role that sperm plays in reproduction, our latest research has discovered that sperm have actually been fooling scientists this whole time.

[Snipped more science history.]

Using state-of-the-art 3D microscopy technology, our team of researchers from the UK and Mexico, were able to mathematically reconstruct the rapid movement of the sperm tail in 3D. Not only does sperm's size make them difficult to study – its tail only measures half a hair's breadth – they're also fast.

Their tail's whip-like movement is capable of beating over than 20 swimming-strokes in less than one second. We needed a super-fast camera capable of recording over 55,000 pictures in one second mounted in a fast oscillating stage to move the sample up and down at an incredibly high rate – effectively scanning the sperm tail while swimming freely in 3D.

What we found surprised us. We discovered that the sperm tail is in fact wonky and only wiggles on one side. While this should mean the sperm's one-sided stroke would have it swimming in circles, sperm have found a clever way to adapt and swim forwards: they roll as they swim, much like the way otters corkscrew through water. In this way, the wonky one-sided stroke evens out as sperm rolls allowing it to move forwards.

The sperm's rapid and highly synchronised spinning causes an illusion when seen from above with 2D microscopes - the tail appears to have a side-to-side movement. However, this discovery shows that sperm have developed a swimming technique to compensate for their lop-sidedness. In doing so they have also ingeniously solved a mathematical puzzle: by creating symmetry out of asymmetry.

The sperm body spins at the same time that the tail rotates around the swimming direction. Sperm "drills" into the fluid like a spinning top by rotating around itself whilst its tilted axis rotates around the centre. This is known in physics as precession, much like the precession of the equinoxes in our planet.

[Continues . . .]

It appears the paper is open access.

"Human sperm uses asymmetric and anisotropic flagellar controls to regulate swimming symmetry and cell steering" | Science Advances

Abstract:

Flagellar beating drives sperm through the female reproductive tract and is vital for reproduction. Flagellar waves are generated by thousands of asymmetric molecular components; yet, paradoxically, forward swimming arises via symmetric side-to-side flagellar movement. This led to the preponderance of symmetric flagellar control hypotheses. However, molecular asymmetries must still dictate the flagellum and be manifested in the beat. Here, we reconcile molecular and microscopic observations, reconnecting structure to function, by showing that human sperm uses asymmetric and anisotropic controls to swim. High-speed three-dimensional (3D) microscopy revealed two coactive transversal controls: An asymmetric traveling wave creates a one-sided stroke, and a pulsating standing wave rotates the sperm to move equally on all sides. Symmetry is thus achieved through asymmetry, creating the optical illusion of bilateral symmetry in 2D microscopy. This shows that the sperm flagellum is asymmetrically controlled and anisotropically regularized by fast-signal transduction. This enables the sperm to swim forward.

[Randy]

Interesting. Whenever I see videos about this subject I'll picture them swimming in a corkscrew manner,

[Ecurb Noselrub]

Starts with a screw, ends with a screw. (Sorry, couldn't resist).

[Dark Lightning]

Starts with a screw, ends with a screw. (Sorry, couldn't resist).

Damn! Ninja'd! 

[Tank]

So there is no rotary motor?

[Recusant]

That is an excellent question, Tank. I went through the references in the paper linked above and found a paper from 2011. Though it's not the initial publication of the information, it does contain the answer (from the bottom of page 506):

Using an elastic/viscous balance argument, Machin (1958) demonstrated that active force must be generated throughout the flagellum rather than being localized to the head-flagellum junction. This differs from the bacterial flagellum, which is driven entirely by a torque motor at its base. This active force arises from the action of inner and outer arm dynein motors, situated on the A subfiber of each doublet, and engaging with the B subfiber of the adjacent doublet (Figure 2i) to exert a sliding force between the doublets.

I also read descriptions noting the utilization of ATP by the elements of the sperm flagellum itself, which indicates that it isn't driven so much as it drives, if you get my meaning. The motive energy isn't generated at the base of the flagellum of the sperm but along the length of the flagellum.

[Randy]

And here I thought Tank was being funny. That's an interesting tidbit of news, Recusant.

[Recusant]

There's been a long-running Creationist "gotcha" regarding the evolution of the rotary motor that drives the flagellum in bacteria. It's not the deadly blow they'd like to believe it is (I consider it essentially an argument from incredulity), but it is an item in their arsenal.

Their focus on that one particular aspect of the physiology of bacteria has meant that many who've spent some time dealing with Creationist argumentation have perforce learned about the bacterial flagellar motor.

[Tank]

That is an excellent question, Tank. I went through the references in the paper linked above and found a paper from 2011. Though it's not the initial publication of the information, it does contain the answer (from the bottom of page 506):

Using an elastic/viscous balance argument, Machin (1958) demonstrated that active force must be generated throughout the flagellum rather than being localized to the head-flagellum junction. This differs from the bacterial flagellum, which is driven entirely by a torque motor at its base. This active force arises from the action of inner and outer arm dynein motors, situated on the A subfiber of each doublet, and engaging with the B subfiber of the adjacent doublet (Figure 2i) to exert a sliding force between the doublets.

I also read descriptions noting the utilization of ATP by the elements of the sperm flagellum itself, which indicates that it isn't driven so much as it drives, if you get my meaning. The motive energy isn't generated at the base of the flagellum of the sperm but along the length of the flagellum.

Good find. Very informative.

[Tank]

And here I thought Tank was being funny. That's an interesting tidbit of news, Recusant.

As Recusant points out the flagella and it's rotary joint/motor have long been a creationist trope extolling irreducible complexity.

[Randy]

It's funny. When I was taught about creationism this sort of thing never came up. I think they wanted to shield us from any talk about sex.