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Developmental Biology - Sperm Locomotion

Secret to Human Sperm's Swimming Prowess

Research discovers what gives human sperm strength to succeed in race to fertilize the egg...

Researchers, from the universities of York and Oxford,in the UK have discovered that a reinforcing outer-layer coats the tails of human sperm giving them the strength to make the powerful rhythmic strokes needed to break through the cervical mucus barrier.
Only around 15 out of the 55 million sperm that embark on the treacherous journey to fertilise an egg are able to make it through the reproductive tract where cervical mucus, which is one hundred times thicker than water, forms part of one of nature's toughest selective challenges.

These findings could lead to better sperm-selection methods in IVF clinics, with the fittest being identified under conditions more closely mimicking nature. 3.5 million people in the UK affected by fertility issues and couples who opt for IVF spend an average of £20,000 or $26,432.98 in the USA.

Dr Hermes Gadêlha, from the Department of Mathematics at the University of York, explains: "We still don't fully understand how, but a sperm's ability to swim could be associated with genetic integrity. Cervical mucus forms part of the process in the female body of ensuring only the best swimmers make it to the egg.

"During the sperm selection process, IVF clinics don't currently use a highly viscous liquid to test for the best sperm as until now it was not entirely clear whether this is important. Our study suggests that more clinical tests and research are needed to explore the impact of this element of the natural environment when selecting sperm for IVF treatments."
Sperm tails - or flagella - are incredibly complex yet measure just the breadth of a hair in length.

Researchers use virtual sperm models to compare tails of sperm from humans and other mammals, which fertilise inside the body. Sea urchins release sperm and eggs, in a reproductive strategy known as broadcast spawning, outside their bodies into sea water.
While the tails of sea urchin and human sperm share the same bendy inner core, the study suggests that the tails of sperm in mammals may have evolved a reinforcing outer layer to give them the exact amount of extra strength and stability required to overcome the thick fluid barrier that exists with internal fertilisation.

Researchers used virtual models to add and remove the features of flagella in different species so that they could identify their function. The work appears in the Journal of The Royal Society Interface.

They tested the ability of virtual sea urchin-like sperm to swim through liquid as viscous as cervical mucus and found that their tails quickly buckled under the pressure, rendering them unable to propel themselves forward. Human sperm on the other hand, thrashed around wildly in a low-viscosity liquid like water, but in thicker liquids began to swim in a powerful rhythmic wave.

Dr Gadêlha: "Using virtual sperm we were able to see how mammalian sperm is specifically adapted to swim through thicker fluids. We don't know which adaptation came first — the stronger sperm or the cervical mucus, or whether they co-evolved - but nothing in nature is by chance and precisely what is required for species to reproduce has been added due to evolutionary pressure over millions of years."
With no central nervous system to make decisions about how to move and when - what controls sperms movement remains a scientific mystery.

"We know that sperm have tiny muscles which allow their tails to bend - but nobody knows how this is orchestrated inside the tail, at the nanometric scale," explains Dr Gadêlha. "Sperm are an architype of self-organisation - movement seems to be happening automatically, perhaps because of a complex combination of many mechanisms."

Eukaryotic flagellar swimming is driven by a slender motile unit, the axoneme, which possesses an internal structure that is essentially conserved in a tremendous diversity of sperm. Mammalian sperm, however, which are internal fertilizers, also exhibit distinctive accessory structures that further dress the axoneme and alter its mechanical response. This raises the following two fundamental questions. What is the functional significance of these structures? How do they affect the flagellar waveform and ultimately cell swimming? Hence we build on previous work to develop a mathematical mechanical model of a virtual human sperm to examine the impact of mammalian sperm accessory structures on flagellar dynamics and motility. Our findings demonstrate that the accessory structures reinforce the flagellum, preventing waveform compression and symmetry-breaking buckling instabilities when the viscosity of the surrounding medium is increased. This is in agreement with previous observations of internal and external fertilizers, such as human and sea urchin spermatozoa. In turn, possession of accessory structures entails that the progressive motion during a flagellar beat cycle can be enhanced as viscosity is increased within physiological bounds. Hence the flagella of internal fertilizers, complete with accessory structures, are predicted to be advantageous in viscous physiological media compared with watery media for the fundamental role of delivering a genetic payload to the egg.

Hermes Gadêlha and Eamonn A. Gaffney.

The authors declare no conflict of interest.

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Mar 26 2019   Fetal Timeline   Maternal Timeline   News  

A comparison between mammalian sperm and axonemal flagellum. (a) Single mammal flagellum show additional reinforcing structures (RED) in cross-section of mid-piece, characterized by nine outer dense fibres exterior to the axoneme with a fibrous sheath (YELLOW). All reinforcing fibres taper along the flagellum, ending prior to the distal tip of the sperm. (b) Sea Urchin sperm possess a flagellum without structures of length approximately 42 µm. (c) Human spermatozoa with reinforcing accessory structures on flagellum of approximately 50 µm length, swim in a highly viscous methylcellulose solution, highlighting suppression of buckling instability. (d) Model predictions for compression and arc length for a naive flagellum undergoing buckling instability in high-viscosity medium [12]. Transition can cause high curvatures and asymmetric waveforms, inducing circular swimming. Note (d) shows high internal compression at regions of ultra-structural components - larger in mammal flagella (black arrows). (e) A smaller circular radius for spermatozoa with larger heads.

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