Scientists have found sperm benefit from co-operation while swimming to the egg, and this can lead to improved IVF treatments.
raditionally, sperm is thought of as nature’s ultimate athlete, designed to win a race of millions to reach the egg first, fertilise it and pass on its genes.
However, scientists have discovered the most successful sperm do not operate alone, but co-operate while navigating toward the egg – and knowing how they do it can lead to better IVF treatments.
Researchers have found biological benefits for sperm working together that may have implications for fertility studies.
It turns out sperm literally “pull together” as they battle to swim against a current of thick fluid in the journey toward the egg.
They often team up to navigate inside the female tract of many species of mammal, scientists say. They reported in the science journal Frontiers in Cell and Developmental Biology on the reasons why sperm link up.
The physics of how they navigate to an egg in mammals, including humans, is now well understood.
Scientists have found sperm tend to cluster together – in a non-random fashion – as they ‘swim’ through the thick, elastic fluid of the female reproductive tract.
“This may resemble the peloton formation in biking, although the fluid mechanics for sperm is drastically different from the bikers,” said Dr Chih-kuan Tung, co-author of the paper and associate professor of physics at North Carolina A&T State University.
The scientists used bovine sperm – similar to that of human – and a device that imitated the make-up of the female reproductive tract in order to observe how well the sperm that clung together in the thick, elastic fluid would swim against various flow currents.
Researchers found three biological benefits to sperm that clustered together as opposed to swimming to the egg solo.
In the absence of current, the clustered sperm swam in a straighter line; against a mild or intermediate current, they sperm were better aligned, like a school of fish heading upstream; and against a strong flow, the clustered sperm were less likely to be carried away.
“Our finding of biological advantages for sperm to swim together suggests that, at least in some part of the female reproductive tract, it is good for sperm to cooperate with each other,” Dr Tung said.
The findings that sperm co-operate does not surprise Dr Edgar V Mocanu, a consultant obstetrician gynaecologist from Dublin’s Rotunda Hospital and the Royal College of Surgeons of Ireland.
“The ejaculate contains millions of sperm,” said Dr Mocanu, who is a specialist in reproductive medicine and surgery.
“The mission is not of a lonely rocket, but a formation of fighter jets.
“The purpose is not to find a lonely egg, but to reach the fallopian tubes and patiently wait for the egg to appear.
“Sperm competency is not in speed, but in sustained progression to the tubes, so swimming in formation makes a lot of sense.”
Human infertility is an issue in the man in one-third of cases, said Dr Mocanu and in the woman another one-third. In the remainder the cause is unknown.
He believes this new knowledge about how sperm co-operate can lead to new, better methods to address infertility.
This can be achieved, Dr Mocanu said, “by possibly identifying better ways to analyse a semen sample, better ways to assess sperm “ability” to swim in vitro and also improve the selection of the most competent sperm for assisted reproductive treatments.”
Dr Tung said: “We certainly hope the knowledge that we provide here will lead to a better male infertility diagnosis.”