Sperm don’t swim anything like we thought they did, new study finds

Under a microscope, human sperm seem to swim like wiggling eels, tails gyrating to and fro as they seek an egg to fertilize. 

But now, new 3D microscopy and high-speed video reveal that sperm don’t swim in this simple, symmetrical motion at all. Instead, they move with a rollicking spin that compensates for the fact that their tails actually beat only to one side. 

“It’s almost like if you’re a swimmer, but you could only wiggle your leg to one side,” said study author Hermes Gadêlha, a mathematician at the University of Bristol in the U.K. “If you did this in a swimming pool and you only did this to one side, you would always swim in circles. … Nature in its wisdom came [up] with a very complex, ingenious way to go forward.” 

Strange swimmers

The first person to observe human sperm close up was Antonie van Leeuwenhoek, a Dutch scientist known as the father of microbiology. In 1677, van Leeuwenhoek turned his newly developed microscope toward his own semen, seeing for the first time that the fluid was filled with tiny, wiggling cells. 

Under a 2D microscope, it was clear that the sperm were propelled by tails, which seemed to wiggle side-to-side as the sperm head rotated. For the next 343 years, this was the understanding of how human sperm moved. 

“[M]any scientists have postulated that there is likely to be a very important 3D element to how the sperm tail moves, but to date we have not had the technology to reliably make such measurements,” said Allan Pacey, a professor of andrology at the University of Sheffield in England, who was not involved in the research. 

The new research is thus a “significant step forward,” Pacey wrote in an email to Live Science. 

Gadêlha and his colleagues at the Universidad Nacional Autónoma de México started the research out of “blue-sky exploration,” Gadêlha said. Using microscopy techniques that allow for imaging in three dimensions and a high-speed camera that can capture 55,000 frames per second, they recorded human sperm swimming on a microscope slide. 

“What we found was something utterly surprising, because it completely broke with our belief system,” Gadêlha told Live Science. 

The sperm tails weren’t wiggling, whip-like, side-to-side. Instead, they could only beat in one direction. In order to wring forward motion out of this asymmetrical tail movement, the sperm head rotated with a jittery motion at the same time that the tail rotated.The head rotation and the tail are actually two separate movements controlled by two different cellular mechanisms, Gadêlha said. But when they combine, the result is something like a spinning otter or a rotating drill bit. Over the course of a 360-degree rotation, the one-side tail movement evens out, adding up to forward propulsion.

“The sperm is not even swimming, the sperm is drilling into the fluid,” Gadêlha said. 

The researchers published their findings today (July 31) in the journal Science Advances.

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Asymmetry and fertility

In technical terms, how the sperm moves is called precession, meaning it rotates around an axis, but that axis of rotation is changing. The planets do this in their rotational journeys around the sun, but a more familiar example might be a spinning top, which wobbles and dances about the floor as it rotates on its tip. 

“It’s important to note that on their journey to the egg that sperm will swim through a much more complex environment than the drop of fluid in which they were observed for this study,” Pacey said. “In the woman’s body, they will have to swim in narrow channels of very sticky fluid in the cervix, walls of undulating cells in the fallopian tubes, as well have to cope with muscular contractions and fluid being pushed along (by the wafting tops of cells called cilia) in the opposite direction to where they want to go. However, if they are indeed able to drill their way forward, I can now see in much better clarity how sperm might cope with this assault course in order to reach the egg and be able to get inside it,” Pacey said

Sperm motility, or ability to move, is one of the key metrics fertility doctors look at when assessing male fertility, Gadêlha said. The rolling of the sperm’s head isn’t currently considered in any of these metrics, but it’s possible that further study could reveal certain defects that disrupt this rotation, and thus stymy the sperm’s movement. 

Fertility clinics use 2D microscopy, and more work is needed to find out if 3D microscopy could benefit their analysis, Pacey said. 

“Certainly, any 3D approach would have to be quick, cheap and automated to have any clinical value,” he said. “But regardless of this, this paper is certainly a step in the right direction.”

Originally published in Live Science.

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‘Stay at home but don’t stay still,’ researchers recommend

The adverse side effects of the social isolation measures implemented to combat COVID-19 include an increase in sedentary behavior and physical inactivity, which can contribute to a deterioration in cardiovascular health even in the short term. Older people and people with chronic diseases tend to be most affected.

The warning comes from a review article published in the American Journal of Physiology by researchers at the University of São Paulo’s Medical School (FM-USP) in Brazil. According to the authors, the slogan “Stay at home” broadcast by governments and chief medical officers is unquestionably valid under the present circumstances but should be coupled with another: “Don’t stay still.”

“You need at least 150 minutes of moderate to vigorous physical activity per week to be considered active by the World Health Organization [WHO] and medical associations. Gyms, fitness centers and sports facilities will be open to a limited extent in the months ahead, even after confinement and quarantine measures are lifted. Physical activity in the home is a worthwhile alternative,” said Tiago Peçanha, first author of the article. Peçanha has a postdoctoral research scholarship from FAPESP.

The article reviews the scientific literature to compile evidence for the effects of short periods of physical inactivity on the cardiovascular system. Some of the studies cited show that between one and four weeks of bed rest can lead to cardiac atrophy and significant narrowing of peripheral blood vessels. Peçanha stressed that this is an aggressive model and does not reflect what happens during social confinement or quarantine. “However, other experiments reviewed in the article are a good match,” he said.

In one of these experiments, volunteers were asked to reduce their physical activity so that they took less than 5,000 steps in a week instead of more than 10,000 steps as usual. At the end of the period, the researchers observed a reduction in the diameter of the brachial artery (the main blood vessel in the arm), loss of blood vessel elasticity, and damage to the endothelium (the inner cell lining of all veins and arteries).

In other experiments, volunteers stayed seated for periods varying between three and six hours. This amount of inactivity was sufficient to cause vascular alterations, an increase in inflammation markers, and a rise in postprandial blood sugar.

“This first group of alterations observed in the studies have to do with functionality. In healthy volunteers, the heart and blood vessels function differently in response to inactivity,” Peçanha said. “In an extended period of inactivity, the alterations tend to become structural and are harder to reverse.”

Prolonged inactivity is particularly harmful for people with cardiovascular diseases and other chronic health problems, such as diabetes, hypertension, obesity or cancer. In older people, it can also aggravate loss of muscle mass (sarcopenia) and increase the risk of falls, fractures and other injuries. The FM-USP group recently published an article on this topic in the Journal of the American Geriatrics Society.

“These groups that are more vulnerable to the effects of inactivity are also high-risk groups for COVID-19 and will be staying at home for months. Ideally, they should find ways of staying active, such as doing housework, going up and down stairs, taking short walks, playing with children, or dancing in the living room,” Peçanha said. “The scientific evidence shows that getting exercise in the home is safe and effectively helps control blood pressure, reduces blood lipids, and improves body composition, quality of life and sleep.”

For high-risk groups, especially people who are not habitually active, Peçanha recommends supervision by health professionals, which can be performed remotely using cameras, smartphone apps and other electronic devices. “Studies show that an online environment favoring social support and interaction tends to motivate people to keep fit,” he said.

Fresh evidence

Data published in recent months by companies that sell smartwatches and exercise tracking apps suggest that the number of daily steps taken by users since the start of confinement has fallen.

“For example, Fitbit’s blog presents data for 30 million users showing a 7%-38% decline in daily step counts during the week ending March 22,” Peçanha said. “In Brazil, an internet survey by Raphael Ritti-Dias involving over 2,000 volunteers showed more than 60% saying they reduced their physical activity after the start of confinement or lockdown. All this evidence is preliminary, but studies are in progress to measure the effects on health of physical inactivity during social restrictions.”

One of these studies is being conducted at FM-USP as part of the Thematic Project “Reducing sedentary time in clinical populations: the Take A Stand For Health Study”. The principal investigator is Bruno Gualano, a co-author of the American Journal of Physiology article.

“We’re working with clinical groups associated with the Thematic Project, such as women with rheumatoid arthritis, patients submitted to bariatric surgery, and elderly subjects with mild cognitive impairment. They’re encouraged to take more exercise in the form of daily activities such as walking the dog or getting off the bus two stops prior to their destination. The effects on their health are being studied,” Peçanha said.

Since the implementation of social restrictions to contain the pandemic, the researchers have monitored a group of female rheumatoid arthritis patients more closely to measure their level of physical activity and compare it with the pre-pandemic level. “The patients are wearing accelerometers [electronic devices that measure physical activity and distance covered in a set period] at home,” Peçanha said. “We call them frequently to ask about quality of life and diet. A few researchers visit them at home to measure body weight, body composition and blood pressure and to take blood samples.”

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