Every male animal that reproduces with meiosis carries sperm. To fertilize an egg, human sperm scales the distance equivalent of Mount Everest if we were to bring a sperm cell to regular human size.
Sperm cells wiggle their tail to propel themselves through the fluid around them. In intercourse, most of the sperm fails to reach the egg, roughly about 50 million of them. Yet, it only takes a single sperm cell to fertilize an egg.
Historically, we learned about sperm cells in 1677. Though, it would take another 200 years for us to figure out how humans actually form. The "preformationists" at the time believed each cell contained a miniature human, and that the female egg was simply a shelter for this tiny human to grow in.
In the 1700s though, the idea of "epigenesis" surfaced, epigenesists argued that the child is the product of both the female and the male. Later in the century, this idea got confirmed.
Even though, we know a lot more about the sperm today, turns out, it was still hiding some of its quirks from us.
In the 17th century, Dutch businessman (and autodidact scientist) Antonie Philips van Leeuwenhoek. At some point, he developed an interest in lenses. He became the first person to observe microorganisms, thus, he's now known as the father of microbiology. Originally, he referred to these tiny organisms as "animacules", which is the Latin for "tiny animal". Leeuwenhoek was also the first to observe muscle fibers, capillary blood flow, and bacteria.
3D imaging of the sperm
A better lens than Leeuwenhoek's was not created for 200 years, furthermore, our perception of how sperm cells navigate in a "snakelike movement, like eels in the water" has not strayed away much from his description, until recently that is.
Utilizing the latest technology in 3D microscopy, researchers were able to reconstruct the rapid movements of the flagellum (the tail). Until now, this had proven a challenge given the tiny size of the cell and its dexterity.
Flagella of the sperm can whip around 20 times per second; however, when we look at the matter with super-fast shutter cameras that can capture 55 thousand frames per second, we notice something's off.
The flagella are actually crooked and this should mean that their strokes in the liquid should have them spin around in circles, but the sperm has an ingenious way of mitigating this effect. They simply roll as they waddle around. Like an otter doing a corkscrew motion underwater.
Computer-aided sperm analysis (CASA) systems used in clinical and research settings both still utilize 2D viewing methods. Thus, they are prone to this illusion and lack the ability to detect whether the cells navigate asymmetrically (which impacts fertility).
More than half the cases of infertility are tied to the male population, increasing our understanding of sperm and the development of more methods like this will surely change the way we diagnose and manage such cases.