Looking to learn what distinguishes the "lucky" one, a team of scientists observed the passage of sperm through man-made micro-channels. Their findings could lead to advances in artificial insemination.
Through micro-channels of varying length and width, scientists were able to adjust the flow of fluids in order to observe the sperm's swimming capacity in different currents.
The first-of-its kind study confirmed that sperm are powerful upstream swimmers, which might not seem like news. Difficulties in observing the sperm voyage in the human body, however, had previously stood in the way of a hypothesis becoming a scientific fact.
"We wanted to know which physical mechanisms could be responsible for navigation," says MIT assistant mathematics professor and research team member Jörn Dunkel. "If you think of salmon, for example, they can swim against the stream, and the question was whether something similar could really be confirmed for human sperm cells."
What draws the sperm to the egg remains a mystery, for prior hypotheses of a rudimentary olfactory capacity were disproven in a 2012 study.
Among the notable observations made by Dunkel's team is the spiral swimming pattern of the sperm, which had been expected to swim in a straight line.
According to Dunkel, this is the reaction of sperm to transitions in current speed within the same channel: The fluid flows more freely in the center and is slowest along the walls.
Dunkel hopes the research could lead to more efficient artificial insemination techniques.
Among the possibilities is the idea that by artificially creating the environment that produces maximal upstream swimming, sperm could be pre-conditioned before insemination to increase the chances of fertilization.
The average man produces around 500 million sperm, the approximate population of Europe, in just one ejaculation.
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