Sperm crawl and collide along walls of uterus in search of egg

Sperm navigate the female reproductive tract by `crawling' along the channel walls and swimming around corners with frequent collisions, according to scientists. The findings provide fresh insight into how sperm might find their way to the egg that will help to inform future innovation in the struggle to treat infertile couples. Scientists led by Dr Petr Denissenko, of the School of Engineering at the University of Warwick, and Dr Jackson Kirkman-Brown, lead in reproductive biology at the University of Birmingham, explored what properties distinguish the tens of cells which make it to the egg from the millions of sperm cells ejaculated. Contrary to popular belief, the researchers indicated that sperm rarely swim in the central part of the three-dimensional female tract, instead travelling along the walls, meaning in the body they are negotiating complex and convoluted channels filled with viscous fluids. To study cell behaviour in confined space, cells were injected into hair-thin microchannels. "When the channel turns sharply, cells leave the corner, continuing ahead until hitting the opposite wall of the channel, with a distribution of departure angles, the latter being modulated by fluid viscosity," the researchers said. "Specific wall shapes are able to preferentially direct motile cells. "As a consequence of swimming along the corners, the domain occupied by cells becomes essentially one-dimensional. This leads to frequent collisions and needs to be accounted for when modelling the behaviour of populations of migratory cells," they added.Dr Kirkman-Brown, who is also Science Lead for the Birmingham Women's Fertility Centre, commented: "Two key questions in reproduction are: how are the millions of sperm selected down to around ten that reach the oocyte?: and Can we use a similar method to select sperm for fertility treatments? "In basic terms - how do we find the `Usain Bolt' among the millions of sperm in an ejaculate. Through research like this we are learning how the good sperm navigate by sending them through mini-mazes," he stated. Dr Denissenko continued: "Sperm cell following walls is one of those cases when a complicated physiological system obeys very simple mechanical rules. "I study fluids in a variety of environments, but moving to work with live human sperm was quite a change. "I couldn't resist a laugh the first time I saw sperm cells persistently swerving on tight turns and crashing head-on into the opposite wall of a micro-channel. "More seriously, it's great being part of an internationally leading team based out of the Midlands addressing a key problem," he added. Dr Kirkman-Brown concluded: "Previous research from the group indicates that the shape of the sperm head can subtly affect how the sperm swim. Combined with this data we believe new methods of selecting sperm, perhaps for quality or even in certain non-human species for sex may become possible." The researchers suggested that the combined effect of the fluid rheology and three-dimensional architecture should be taken into account in future in-vitro studies. The results were recently published in the Proceedings of the National Academy of Sciences of the United States (PNAS).