Every mammal on this planet starts in the same way: a sperm encounters an egg and fuses with it. This process is familiar to every eighth grade biology student, and pictures of the event can be found in every biology text book. However, despite this ubiquity, the detailed mechanics of the process itself is still somewhat of a mystery.

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The researchers filmed the gradual fusing of a mouse sperm cell with a mouse’s egg cell; these images show over 100 minutes of the sperm cell slowly being assimilated into the membrane and the cytoplasm of the egg cell.
Image credit: Benjamin Ravaux

Now, new techniques – featuring an “IVF chip” – presented recently at the Biophysical Society’s 60th annual meeting in Los Angeles, CA, promise to reveal new insights into how a single sperm cell fuses with an egg cell.

The researchers hope the new techniques will help us better understand the causes of infertility and improve treatments.

At the meeting, Benjamin Ravaux, a physics graduate student at the Ecole Normale Supérieure de Paris in France, described how, using the “completely new approach,” he and his colleagues captured high-resolution images of the events that unfold at the membrane of the egg cell during mammalian fertilization.

Ravaux says the “IVF chip” is a “unique tool to observe the cascade of molecular and membrane events occurring during the fertilization process,” under conditions that mimic what happens in nature.

The idea and design of the device are the product of expertise in biophysics and fertilization and assisted reproduction technologies (ART) – including in vitro fertilization (IVF).

At the heart of the new approach is an “IVF chip” – a microfluidic device made from an electronic chip comprising several layers of silicon polymer sealed on a glass slide.

The design of the chip allows a sperm cell to be held in the bottom layer with an egg cell held above it, inside an “egg cup.” At the bottom of the egg cup is a tiny opening, with a width of about 30 microns (roughly half the width of human hair).

When inserted in the lower layer of the chip, a sperm cell swims through the opening and fuses with the egg held in the egg cup.

The chip is compatible with confocal microscopy and other imaging systems, allowing the researchers to capture high-resolution images and movies of the fertilization process as it occurs.

The images show what happens to the sperm cell when it encounters the membrane of the egg cell. They show the two cells merging their membranes over time and the sperm cell gradually sinking into the egg cell.

The scientists also saw how the DNA in the sperm was assimilated into the egg’s cytoplasm – the fluid surrounding the nucleus of the egg cell.

Ravaux explains that the new technique offers scientists the chance to investigate an area of reproductive biology that has remained largely unexplored due to lack of tools.

The IVF chip is different to what has been tried before because it allows scientists to observe what happens when just one sperm cell fuses with an egg. Other attempts to do this have had to settle with observing multiple sperm cells coming into contact with the membrane of the egg cell.

Ravaux says the technique could be combined with other approaches – such as fluorescent antibodies or genetically modified animals – to offer new insights into the membrane events of the sperm-meets-egg process. He concludes:

An enhanced understanding of the molecular and physical mechanisms responsible for fertilization could ultimately lead to better methods to diagnose the causes of infertility, and improved personalized medicine treatments.”

From a study published recently, Medical News Today learned how scientists in China have created functioning sperm from stem cells, raising hope that the approach may one day be used to treat male infertility.