Fertilization process captured for first time ever

Highly detailed images of what happens when sperm and egg first touch have been caught on tape for the first time with the help of a newly-developed tool, shedding some light on the still-mysterious process of fertilization.

Researcher Benjamin Ravaux and his colleagues work in collaboration with biologists and physicians involved in fertilization and assisted reproduction technologies such as in vitro fertilization (IVF), developing multidisciplinary tools that will help to elucidate fertilization in mammals.

To that end, Ravaux designed a new microfluidics device that allows him to precisely control the membrane location where a sperm cell fuses with an egg. The device consists of a microfluidic chip made from a multilayer silicon polymer sealed on a glass slide; a sperm cell is located in the bottom layer and the egg is positioned in the top, inside an eggcup.

At the bottom of the eggcup is a tiny opening, 30 millionths of a meter wide, forming a connection to the chip's lower layer. When added to the lower layer, the sperm cell swims through the opening and adheres to, fuses with, and fertilizes the egg. The chip is compatible with optical imaging technologies such as confocal microscopy, allowing the researchers to obtain high-resolution images and movies of the fertilization process, as it occurs.

"This is a completely new approach," Ravaux said. With the device, the researchers were able to obtain the first images with both high spatial and temporal resolution of the onset of contact between a single sperm and egg, the merger of their membranes, and the engulfment of the sperm by the egg. In addition, they were able to observe the incorporation of sperm DNA inside the cytoplasm of the egg.

"Our approach has the potential to provide new knowledge in a research field that remains largely under-explored because of a lack of adequate technologies. This 'IVF chip' is a unique tool to observe the cascade of molecular and membrane events occurring during the fertilization process," Ravaux said.

The new technique will be described in a talk at the Biophysical Society's 60th annual meeting at the Los Angeles Convention Center in Los Angeles, Calif.