Using 3D in situ imaging of the female reproductive tract, researchers have revealed molecular signatures of fertilising sperm in mice.
When mammals mate, males ejaculate millions of sperm cells into the females’ reproductive tract. However, as the sperm travel up the reproductive tract, only a handful of the ‘fittest’ sperm reach the egg. This process of elimination prevents the egg from being fertilised by multiple sperm cells. It also stops the eggs from being fertilised outside of the womb.
Most of our understanding about fertilisation in mammals comes from studying how sperm and egg cells interact within Petri dishes. Nonetheless, this approach does not provide us with information about how sperm are selected and eliminated as they travel towards the egg.
Previous work has suggested that a core component of calcium channels – CatSper – may provide some answers. CatSper is made up of four proteins arranged into a unique pattern. Without this specific arrangement, sperm cells cannot move forward and fertilise the egg.
In a study, published in eLife, researchers established a new way to image the minute structures of sperm cells in the reproductive tract of female mice.
Experiments in a Petri dish have revealed that some sperm cells have cleaved CatSper1. Whereas in other cells this protein remains intact. CatSper1 is one of the proteins that make up the calcium channel. Visualising this, the researchers observed that sperm cells closer to the site of fertilisation contained non-cleaved CatSper1. Whereas, sperm cells further away were more likely to contain broken CatSper1.
These findings and the new imaging platform may allow scientists to learn more about the steps in the fertilisation process and what happens afterwards, i.e. when the egg implants into the mother’s uterus. The team suggest that further studies need to address what happens once the CatSper1 protein is cleaved and how this impacts the movement and lifespan of sperm. In turn, this could help identify new targets for contraception and improve current strategies for assisted reproduction.
Image credit: By kjpargeter – www.freepik.com