Decoy eggs to trap sperm

29 April 2016
Posted by Chris Smith.

Microscopic beads coated with part of the outer shell of human eggs can sequester sperm to prevent fertilisation, potentially ushering in a newSperm and Egg form of contraceptive.

Surprisingly, the same tool might also be useful in selecting the fittest sperm for fertility treatments, a new US study has shown.

Mammalian eggs, humans included, are surrounded by an outer shell called the zona pellucida.

This is made up of four proteins, designated ZP1-ZP4, woven together.

The ZP2 component plays the linchpin role of recognising and binding to sperm and triggering fertilisation of the egg.

In essence, it works like the sperm equivalent of velcro.

Now, writing in Science Translational Medicine, National Institutes of Health, Maryland, scientist Matteo Avella and his colleagues have come up with a way to coat tiny beads of agarose with ZP2, producing a harmless contraceptive system that, in tests on mice, reversibly prevented pregnancy by acting as a sperm decoy.

The team made the ZP2 by expressing the gene for the protein in a cell culture system and then soaking the beads in the solution overnight.

Squirted into the uteruses of experimental mice, a million or so of the tiny beads, which are not harmful to health, soaked up sperm from matings with male littermates and prevented pregnancy for more than 10 oestrus cycles, after which normal fertility gradually returned as the beads slowly left the body.

The system works by decoying and sequestering sperm in the uterus, rather than permitting them to make their way into the fallopian tubes where fertilisation normally takes place.

Another unexpected twist in the story was that the researchers also found that they could use the system to improve the overall quality of sperm in a sample that might be used for in vitro fertilisation (IVF).

Mixing sperm with the beads for a short while and then pipetting them off led to an enrichment in the sample of motile, fertilisation-friendly sperm.

Although the results are preliminary, and so far have been tested only in mice, this new technique might provide a novel means of contraceptive that doesn't rely on manipulating female hormonal cycles, which can have side effects and is not acceptable to some women.

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