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Decoy eggs used to provide birth control for mice

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Scientists have created a novel method of contraception, using polymer beads coated in a special protein as "decoy" eggs in mice. In experiments described in Science Translational Medicine, researchers deposited the beads in the uteruses of mice. When the mice mated, sperm cells bound themselves to the fake eggs, preventing the real eggs from being fertilized. The scientists from the National Institutes of Health say it’s extremely unlikely the beads would be used in their current form as human contraception, but that they do show promise as a better way to select sperm for use in fertility treatments.

The key to the scientists’ discovery is a selection of peptide proteins that coat the eggs of mammals and bind sperm to their exterior. These peptides make up what is called the "zona pellucida" — a sort of extracellular eggshell that surrounds the egg in a transparent layer. The exact mechanism that causes sperm cells to attach to the pellucida is still unknown, but the peptides that comprise this layer effectively act as a glue trap for the mobile sperm.

The peptide beads reduced fertilization rates in mice to just above 1 percent

Scientists tested whether the peptide-covered beads acted as decoys for sperm first in external containers, and then in living mice. Beads with and without the peptide coating were implanted in healthy mice before breeding. (Mice without any bead implants were used as a control group.) After breeding, the mice were anesthetized and their uteruses examined for embryos. Fertilization rates in the untreated mice and those implanted with uncoated beads was above 80 percent, but in the mice implanted with peptide-covered beads it was just above 1 percent. After several weeks, the mice regained normal fertility as the beads were naturally discharged, suggesting the method offers reversible contraception.

The peptide coating could be added to existing contraception

However, this doesn’t mean that these peptide-covered beads could be used as a method of contraception for humans. Speaking to The Verge, Jurrien Dean, one of the study’s co-authors, said the beads need "a lot more development before they can be tested in anything other than mice." He says that it’s more likely that the beads or their peptide coatings would be added to existing contraception methods to improve their effectiveness. "You can imagine attaching the peptides to spermicidal sponges, or to vaginal rings impregnated with hormones," Dean says.

He adds that there are still many unknowns about the beads, including whether or not there are adverse affects from repeated use as contraception, and whether the treatment is reversible over longer periods of time. "It could be used to control fertility in pets and farm animals, and […] if it was irreversible, it might be a desired outcome," Dean says.

The beads need clinical trials before BEING used for infertility treatments

A more immediate application for the beads might be selecting sperm for fertility treatments. Currently, the process of choosing the best sperm relies on analyzing sperm concentration or sperm motility. The peptide-covered beads effectively act as try-outs to find which sperm can successfully bind to eggs. Human sperm mixed in with the peptide-covered beads quickly bind together, but remain detachable "by gentle pipetting" for the first 30 minutes. In the study, these selected sperm cells were compared to the starting population for their ability to bind to donor eggs. The number of cells that penetrated the eggs "substantially increased after selection," say the study's authors.

As with the beads used as contraceptive, this sperm selection technique will need further tests to prove its utility, but the scientists involved say as a proof of concept their study is a success. "Going forward we’d like to see controlled clinical trials in fertility clinics to determine if the peptide beads could select sperm that were superior for the treatment of infertility," says Dean.