Tiny hairs move eggs down the fallopian tube

An old question about how the egg gets from the ovary to the uterus has been solved – it is all to do with little hairs in the fallopian tube.

The fallopian tube connects the ovary with the uterus. This tube has two purposes; the first is to allow the egg to travel from the ovary to the uterus, and the second is to provide a place where sperm can fertilise the egg – called the ampulla.

The unfertilised egg is first taken to the ampulla, where it stops to get fertilised by a sperm. Then, the new embryo cell travels down the fallopian tube to implant in the uterus, but if it cannot get there it will not grow (or will grow in the wrong place). This can be a cause of infertility.

Key research points

• Hair-like structures help move the unfertilised egg.
• Mice without cilia near the ovary were infertile.
• Cilia were essential for moving unfertilised eggs, but smooth muscle moved sperm and embryos.
• This could lead to non-hormonal targets from infertility treatment.

The egg doesn’t travel passively – instead, the fallopian tube does the major pushing. However, there was debate about how it did this. Either the smooth muscle contracted and squeezed the egg down the tube (like toothpaste), or tiny hair-like structures called motile cilia rhythmically wave like grass in the wind and help ‘sweep’ the egg in the right direction.

The answer to these questions is very important in determining the reasons and mechanisms behind infertility caused by inhibited implantation of the egg.

A brief tour of the fallopian tube

• Ovary: The place that makes eggs.
• Fallopian tube: the tunnel between the ovary and uterus.
• Motile cilia: hair-like structures in the fallopian tube.
• Ampulla: the place where sperm fertilises the egg.

Now, a team of researchers, led by Wei Yan of the Lundquist Institute, US, found that the motile cilia do the heavy lifting when it comes to getting the egg to the ampulla.

“We are very excited about solving this longstanding mystery,” said Yan. “It is now clear that both motile cilia and smooth muscle participate in the transport function, and motile cilia are required for egg pickup, whereas smooth muscle contraction is more important for sperm and embryo transport.”

Using mouse models, the team found that mice without motile cilia in the part of the fallopian tube that connected to the ovary couldn’t pick up eggs and take them away to be fertilised. This led to infertility.

But, when mice had poor quality motile cilia near the uterus end of the tube, the embryos were still able to move out into the uterus, because the movement was mostly done by the smooth muscle instead. Likewise, the sperm were still able to swim up the fallopian tube to the ampulla, regardless of motile cilia.

This means that the motile cilia are essential for the first stage of egg transport, but not so much for the second stage.

“Knowledge derived from the present study helps us understand the causes of female infertility and ectopic pregnancy, and the molecules essential for motile cilial functions represent a good target for developing nonhormonal female contraceptives,” says Yan.

Their study, “Oviductal motile cilia are essential for oocyte pickup but dispensable for sperm and embryo transport” is published in PNAS.

See more:

• Patchwork placenta
• Inside Biology’s Black Box
• What’s behind the belly button?