Motile cilia are needed to transport ovulated eggs in the fallopian tube, according to new research.
The study, published in the Proceedings of the National Academy of Sciences (PNAS), used mice lacking motile cilia, which are little hairs found in the fallopian tube, to show that they are essential for transporting eggs from the ovary to the uterus. The study further clarified that smooth muscle, which was also thought to move eggs in the fallopian tube, is instead more important for moving sperm and embryos.
'We are very excited about solving this longstanding mystery,' said Dr Wei Yan, an investigator at the Lundquist Institute in Torrance, California, who led the study. '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.'
The cilia at the end of the fallopian tube nearest the ovary, picks up and transports ovulated eggs, called oocytes, to the widest part of the fallopian tube called the ampulla, in the middle of the fallopian tube. Here, an egg can be fertilised by sperm. A fertilised embryo is then transported further down the fallopian tube to the uterine cavity where it can eventually implant and grow. This process is achieved by cells which have motile cilia called multiciliated cells and smooth muscle cells.
Motile cilia 'beat' and smooth muscle contracts to move objects, but which cell type drives the movement of ovulated eggs and embryos at different points in the fallopian tube has been unclear. Clarifying this mechanism is important, because disrupted transport can result in eggs being unable to be fertilised, or embryos being unable to grow or growing in the wrong place, which can lead to infertility and an ectopic pregnancy.
The results of the study have resolved the mechanism underlying the transport of oocytes in the fallopian tube from the ovary to the ampulla, in the middle of the fallopian tube. Using mice that lacked motile cilia, Dr Yan's team showed that oocytes could not be picked up in the part of fallopian tube that connects to the ovary, called the infundibulum, and be transported to the ampulla for fertilisation. Thus, disruptions to either the structure or beating patterns of motile cilia lead to fertility loss.
The study also showed that motile cilia in other parts of the fallopian tube closer to the uterus facilitated sperm and embryo transport but were not essential for fertility. This is because embryos could also be moved due to smooth muscle contraction in that areas. Sperm were also able to reach the ampulla regardless of whether motile cilia were present or not, due to the contraction of smooth muscle.
According to the authors, these findings may contribute to developing non-hormone female contraceptives, and understanding the cause of ectopic pregnancies and other conditions.
'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 non-hormonal female contraceptives,' said Dr Yan.