Researchers from both the University of Manchester and Stockholm University, Sweden showed that follicular fluid which surrounds eggs in the ovary and is released with the egg at ovulation, contains chemical signals that improve the chances of successful fertilisation in humans.
'Human eggs release chemicals called chemoattractants that attract sperm to unfertilised eggs. We wanted to know if eggs use these chemical signals to pick which sperm they attract,' said lead author Dr John Fitzpatrick from Stockholm University.
The team used samples of sperm and follicular fluid from 16 couples undergoing assisted reproductive treatment. Sperm were able to swim towards either of two follicular fluids in a dish, or one follicular fluid and a control substance.
The control experiments proved that the follicular fluids have a strong chemo-attractant effect, attracting ten-times more sperm than the control samples. However, when sperm were exposed to two different follicular fluids there was often a marked difference in how many sperm were attracted to each sample – up to 18 percent.
Each sample of follicular fluid attracted more sperm from some samples than others, but the pattern of attraction appeared random and was not correlated with whether the donors were partners or not. The researchers hypothesise that eggs are likely to attract sperm that is more genetically compatible.
The team demonstrated that the differences in sperm accumulation between follicular fluid samples were predominantly caused by this attraction effect, as opposed to the quality of male ejaculate or the quantity of chemical signals in the female follicular fluid.
'The idea that eggs are choosing sperm is really novel in human fertility,' said senior author Professor Daniel Brison, from the Department of Reproductive Medicine at Saint Marys' Hospital, Manchester. 'Research on the way eggs and sperm interact will advance fertility treatments and may eventually help us understand some of the currently 'unexplained' causes of infertility in couples.'
Future studies need to explore whether the same interactions are also present in regular mammalian (particularly human) reproductive cycles, away from the context of assisted reproductive treatment.
The research was published in the Proceedings of the Royal Society.