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Female's cervical mucus selects best sperm

24 August 2020
Appeared in BioNews 1061

Finnish researchers have found that sperm function better in cervical mucus of women with less similar immune genes

Professor Jukka Kekäläinen and her team at the University of Eastern Finland collected cervical mucus samples from nine women and sperm samples from eight men and tested them in all possible combinations to see how the sperm behaved. 

'Cervical mucus is the first major barrier that sperm encounter on their way to fertilise the egg', andrologist Professor Allan Pacey of the University of Sheffield, who was not involved in the present study, told the Australian. 'It's a complex structure that both helps to protect the uterus from infection but also acts as a sperm-filter and possibly as a sperm storage site.' 

The team found that sperm fared better in cervical mucus of females whose genetic profiles were less similar to the genetic profiles of the males the sperm came from. The researchers also looked at each subject's human leukocyte antigen (HLA) genes. HLAs are a group of proteins that reside on the surface of cells and help the immune system differentiate between the body's own cells and invading threats, such as bacterial cells. 

'These findings can have important implications for a deeper understanding of sexual selection and the fertilisation process in humans and other mammals,' said Professor Kekäläinen.

It is not always obvious why some couples have trouble conceiving and there is still much to be learned about the compatibility of human gametes. Unfortunately, there is also no routine test for immunological compatibility at the moment. 

Previous studies have established men and women prefer the smell of those with a less similar HLA sequence. HLAs were also detected on the surface of sperm cells and in cervical mucus, supporting the idea that the immunological matching of egg and sperm might play an important role in successful fertilisation. 

'Quite a lot of these type of effects have been shown in other animals... but obviously there's a different importance to understanding this in humans,' says Professor David Richardson, a biologist at the University of East Anglia in the UK not involved in the study. 'Unlike in birds or fish, it's hard to determine whether these mechanisms produce more viable or more healthy offspring in people', said Professor Richardson. 'It wouldn't be that surprising, given what we know about other species, but it would obviously open up lots of opportunity for [...] fertility treatment.'

The team is now planning to try and identify what mechanisms underlie this post-copulatory selection process.

'These findings can have important implications for a deeper understanding of sexual selection and the fertilisation process in humans and other mammals,' said Professor Kekäläinen. 'Since the gametes of some partners may be immunologically more compatible than others, our results may also open up novel possibilities for the development of more accurate infertility diagnostics,' co-author Annalaura Jokiniemi added.

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