Human Eggs – Preference of Sperm

Human Eggs – Preference of Sperm

How extraordinary! Did you know that the identities of a man and woman can play an important part in the interaction between a human egg and sperm?

“Human eggs release chemicals called chemoattractants, which leave a sort of chemical breadcrumb trail that sperm use to find unfertilized eggs,” said study author John Fitzpatrick, an assistant professor in the department of Zoology at Stockholm University in Sweden.

It was only when a study was conducted that it was found that the chemicals act differently on sperm from different males, deciding on the choice of sperm which is successful. And the strange part is that a woman’s egg does not always agree with her choice of partner! In the study conducted, about 50% of the cases showed that the eggs were attracting more sperm from a random male – and the presumed conclusion was that more genetically compatible males are chosen by females with the “permission” of these chemoattractants!

The hormones present at ovulation can cause a woman to choose what type of sperm she permits – those of a confident man or a more masculine looking man. Women paired with feminine-faced men were, when ovulating, more attracted to men other than their partners!

In the case of males, they can also “sense” when a woman is ovulating and can become more possessive or loving. The sense of smell also plays a part in the testosterone levels! In a study, over 100 undergraduate men smelled worn T-shirts of young women who were near ovulation and others smelled unworn T-shirts which served as the “non-ovulating” control. It was found that the men who sniffed the ovulation-scented shirts had higher levels of testosterone than those who smelled the “control” shirts.

Diverse genes: The MHC (major histocompatibility complex) is a group of genes that, basically, help the immune system by fighting infection and diseases. The more diverse the genes are, the more diseases can be fought by the immune system. And, if one partner has a slightly different combination of these genes, then the higher the immunity of the child to fight off even more infections and diseases.

Because our bodies are programmed to give our children the best chance of survival, they develop many methods to find the strongest, most diverse and compatible male. One of these methods is the female reproductive tract, where the fluids flow downwards, forcing the sperm to swim upwards. The female’s immune system attacks the sperm as an invader and, once they pass through the cervix and uterus, they have to choose one of the 2 fallopian tubes. Only one will have an egg in it and, if the fertilized egg attaches to the wall of the uterus, the female becomes pregnant. So, despite millions of sperm that a male deposits, only about 250 sperm actually get to the egg for fertilization. And only about 10% of those sperm (only about 20 or 30 cells) are able to fertilize at any given time. And, finally, the egg, by the chemicals it releases in the follicular fluid that surrounds it, can affect which sperm will be the winner! According to Dr. Fitzgerald, “It’s only in the last two centimeters between a sperm and the egg that these chemical signals matter since it’s the final phase of this long journey where females continue weeding out less acceptable sperm.”

Although the finding that eggs are choosing sperm is rather novel in human fertility, research on this subject will advance fertility treatments and may help with understanding the causes of “unexplained infertility” in couples.

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