Conception not as simple as a sperm race: research reveals how female reproductive system chooses most ideal sperm
The study found that human eggs release certain chemicals to attract the right sperm and that the chosen sperms may be from an entirely different male than the one chosen by the woman to be her partner
Evolution is a brutal process which only allows the strongest and the fittest to survive. The subtle signs of this natural selection theory are seen in the complexities of human mating and sexual behaviour, where women are shown to chose specific partners that are genetically suited to producing a fit offspring with more chances of survival. Various theories exist about how a woman or a man chooses the right partner.
And even after that, the process of fertilization weeds out the unfit sperms before finding the right sperm cell for fertilisation.
The process of fertilisation is often considered to be an olympian race where the sperm that reaches the egg first is the one that fertilises it. In recent research though, a group of researchers at the Stockholm University, Manchester University NHS Foundation Trust and The University of Manchester said that a woman’s egg might reject the sperm of her chosen partner.
Explaining the findings of the study, Dr John Fitzpatrick, an Associate Professor at Stockholm University said in a press release that human eggs release certain chemicals to attract the right sperm and that the chosen sperms may be from an entirely different male than the one chosen by the woman to be her partner.
The obstacle race to fertilisation
Millions of sperms are released inside the uterus with a single ejaculation. However, the uterus has an entire obstacle course to help narrow down the choices the ova/egg has. These include the acidic pH of the uterus, the thick lining of mucus inside the uterus wall and the various white blood cells (WBCs) that the uterus uses to kill sperms. The female body considers sperms to be a foreign object and hence her immune system reacts to eliminate them.
Also, sperm can stay inside the uterus for about 3-5 days, but an egg only becomes viable for 24 hours.
Studies suggest that some birds even eject out sperms to avoid the unfit ones from fertilizing the egg.
The first sperm to reach the egg does not always fertilise it. This is because ova have two thick protective layers surrounding it and the sperms have to hydrolyse (break down) these layers enough to be able to enter the ova. So, hundreds of sperms break down the layers and only when there is enough space through the two layers can a sperm enter into and fuse with the ovum.
Usually, the female reproductive system prepares the sperm for this through a process called capacitation. Again, those sperms that do not complete this process cannot fertilise the ova.
The role of the egg
Researchers have observed that the otherwise passive looking egg has much to do when it comes to choosing the right sperm. The egg releases specific chemicals to attract the sperm that is genetically compatible with it and would reject the sperm that is incompatible.
The genes that make this choice are usually related to the immune system, more specifically the MHC (Major histocompatibility complex) present in human cells. MHC plays an important role in recognition and elimination of pathogens from the body. So, if an egg fuses with a sperm with dissimilar MHCs, the progeny would have a stronger immune system than either of the parents and would hence be fitter to survive.
It has previously been noted that the MHCs have an association with body odour and humans can detect the variations in MHC through body odours. They also tend to choose perfumes that most match their MHCs.
For more information, read our article on In Vitro Fertilisation
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