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We recently posted an ovary dissection video (which you can check out here), where we explained the anatomy and physiology of the ovary, as well as the process of fertilization in the mammalian reproductive system. As we briefly mentioned in the video, there are a lot of pre-sex factors that determine mate choice in humans–men can subconsciously determine whether a woman is ovulating merely by smelling her T-shirt, women are driven by hormones to select confident men with a masculine presentation, etc. However, recent research conducted by Stockholm University and Manchester University NHS Foundation Trust has revealed that the game of mate choice continues even after sex–human egg cells use chemical-based preferences to, in a sense, “choose” which sperm cell it wants to fuse with.
John Fitzpatrick, an assistant professor of zoology at Stockholm University, and one of the authors of the study, explained that "Human eggs release chemicals called chemoattractants, which leave a sort of chemical breadcrumb trail that sperm use to find unfertilized eggs.” A primary focus of the study was to determine whether the chemoattractants from one egg cell attracted sperm from some men more so than the sperm of others. To accomplish this, the researchers examined how samples of sperm from different men responded to the follicular fluid of egg cells, which is the chemoattractant containing fluid surrounding egg cells. Surprisingly, the results of comparing the response of different sperm cells to a particular egg cell's chemoattractants revealed that, in the words of Dr. Fitzpatrick, “Follicular fluid from one female was better at attracting sperm from one male, while follicular fluid from another female was better at attracting sperm from a different male.” In short, these results show that the specific identity of the man involved plays a role in determining the interaction between the sperm cell and the egg cell’s chemoattractant–by having a greater attraction to certain sperm cells, the chemoattractant “chooses” which sperm is going have a much higher chance of successfully reaching the egg cell. In a sense, the chemoattractants, and more generally the egg cell that secretes the chemoattractants, are giving the woman a second chance at mate selection at the molecular level, long after the woman has made the initial mate choice at the organismal level.
Although we might expect the molecular mate choice of the egg cell to align with the mate choice made by the woman, surprisingly that is often not the case. Researchers went into the study expecting a “partner effect”–where the sperm cell of a woman’s partner would be preferred by the woman’s chemoattractants over a random sperm cell–but discovered that in nearly half of the cases, the chemoattractant of a woman’s egg cell was more attracted to a stranger’s sperm over her partner’s. Why? Scientists hypothesize that the most probable explanation for the lack of a “partner effect,” is that at the molecular level, the egg cell is most attracted towards sperm cells with which it is most genetically compatible (genetic compatibility is not something the woman can use as a factor for mate selection in an organismal level, which provides more merit to the argument that it plays a significant role in the molecular level). Counterintuitively, when we talk about egg cells wanting a sperm cell that it is “genetically compatible” with, we mean that egg cells seek out sperm cells with which it is most genetic different from. Mating with an individual possessing a set of genes distinct from oneself has many evolutionary advantages, one of the most prominent advantages being MHC diversity. Major Histocompatibility Complex (MHC) are genes that help the immune system protect the body, and greater diversity in the MHC genes allows an individual to better fend off a greater variety of diseases/infections. Everyone has a slightly different combination of the MHC genes, and thus the more different a partner’s MHC gene combination is from an individual’s, the more diseases/infections the offspring between the two would be able to fight.
(Video demonstrating the sperm cells' arduous journey inside the female reproductive tract)
Although the focus of this article is on the role chemoattractants play in mate selection, humans, and mammals in general, value genetic diversity in their offspring to a point where the entire process of fertilization is an “obstacle course” designed to weed out less genetically diverse, less ideal sperm cells. When the sperm cell first enters the female reproductive tract, they face against a downwards stream of reproductive fluid, much as salmons face a downwards current on their journeys upstream. In addition to that, the female immune system perceives the sperm cells as foreign, thus launching full-out immune attacks that the sperm cells must endure. The sperm cells that manage to defeat the downstream current and the immune attacks are then faced with the muscular contractions of the cervix. When the significantly smaller pool of sperm cells finally reach the uterus, they must choose to travel in one of two fallopian tubes–unfortunately, approximately half the surviving sperm cells won’t even get anywhere near the egg cell, as the ovulated egg cell only travels down one of the two tubes. The conditions leading up to the final steps of fertilization are so extreme, that as Dr. Fitzgerald states “of the tens of millions of sperm a male might deposit … only about 250 total sperm get to the site of fertilization where the egg is.”
However, the competition is far from over. Of the ~250 sperm cells that make it to the fallopian tube where the egg is present, only 20-30 are ready to fertilize an egg at any given moment. This is because sperm cells, in a sense, are continuously turning on and off their ability to fertilize an egg, as you would flip a switch–the duration of the period a sperm cell has its fertilization capacity turned off makes it so that at any particular instance, only about 10% of the ~250 sperm cells can fertilize the egg. When only about 20-30 cells remain as potential candidates, this is when the egg cell gets a say in the selection process. In the final phase of this arduous journey, when the sperm cells are mere two centimeters away from the egg, the egg cell starts releasing follicular fluid that contains chemoattractants. Studies have found that sperm cells have odor receptors on their head that respond to the chemoattractants secreted by egg cells, the strength/type of chemical signal determining how fast/effectively the sperm cell swims. If an egg cell wants a particular sperm cell to travel fast, it will send a strong chemical signal that encourages vigorous swimming, and if an egg cell does not want a particular sperm cell to travel fast, it will send chemical signals discouraging the sperm cell from swimming in a fast, straightforward manner. Ultimately, the winner of the race is chosen–after heavy last-minute input from the egg cell–when a single sperm cell fertilizes the egg.
Aside from being a fascinating scientific discovery, the knowledge of the role egg cells play in mate selection has potential real-life applications. Currently, ~30% of all infertility cases have “unexplained” causes, and scientists hypothesize that the newfound information on the role egg cells play in fertilization could provide new insight into some of these “unexplained” infertility cases.
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