Mucus—every wet surface of our bodies is coated in it. It helps to lubricate our organs, protect us from our environments, and prevents infection by microorganisms. While mucus is 90% water, the other 10% is a complex mixture of biological molecules whose composition and consistency depends on where in the body you find it. For example, the mucus that coats our eyes is tethered to the surface of the ocular cells whereas the mucus in our respiratory system is secreted. Another place you can find secreted mucus is the cervix.
The female reproductive system is a bit of a maze whose primary function is to support fertilization of an egg and the growth of an offspring. Basically, an egg is released from the ovaries into the fallopian tubes where it awaits fertilization by sperm. These sperm enter the vaginal canal during intercourse, then move through the uterus to meet the egg where the two join to create an embryo. It’s an amazing process that is responsible for the conception of most people reading this post. But the first gate keeper that the sperm have to pass in their journey is the cervix—a very small opening between the vaginal canal and the inner uterus. And what do you imagine the cervix is filled with? Cervical mucus.
Cervical mucus changes throughout the menstrual cycle
Cervical mucus is an interesting material for a number of reasons. First, depending on where the female is in their menstrual cycle, the cervical mucus changes in consistency. During ovulation—when an egg is ready to be fertilized—there is a lot of thin, watery mucus. In the fertility world, this is called egg white cervical mucus because of the resemblance. Outside of ovulation, however, the cervical mucus becomes thick and opaque. Why does this happen, you might ask? The reason is sperm.
While the whole purpose of the reproduction system is to facilitate sperm meeting egg, the when is really important. Both the vaginal environment and seminal fluid have associated bacteria that are normal and cause no problems in their natural environments, but could pose a threat to the new embryo or upper reproductive tract. Then you have to think about the various sexually transmitted diseases that can be carried by semen. For that reason, the uterus tries to limit its exposure to sperm to only the times when an egg is present for fertilization. So the cervical mucus in easily traversed by the sperm during ovulation, but virtually impenetrable outside of ovulation.
This is one of the reasons that people trying to get pregnant often track their cervical mucus consistency to monitor the best time to have intercourse. In fact, problems with this system can actually cause infertility in women who have what is called “hostile” cervical mucus. In these cases, the cervical mucus remains thick even during ovulation, thus preventing a couple from getting pregnant. For some women, like those who expressly do not want to get pregnant, this change in cervical mucus is a good thing. Thickening of the cervical mucus is actually one of the benefits of being on hormonal birth control.
Cervical mucus filters out “bad” sperm
But that’s just the first thing that is interesting about cervical mucus. The second is that cervical mucus can filter out “bad” sperm. It has been shown that sperm with damaged DNA are often bad at swimming. Now imagine that poor swimmer trying to make it through a thick, gel material like cervical mucus—they don’t make it through. Only the good swimming sperm with undamaged DNA are able to move on to the next stage of fertilization, thus preventing the female from wasting a valuable egg on an unviable offspring. In this way, cervical mucus helps the female select for the strongest sperm.
Cervical mucus changes its sugar composition
The third interesting thing, the one that I am actually researching, has to do with the molecules that give mucus its consistency—mucins. Mucins are very large biomolecules composed of a protein backbone that is covered in sugars. These sugars actually make up 80% of the mucin molecule by mass. Contrary to popular knowledge, sugars are not just used for energy. They actually serve a lot of functional roles in our bodies. For example, differences in sugar composition on our red blood cells is the reason we have different blood types. In the case of mucin molecules, these sugars help the molecule hold onto water and prevent bacteria from breaking the molecule apart.
In cervical mucus, the composition of these sugars also changes throughout ovulation, which is weird. Well, weird if holding onto water and protecting the molecule is the only role the sugars play. In that case, any old sugar should do and there would be no need for cyclic change.
The study of functional sugars—called glycobiology—is still a relatively new field, so scientists don’t yet understand the true importance of sugars in mucus. It’s possible that the changes in sugar composition could contribute to the changes in consistency of the cervical mucus. It could also be that the sugars are directly interacting with the sperm themselves to either help or prevent the movement of sperm through the material. Sugars could be one of the ways that the reproductive system selects for viable sperm. We simply don’t know. But scientists like myself are investigating this phenomenon and maybe someday we’ll know the answer.
It is a fairly recent development that scientists are looking at the female reproductive system at all. For a long time, it was believed that the primary selection process in fertilization was differences in sperm: there are more of them and they move. Studying cervical mucus proves that that is not the case. The female reproductive system definitely plays a role in assuring that only the most fit offspring are created.
Overall, cervical mucus is an amazing, dynamic material that plays an important role in human fertilization and reproduction. Hopefully with more research, we will learn even more about ways that the female reproductive system plays an active role in offspring selection.