The Design of the Seminal Fluid and Sperm Capacitation
In a previous article, I discussed the ways in which sperm cells exhibit irreducible complexity. Here, I will discuss the importance of the seminal fluid and how it contributes to the irreducibly complex core of components needed for successful reproduction. I will then consider the process of sperm capacitation, the mechanism that prepares the sperm cells for successful fusion with the egg.
The Seminal Fluid
As I mentioned in my previous article, between two hundred and five hundred million sperm, surrounded by seminal fluid, are released with each ejaculation. Such huge numbers are necessary in order to have a significant chance of fertilizing the egg, since many hazards confront the sperm cells as they swim through the uterus and uterine tubes. Following ejaculation, millions of the released sperm cells will either flow out of the vagina, or else die in its acidic environment. Sperm cells also need to pass through the cervix and opening into the uterus, which requires passage through the cervical mucus. Though the mucus is thinned to a waterier consistency during the fertile window, making it more hospitable to sperm, millions of sperm cells will nonetheless die attempting to make it through the mucus. Furthermore, the female reproductive tract has immune defenses that protect against pathogens. These defenses can also target and destroy foreign cells like sperm. Antibodies may recognize sperm as foreign invaders and lead to their inactivation or elimination. There are also tiny cilia in the fallopian tube that propel the egg towards the uterus. Some of the remaining sperm will become trapped in the cilia and die. Only a small handful of the original sperm cells will make it as far as the egg. Thus, it is necessary that hundreds of millions of sperm cells are released in order to have a reasonable chance of the egg cell being fertilized.
Seminal fluid also provides essential nutrients to support the survival and motility of the sperm. These include fructose — which serves as a source of energy for the sperm, fueling the mitochondrial production of ATP — as well as other sugars, amino acids, and enzymes. If the seminal fluid did not contain fructose, to power the mitochondria, this would have drastic implications for sperm cell motility and viability.
The seminal fluid is also alkaline. This is important because the vagina has an acidic pH, produced by the normal flora (bacterial populations) of the vagina. This environment would be unfavorable to sperm cells. But the alkalinity of the seminal fluid helps to neutralize the vagina’s acidic pH, assisting the survival of the sperm.
Following ejaculation, the seminal fluid initially coagulates to form a gel-like consistency. This coagulation helps to keep the semen in the vagina and cervix, preventing it from immediately leaking out and thereby greatly increasing the odds of a successful fertilization. This occurs upon exposure to the air or the alkaline environment of the female reproductive tract, activating clotting factors present in the seminal fluid, including tissue transglutaminase. The transglutaminase converts semenogelin (a major protein in seminal fluid secreted by the seminal vesicles) into a sticky protein called fibrin. Fibrin forms a network-like structure that entraps sperm and other components of the semen.
If the semen remained in this state, the sperm would be permanently immobile and unable to fertilize the egg. Over time, however, the coagulated semen liquefies due to enzymes present in the fluid that slowly break down the fibrin network, allowing the sperm to move more freely. Anamthathmakula and Winuthayanon note that “The liquefaction process is crucial for the sperm to gain their motility and successful transport to the fertilization site in Fallopian tubes (or oviducts in animals). Hyperviscous semen or failure in liquefaction is one of the causes of male infertility.”1 In fact, targeting these serine proteases has been suggested as a target for novel non-hormonal contraceptives.2From an evolutionary perspective, it is difficult to envision a scenario where semen coagulation evolved, without simultaneously having a mechanism for liquefaction. This is a prime example of a non-adaptive intermediate that is prohibitive to evolution by natural selection.
Sperm Capacitation
In order for a sperm cell to fertilize an egg, it has to undergo capacitation. This takes place in the female reproductive tract. The process of capacitation involves a series of biochemical and physiological changes that prepare the sperm for successful interaction with the egg and is crucial in order for the sperm cell to acquire the ability to fertilize.
When sperm are initially ejaculated, they possess certain molecules and proteins on their surface that inhibit their ability to fertilize an egg. During capacitation, these surface molecules, such as cholesterol and glycoproteins, are removed or modified, allowing the sperm to become more receptive to the egg. As capacitation progresses, the motility pattern of sperm also changes. They undergo hyperactivation, which is characterized by increased amplitude and asymmetrical beating of the tail. Hyperactivated sperm exhibit vigorous movements, which help them to navigate through the female reproductive tract and reach the egg. Capacitation also involves changes in the composition and fluidity of the sperm cell membrane. These changes allow the sperm to better interact with the egg’s zona pellucida. The acrosome becomes primed for the acrosome reaction, which releases these enzymes to allow penetration of the egg membrane.
Capacitation is associated with an increase in calcium ion influx into the sperm. Calcium plays a crucial role in various intracellular signaling processes that are necessary for sperm function and fertilization. For a much more detailed treatment of what is known about the mechanisms of sperm capacitation, there are good reviews of this subject, to which I direct readers.3,4
Conclusion
In summary, various features of the head, middle piece, and flagellum, together with the properties of the seminal fluid, are critical to the sperm cell’s function of reaching and fertilizing an egg. If any one of these parts is not present or fails to function properly, the sperm cell is rendered completely impotent, and reproduction cannot occur. The phenomenon of human reproduction points to a cause with foresight — one that can visualize a foreordained outcome and bring together everything needed to realize that end goal. There is no cause in the universe that is known to have such a capacity of foresight other than intelligent design.
Notes
Anamthathmakula P, Winuthayanon W. Mechanism of semen liquefaction and its potential for a novel non-hormonal contraception†. Biol Reprod. 2020 Aug 4;103(2):411-426.
Ibid.
Puga Molina LC, Luque GM, Balestrini PA, Marín-Briggiler CI, Romarowski A, Buffone MG. Molecular Basis of Human Sperm Capacitation. Front Cell Dev Biol. 2018 Jul 27;6:72.
Stival C, Puga Molina Ldel C, Paudel B, Buffone MG, Visconti PE, Krapf D. Sperm Capacitation and Acrosome Reaction in Mammalian Sperm. Adv Anat Embryol Cell Biol. 2016;220:93-106.
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