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The process of fertilization is an inevitable one as it the main contributing factor to procreation. The outcomes of fertilization are the offspring who grow up and repeat the process thus coming up with other offspring. It is therefore the cycle that leads to the existence of living things. Fertilization is a process that essentially entails the fusion of two gametes thus producing a new organism.
This process can either take place inside or outside the body of the organism depending on the species. In mammals, fertilization mostly takes place inside the body of the female animal whereby the gamete from the male animal known as the sperm fuses with the female gamete (egg) in the oviduct forming a zygote.
The zygote undergoes development and after the gestation period, it is released to the environment as a mature organism. For a long period, the process of fertilization had been a mystery until recently when the process was extensively investigated to unveil the mystery behind it. This paper is therefore an exploration of the process of fertilization in mammals by looking at what initiates it, how it takes place and the scientific explanation behind the process.
The process of fertilization in mammals
The process of fertilization begins with coitus whereby the mammalian sperm is released into the vagina of the female mammal. Once inside, the sperm finds its way into the egg by swimming. The egg is normally placed in the oviduct. On reaching the egg, the sperm fights to get in through fusion.
The mammalian oocyte or egg comprises of three layers that encloses it. These layers include; the cumulus layer, zona pelllucida and the plasma membrane (Evans 297). For the process of fertilization to be successful, the mammalian sperm ought to penetrate through the three layers. The interaction of the mammalian sperm with the membranes of the oocyte involves several interactions.
The membrane interactions of the sperm and oocyte
To find out on the sperm-oocyte interactions, microscopic analyses were conducted. The study indicated that the plasma membrane of the mammalian oocyte is covered with microvilli (Evans 298). Therefore, once the acrosome reactions take place, the posterior end of the sperm is attached to the membrane of the oocyte thus fusing.
After the fusion, the sperm’s motion is inactivated. The study also indicated that the sperm with the highest motility is the one that penetrates the oocyte membranes. Those with low motility get lost into the body of the female mammal. However, once a sperm has fused with the oocyte the membrane prevents other sperms from fusing with the oocyte again.
It was also found out that the gamete membrane interactions are mediated by various molecules. Some of these molecules include a protein known as DE, CRISPI, ARP and AEG-1 for the sperm. The oocyte contains molecules such as integrins, which play a great role of adhesion. Other components that assist in the reaction are Ca2+, phospholipase, and IP3 receptors just to mention a few. Essentially, the process of fertilization in mammals takes place in the five steps discussed below.
The five steps of fertilization
The first step is that of attachment of the sperm to the egg. This is species-specific as eggs and sperm will only bind if they are from the same mammalian species (Wassarman et al 1). For instance, it is not possible for the sperm of a rabbit bind and fuse with the egg from a dog, since they are not of the same species. However, recent developments have indicated that fusion of gametes from different species would only fuse if the zona pellucida is removed from the egg using buffers or proteases.
The second step occurs after the sperm binds to the zona pellucida of the egg therefore undergoing an acrosome reaction referred to as cellular exocytosis. In this reaction, multiple fusions are bound to occur between the plasma membrane and the external membrane. Thereafter, the contents of the interior part (acrosome) are exposed thus enabling fusion to take place. The sperm that manages to fuse with the acrosome contents is the one that has fully completed the acrosome reaction to penetrate the zona pellucida.
Penetration of the sperm into the egg zona pellucida is the third step. The sperm that has undergone the acrosome reaction now binds to the zona pellucida for penetration. This step is facilitated by sperm motility as well as enzymatic hydrolysis. The catalyst in action is known as acrosin, which helps the sperm to penetrate the zona pellucida of the egg successfully.
The fourth step is the fusion step whereby the successful sperm fuses with the contents of the egg. This process is partly mediated ADAM proteins and the integrins, which are adhesive compounds (Primakoff and Myles 84). Others are fertilin-α and fertilin-β which support the binding process of the sperm to the egg contents.
The last step in the fertilization process is a continuation of the latter stage whereby the fusion process goes on. The only difference is that the facilitating proteins are now called CD9. These act with the combination of β1 integrins to complete the fusion process (Wassarman et al 3).
Once the process is completely done, it leads to the establishment of the zygote, which is a diploid cell (Bruce). The zygote then slowly develops to form a blastocyst, which then comes down from the oviduct to the uterus (Bruce). Once in the uterus, implantation occurs, marking the beginning of pregnancy.
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In a situation where implantation occurs in a different location other than the uterus, ectopic pregnancy is evidenced. Because, pregnancy cannot take place to term in any other place other than the uterus, such a pregnancy is terminated.
From the above discussion, it is evident that the process of fertilization is one of the complicated processes in the body of mammals. This is because it encompasses a series of subsequent reactions and steps that take place inside the body of the female mammal. It is also clear that the process of fertilization occurs in five major steps. Each of these steps requires different compounds and substrates to complete every step. Despite the fact that the process is a series of reactions, the time taken to complete them is very short.
However, this depends on the species of the animal just like the differences in the gestation period. It has also been seen that penetration of the egg zona pellucida by the sperm is species specific, such that penetration only occurs in mammals of the same species. In case of inter-specie penetration, the zona pellucida is removed so that the sperm only penetrates the plasma membrane.
Bruce, Carlson. Patten’s Foundations of Embryology -6th Edition. (1996). McGraw-Hill, Inc., New York. Print.
Evans, Janice. “The molecular basis of sperm-oocyte membrane interactions during mammalian fertilization.” Human Reproduction 8.4 (2002): 297-311. Print.
Primakoff, Paul and Myles, Diana. “The ADAM gene family: surface proteins with adhesion and protease activity. Trends Generation. 16.1 (2000): 83-87. Print.
Wassarman, Paul, Jovine, Luca and Litscher, Eveline. A profile of fertilization in mammals. 2001- September 28, 2011. <http://www.life.illinois.edu/>