Living things are equipped with sexual and asexual reproduction systems to enable life to continue. Sexual reproduction requires the presence of two opposite genders, male and female. Special reproductive cells, the sperm and egg, are produced in the reproductive organs of these individuals. The shape, size and fertilization models of the gametes display differences. Reptiles usually produce oval shaped and hard shelled eggs; this is also true of birds, including chickens. In aquatic species like frogs and fish, smaller, softer eggs are released into water in a jelly-like mass. For a majority of mammals, the union of sperm and egg takes place inside the body and the baby completes its development inside the mother’s body.
Different tasks are assigned to the egg and sperm. In recent years researchers have focused on the type of mechanisms that are utilized by a single sperm, which is in a race with millions of others to reach and enter the egg to fertilize it. Fertilization is not an event that happens by itself. It occurs depending on the interactions of sperm and egg in a very systematic way. It has been studied intensely how the molecules on the surfaces of the egg and sperm behave so selectively like orchestra conductors during fertilization. They fit together like a matching lock and key.
The proteins on the outer membrane of the egg
Mammalian sperm cells resemble a chubby arrow with a long tail. However, egg cells are found in different shapes and sizes. Millions of sperm in mammals compete with each other to be the first to reach the egg through the reproductive canal. The human egg is kept in a jelly like covering and a strong, protective membrane (or coat) called the Zona pellucida lies underneath this covering. It wraps the outer perimeter of the cell. Glycoproteins (ZP1, ZP2, ZP3, and ZP4) that function as the locks in the key-lock model are located at this protective layer. Sialyl-LewisX is a special type of tetrasaccharide carbohydrate (sugar) that binds to the proteins and lipids on this membrane. This carbohydrate is assigned with the task of making the egg’s surface sticky and this helps with the facilitation of sperm attachment to the outer receptors of the egg. In 2011, it was proved that this sugar molecule plays a critical role during fertilization.
Only a few of the sperm among the millions that joined the contest can make it to this layer. A lower sperm count as well as a sperm’s failure to cross this barrier can play a role in unsuccessful unions with the egg.
During the union of sperm and egg, the sperm becomes very active and its outer proteins are rearranged, thus getting recognized by the proteins of the outer layer of the egg (Zona pellucida). The ZP3 glycoprotein, working like a terminal gate, assists with the entry of the mammalian sperm into the egg. There is no chance for a sperm to cross the membrane once it has failed to bind to this receptor. The chemical changes initiated inside the sperm when bound to this protein trigger the secretion of acrosomal enzymes. Upon the release of enzymes, the Zona pellucida is broken down and the entry of sperm is permitted.
There are two distinct sections, called the ZP3-N and the ZP3-C, of the ZP3 molecule which is composed of 424 amino acids. The ZP3-N section of the ZP3 protein is conserved strictly as it is in the outer membranes of eggs of all animals, from fish to amphibians, birds, and mammalians. This suggests of a critical function for this protein. New birth control pills can be developed if small molecules that can attach to the ZP3 binding regions of the sperm can be discovered.
The second receptor required for the fertilization of the egg is the glycoprotein called ZP2. Certain sections of the ZP2 are responsible for sperm recognition and enabling the union. During experiments with mice, eggs were found to secrete an enzyme called ovastacin right after fertilization, destroying the binding protein (ZP2) which works outside the egg like a lock, thus disabling the egg’s ability to take up another sperm. Without this enzyme, sperms will continue to bind to the egg, even after divisions, leading to multiple fertilizations and eventually killing the embryo.
The process of fertilization is wondrous, a perfect meeting of time and circumstance. New research is revealing just how refined reproduction systems really are as they are found in nature.
How does an egg find the right sperm?
After an investigation regarding 1,880 protein encoding genes and proteins involved in reproduction, regions prone to mutation have been discovered on these genes. Mutations are found to occur frequently in ZP2 and ZP3 proteins that are directly involved with the egg and sperm interaction, along with proteins required for acrosomal reactions. There are various hypotheses being developed to unveil the causes behind the rapid mutation rate of proteins taking part in the reproduction system. These speculations range from gender dependent selection, to the development of traits by males that will be more attractive to females, and the avoidance of diseases. Reproduction is not collaboration; it is a race between millions of sperm, and only one can fertilize the egg.
At the molecular level, sperms try to accelerate this process and complete it as soon as possible, whereas the egg tries to slow it down. Therefore the needs of egg and sperm differ during fertilization: sperms fight to reach the egg and be selected, whereas the task of the egg is to be selective. The egg needs time to pick the most suitable sperm. After this selection, it must ensure the entry for the selected sperm and block access for the rest of them. Therefore, one way to achieve this successfully is via small mutations that lead to selective surface variations occurring in the structures of receptor proteins in charge of sperm recognition located on the egg membrane.
There are interesting results obtained with experiments performed on the eggs of Strongylocentrotus franciscanus, the red sea urchin. Sea urchins free their eggs into open sea for fertilization. The wisdom behind this is to ensure the fertilization of different eggs by different males. Being released into open water gives the eggs many options and puts pressure on the sperms. Variations are observed in the egg recognition proteins (Bindin) of each sperm; these proteins work like a key on the outer surface to attach, punch, and cross the egg membrane. Sea urchin eggs are also equipped with molecular blocking mechanisms to prevent the simultaneous entry of two sperms. Sperms thus go through mutations regarding egg recognition proteins and new keys are developed. The mutations occurring in the genes that encode the proteins, both on the egg and sperm membranes, result in slightly different matches that do not fully conform to one another. This situation causes a delay in the entry process. This meaningful delay is to grant the egg the necessary time to close the molecular gates to block access to a second or third sperm.
The studies carried out over sea urchin populations confirmed that such events take place in nature. The mutations are accelerated in genes that encode egg-recognition proteins and variants of such proteins are produced on the surface of sperms that exist in waters with high population density. In parallel to this, mutations also take place on receptor proteins of sea urchin eggs; these act like a lock. Studies confirmed that there are sperms that both lock perfectly and just shy of perfectly with these eggs. In cases of lower sperm density, it has been revealed that conforming and easily matching variants of the egg-recognition proteins of sperms are synthesized to facilitate fertilization. And in cases of higher sperm counts, a different form of receptor protein is found to be synthesized. This adaptation is interpreted as cautionary, as it prevents the entry of multiple regular-form sperms into the egg. In other words, a mutation regarding the sperm binding receptors can develop in females, thus establishing a prevention system against fertilization. Therefore, the role of the egg in reproduction is too critical to neglect.
These findings contain significant messages for human reproductive science. For instance, these new models can explain the role of the egg in certain infertility situations. It is possible that numerous variations have developed over time for human egg and sperm proteins. Mismatches in the key-lock system (protein-protein binding) may increase with these variations, and this would inhibit fertilization. This can eventually result in a failure to reproduce. Consequently, the know-how of the egg-sperm recognition systems awaits researchers as a new field of research. The most interesting development of this research is that eggs and sperms that are far away from each other still develop key and lock mechanisms, as if they are equipped with high level intelligence, willpower, and knowledge.
In summary, fertilization depends on suitable alterations and the necessary overlap of these changes in both sperms and eggs. It does not mean that all nonconforming matches are resulting from low quality eggs. In some cases, one reason for the mismatch is due to the defense mechanism of the egg against fertilization by male members of different species.that the egg is preventing itself from a bad match. The fact that the egg is equipped with a protective system at the molecular level simply shows that nothing is left to chance in nature.
Gaidos, Susan. 2012. “As Told by the Egg: The story of fertilization from the female point of view.” Science News Prime. December 10, 2012. www.sciencenews.org