Fertilization (zoology)

For fertilization to occur, several things must happen: Sperm and eggs must be in close proximity, the gametes need to be compatible, the sperm must be able to penetrate the egg, and the haploid egg nucleus must combine with the haploid sperm nucleus. If any one of these is missing, fertilization will not occur.

Assuring Eggs and Sperm Are in Proximity

Animals have many mechanisms to ensure that sperm and eggs are in close proximity. This can be a major concern for aquatic organisms with external fertilization, and many release gametes in the millions or even billions to assure that at least some sperm reach the appropriate eggs. To increase the chances of a meeting between same-species gametes, animals often have specialized mating behaviors. Corals are among those animals that release their gametes into the water and depend on currents to bring egg and sperm together. This is not, however, as random as it may seem. As the first coral releases its gametes, it also releases hormones that induce nearby corals of the same species to release their gametes. These also release the same chemicals with their gametes, and soon there are clouds of eggs and sperm, and the chances of a proper meeting are increased dramatically. One species of polychaete annelid, Eunice viridis, or the palolo worm, has another method of assuring male and female gametes are in the same place. In this species, sexually mature worms, called epitokes, swarm together at the ocean’s surface in response to the lunar cycle. Females then secrete a hormone that induces males to release sperm, and the sperm induce the females to shed eggs. Many fish go through elaborate courtship rituals, during which males and females release gametes at a specific point, thus assuring that egg and sperm are together. Other fish build nests where females lay eggs and males deposit sperm. Frogs and toads usually breed in the water, but the female will only release her eggs when the male is clasped to her back in amplexus. Thus, sperm are deposited on the eggs as they are being laid.

Males of other species place sperm directly in the female’s reproductive tract. The male octopus has a special tentacle that is used to place one of his sperm packets in the mantle cavity of the female. Some salamander males deposit their sperm packets on the substrate during a squat dance courtship ritual. Females also do the squat dance and pick up the packet with the lips of their cloacae. In some species of water mites, females mount a special saddle-shaped extension of the males’ abdomen. The male squats to deposit a sperm packet, moves ahead slightly, and then squats again when the opening of the female’s reproductive system is over the packet, forcing the packet into her reproductive system. Another interesting way to assure fertilization is seen in the sea horse. In these animals, a female deposits her eggs into a pouch on the male’s abdomen, and the male releases sperm into the pouch at the same time. The most common way to introduce sperm into a female’s reproductive tract is through copulation, where the male ejaculates sperm directly into the female’s reproductive tract. The motile sperm then travel to the egg. For a sperm to gain full motility, it usually must undergo a little-understood process called capacitation.

Penetration

Once eggs and sperm are in close proximity, the sperm must begin to penetrate the egg’s protective layers. All eggs have at least one protective layer outside the cell membrane. Called the vitelline envelope in most organisms, it is synthesized in the ovary and composed primarily of polysaccharides and glycoproteins. The oviducts and uterus often secrete other protective layers around the egg. In some instances, the sperm must also penetrate these layers, for example, the jelly layers that surround sea urchin and frog eggs. In other instances, the egg is fertilized before these layers are added, as is the case with the many protective layers that surround bird and reptile eggs. A protective layer made up of cells is seen in most mammals, since the egg is released by the ovary with cells of the cumulus oophorus still attached. For the sperm to penetrate these layers, its acrosome must contain the appropriate enzymes to lyse (disintegrate) the chemicals that block its way. The acrosomal reaction must also take place in order to expose the digestive enzymes of the acrosome. This reaction depends on changes in membrane permeability to ions and subsequent changes in pH.

Once through the protective layers, the sperm makes contact with the egg’s plasma membrane. If the sperm and egg are of the same species, sperm receptor molecules on the egg membrane attach to complementary molecules, called bindins, on the sperm membrane and the two membranes fuse. If the bindins on the sperm do not complement the receptors on the egg, there is no fusion and fertilization does not continue, thus preventing most interspecies crossings. However, closely related species often have bindins and receptors sufficiently alike to allow some fertilization to proceed. The products of these interspecific matings are hybrids, such as the mule.

Once the first sperm fuses with the egg, mechanisms to prevent polyspermy, the fertilization of an egg by more than one sperm, are put into place. The first block to polyspermy is common to most animals studied: a very quick and only temporary depolarization of the plasma membrane. In sea urchins, the resting membrane potential of the egg plasma membrane is approximately -70 millivolts, the inside being more negative than the outside. Fusion of the sperm plasma membrane with the egg cell membrane causes a rapid influx of sodium ions. The positive charges neutralize negative charges in the egg until the membrane potential is raised to +10 millivolts. All this happens in less than five seconds and lasts for about one minute before the egg cell has actively transported enough sodium out of the cell to repolarize it. While the cell is depolarized, no further sperm membranes can fuse with the egg membrane. This is often referred to as the fast or temporary block to polyspermy and seems to occur in all animals thus far studied. The fast block also sets into motion the slow or permanent block to polyspermy. The changed membrane potential of the fast block and the release of nitrous oxide by the sperm allows cells to release calcium ions from storage. The initial calcium ion release causes the egg to release nitrous oxide, which then increases the egg’s release of calcium ions. The release of calcium ions induces the cortical reaction by which cortical granules move to the surface of the cell, fuse with the cell membrane, and empty their contents into the space between the cell membrane and the vitelline envelope. In sea urchins, the first acrosomal enzymes released break the bonds between the cell membrane and the vitelline envelope. In the presence of water, other chemicals released by the cortical granules swell, lifting the vitelline envelope away from the cell membrane.

Finally, other enzymes released by the cortical granules alter the vitelline envelope, knocking off any attached sperm and causing the release of peroxide ions, which harden the envelope, making it impermeable to sperm. This impermeable barrier is renamed the fertilization membrane. The released peroxide may also provide another benefit. Any sperm that had penetrated the vitelline envelope before it hardened would be killed by the peroxide and would thus not lead to polyspermy. In other animals studied, although cortical granules do empty their contents into the perivitelline space, the permanent block to polyspermy does not seem to involve the same extensive changes to the vitelline envelope (or zona pellucida in mammals) that are seen in the sea urchin. In large, yolky eggs, some polyspermy does occur, but the extra sperm remain in the yolk and never reach the egg nucleus for fusion.

Cell Metabolism and Meiosis

Concomitant with the cortical reactions is an increase of metabolism in the egg, which is necessary for nuclear fusion and cleavage, the process in which mitotic cell division takes place. In species where the egg has not completed meiosis, it does so at that time. Which parts of the sperm enter the egg is dependent on the species. In many mammals, the entire sperm enters, while all but the tail enters in echinoderms. In other organisms, the head with the nucleus and centrioles seem to be the only things that enter. There is no evidence that any parts of the sperm other than the nucleus and centrioles are used by the zygote, and other parts that enter most probably degenerate, and their components are recycled.

Studies on the mitochondria of sperm indicate that soon after entering the egg, the sperm’s mitochondria are tagged by ubiquitin, the first step in breakdown and recycling. After entry, the sperm nucleus imbibes water and is converted into the male pronucleus. At the same time, the egg nucleus becomes the female pronucleus. In most animals, the male pronucleus and the female pronucleus fuse to form the diploid zygote nucleus. In some nematodes, mollusks, and annelids, however, the pronuclei remain separate until after the first cleavage division. In a few others, like the copepod Cyclops, the pronuclei divide separately for several cleavage divisions.

The fusion of the sperm with the egg nucleus affects many other cellular processes. One of the most interesting is the displacement of some cytoplasmic constituents. These constituents of the egg determine the fate of cells derived from the parts of the egg in which they were located and probably determine the plane of bilateral symmetry. Sperm attachment and entry often cause shifts in the position of the viscous cortical and subcortical cytoplasm, where many of the fate-determining chemicals are located.

Artificial Insemination

Artificial insemination, or the manual fertilization of animals, is the process by which semen is removed from a male and is directly deposited into a female's vagina. In humans, this process is known as in vitro fertilization, and instead of the deposit directly into the female, an egg is removed from a woman's ovary and fertilized with sperm in a laboratory dish. If fertilization is successful, the embryo is then implanted into the uterus. This process has been utilized in humans for a variety of reasons, including to avoid certain genetic abnormalities or because fertilization or implantation was unsuccessful through intercourse.

In animals, though, the process of articifial insemination is often used in livestock, particularly cattle, to improve genetics by selecting superior males and efficiently breeding large herds. It can also be used to avoid disease and to, more generally, control and streamline the mating and birthing of animals.

Principal Terms

Corona Radiata: The layers of follicle cells that still surround the mammalian egg after ovulation

Vitelline Envelope: The protective layers that form around the egg while it is still in the ovary

Zona Pellucida: Mammalian protective layer analogous to the vitelline envelope

Bibliography

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Oliver, Rebecca and Hajira Basit. "Embryology, Fertilization." NCBI, 17 Apr. 2023, www.ncbi.nlm.nih.gov/books/NBK542186. Accessed 3 July 2023.

Springate, Leah, and Timothy R. Frasier. “Gamete Compatibility Genes in Mammals: Candidates, Applications and a Potential Path Forward.” Royal Society Open Science, vol. 4, no. 8, 30 Aug. 2017. doi:10.1098/rsos.170577. Accessed 12 Sept. 2024.