Fertilization

Fertilization

Life can persist in one of two ways. First, living things can spurn death in the hope of living forever. This is usually not possible, because death by predation or other unintentional causes is virtually inevitable. Second, living things can make copies of themselves, or reproduce, as a means of "hedging their bets" against death. Then, all that is required for life to persist is for at least one of the copies, or offspring, to remain alive at any one time. Thus, all living things reproduce.

How does reproduction occur? Some organisms reproduce asexually. The single-celled prokaryotes undergo binary fission, meaning literally "to split in half." Binary fission occurs after the cell has doubled the amount of its cellular constituents. The single circular chromosome is replicated from a single initiation point, the replication fork moving bidirectionally around the chromosome. Finally, inward growth of the plasma membrane separates the large cell into two equal halves. Some multicellular organisms reproduce asexually through budding. For example, in hydra, a relative of the jellyfish, a mass of mitotically dividing cells grows on the parent's side and eventually detaches as a small copy of the parent. However, most multicellular organisms reproduce sexually. Sexual reproduction takes place after the production of special reproductive cells termed gametes . Gametes are typically labeled as being of one sex or the other, male or female, with the female gamete being the larger of the two types. Fertilization is the union of a male and a female gamete to form a zygote , the developing offspring. There are many ways in which fertilization can occur in nature.

The sexual organs of animals are called gonads. Gametes are made in the gonads—the female gonads produce eggs, the male gonads produce sperm. Human males produce millions of sperm every day. Females produce one mature egg in each menstrual cycle. When a sperm and an egg fuse during fertilization, a diploid zygote is formed. This zygote divides mitotically until it is an adult organism. Animals are unique in that they have a special type of cells called germ cells , in the gonads. The sole function of these cells is to undergo meiosis to form the precursors of eggs and sperm, oocytes and spermatocytes, which differentiate into the mature gametes necessary for reproduction. One can think of animal bodies, which are made of somatic cells derived from the original germ cells, as the machines that germ cells use to ensure their successful passage to the next generation.

Animals may release gametes into the external environment to be fertilized, or the male may deposit gametes into the female, allowing fertilization to take place inside the female reproductive tract. External fertilization occurs only in aquatic or moist habitats where gametes will not dry out. Sessile animals such as corals often release millions of gametes into the water at one time ensuring that at least some will be fertilized. In such cases, fertilization does not require that members of the opposite sex be near each other, although it is necessary that both males and females release their mature gametes at the same times. They do this by responding to species-specific environmental cues such as light cycles or temperature.

Some animals, including fish and amphibians, use external fertilization but do so only with a particular mate. When a female is receptive to a particular male, she will lay her clutch of eggs in the water and the male will distribute his sperm over them. In this form of external fertilization, unlike internal fertilization in which the female can store and use sperm from many males, the male is assured of paternity and is therefore much more likely to take care of the offspring.

Internal fertilization requires that the male introduce his sperm directly into the female, so there is a much greater probability that any particular gamete will be fertilized. Furthermore, animals are no longer dependent on water for fertilization and may become completely terrestrial. Animals with internal fertilization, especially females who incur most of the cost of reproduction, are selected to be extremely choosy with whom they mate. If they do not pick healthy mates they may spend precious energy and time raising an offspring that cannot compete with the offspring of choosier parents. There are usually species-specific behaviors, courtship displays, and other physical cues that allow females to pick healthy mates of their own species, a phenomenon known as sexual selection.

At the cellular level, a mammalian sperm must undergo several steps before it can fertilize the egg. The first step is termed the acrosomal reaction, in which enzymes from the sperm cap, or acrosome, are released. These enzymes serve to break down the barrier of follicle cells that surround the egg, as well as the zona pellucida, a glycoprotein envelope that encases the egg. The sperm can tunnel through the zona pellucida only if the acrosomal enzymes recognize species-specific molecules of the female's egg. Eventually the sperm gains access to the egg itself, and the sperm and egg plasma membranes fuse. At this stage the egg becomes activated and initiates a rapid sequence of events.

First, the activated egg blocks entry to other sperm, as polyspermy (the fertilization of an egg by more than one sperm) is generally lethal to the developing embryo. When the first sperm fuses with the egg plasma membrane, the egg begins to increase its concentration of positively charged sodium ions from the surrounding environment of the female oviduct. The change in the electric potential across the plasma membrane (the excess positive charge inside the egg) prevents further sperm/egg fusions. Next, development processes begin; the egg increases oxygen consumption and begins protein synthesis. Eventually the nuclei of the sperm and egg fuse to form the diploid nucleus of the new zygote.

see also Reproduction, Asexual and Sexual.

Todd A. Schlenke

Bibliography

Austin, C. R., and R. V. Short, eds. The Evolution of Reproduction. Cambridge, U.K.: Cambridge University Press, 1976.

Dale, Brian, ed. Mechanism of Fertilization: Plants to Humans. New York: Springer-Verlag, 1990.

Gwatkin, Ralph B. L. Fertilization Mechanisms in Man and Mammals. New York: Plenum Press, 1977.

Longo, Frank J. Fertilization, 2nd ed. New York: Chapman and Hall, 1997.