FERTILIZATION

union of a spermatozoal nucleus, of paternal origin, with an egg nucleus, of maternal origin, to form the primary nucleus of an embryo. In all organisms the essence of fertilization is, in fact, the fusion of the hereditary material of two different sex cells, or gametes, each of which carries half the number of chromosomes typical of the species. The most primitive form of fertilization, found in micro-organisms and protozoans, consists of an exchange of genetic material between two cells. The first significant event in fertilization is the fusion of the membranes of the two gametes resulting in the formation of a channel that allows the passage of material from one cell to the other. Fertilization in advanced plants is preceded by pollination, during which pollen is transferred to, and establishes contact with, the female gamete or macrospore. Fusion in advanced animals is usually followed by penetration of the egg by a single spermatozoon. The result of fertilization is a cell (zygote) capable of undergoing cell division to form a new individual. The fusion of two gametes initiates several reactions in the egg. One of these causes a change in the egg membrane(s), so that the attachment of and penetration by more than one spermatozoon cannot occur. In species in which more than one spermatozoon normally enters an egg (polyspermy), only one spermatozoal nucleus actually merges with the egg nucleus. The most important result of fertilization is egg activation, which allows the egg to undergo cell division. Activation, however, does not necessarily require the intervention of a spermatozoon; during parthenogenesis, in which fertilization does not occur, activation of an egg may be accomplished through the intervention of physical and chemical agents. Invertebrates such as aphids, bees, and rotifers normally reproduce by parthenogenesis. In plants certain chemicals produced by the egg may attract spermatozoa. In animals, with the possible exception of some coelenterates, it appears likely that contact between eggs and spermatozoa depends on random collisions. On the other hand, the gelatinous coats that surround the eggs of many animals exert a trapping action on spermatozoa, thus increasing the chances for successful sperm-egg interaction. The eggs of marine invertebrates, especially echinoderms, are classical objects for the study of fertilization. These transparent eggs are valuable for studies observing living cells and for biochemical and molecular investigations because the time of fertilization can be accurately fixed, the development of many eggs occurs at about the same rate under suitable conditions, and large quantities of the eggs are obtainable. The eggs of some teleosts and amphibians also have been used with favourable results, and techniques for fertilization of mammalian eggs in the laboratory may allow their use even though only small numbers are available. the reproductive process in which a male sex cell (spermatozoon) unites with a female sex cell (ovum, or egg) to form a zygote, which will divide to form an embryo. Before fertilization can occur, an egg must be surrounded by appropriate egg coatings, which stimulate acrosome reactions in sperm. In mammals, sperm must go through physiological changes (called capacitation) that occur during their passage through the female genital tract. Without capacitation, the sperm cannot penetrate the egg coating. Fusion between a sperm and an egg is initiated by a prominence in the tip of the sperm's head that is called the acrosome. An opening forms in the sperm tip touching the egg, and an acrosomal granule disappears, apparently releasing lysin, a substance that dissolves the egg's protective coatings and allows the sperm to reach the egg. The sperm's acrosome membrane elongates into a tubule that extends to the surface of the egg and fuses with the egg's plasma membrane. The tubule provides a passageway for the sperm nucleus to reach the interior of the egg. This membrane fusion instigates the formation on the egg's surface of a resistant fertilization membrane that blocks the entrance of any more sperm. Once the sperm nucleus, or male pronucleus, has been absorbed by the egg, it begins to swell and disperse its chromosomes. It moves toward the female pronucleus and the two pronuclei meet in the centre of the egg, where their threadlike genetic material organizes into chromosomes. The chromosomes of the two pronuclei then merge, creating a fertilized egg, or zygote. The zygote contains all the components necessary to make the proteins comprising the embryo. Immediately after fertilization, the rate of protein synthesis increases greatly in the zygote. Because this protein production is directed by maternal RNA already in the unfertilized egg, an inhibitor to protein synthesis is probably removed by fertilization. At later states of embryonic development, protein synthesis is governed by the RNA of the new individual. Additional reading The process of fertilization is discussed by Charles B. Metz and Alberto Monroy (eds.), Fertilization: Comparative Morphology, Biochemistry, and Immunology, 2 vol. (196769), a major source of information, and Biology of Fertilization, 3 vol. (1985); Edmund B. Wilson, The Cell in Development and Heredity, 3rd ed. with corrections (1928, reprinted 1987), a classic work; John F. Hartmann (ed.), Mechanism and Control of Animal Fertilization (1983); and Paul M. Wasserman, The Biology and Chemistry of Fertilization, Science, 235(4788):553560 (Jan. 30, 1987), and Fertilization in Mammals, Scientific American, 259(6):7884 (December 1988). Alberto Monroy The Editors of the Encyclopdia Britannica