Fertilization
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What is fertilization in reproduction?
Fertilization is the union of a sperm cell with an egg cell, resulting in the primary nucleus of an embryo. The essence of fertilization lies in the fusion of genetic material from two different sex cells, known as gametes. Each gamete carries half the number of chromosomes typical of the species. The result of fertilization is a zygote, a cell capable of undergoing cell division to form a new individual. In cross-fertilization, the sex cells fuse from different individuals of the same species.
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Why is fertilization important for living things?
Fertilization is the fusion of male and female gametes to form a zygote, initiating the development of a new individual. Each gamete carries half the number of chromosomes typical of the species; fertilization restores the full complement, ensuring the offspring receives genetic material from both parents.
Sexual reproduction, which includes fertilization, allows for genetic reshuffling, increasing diversity within a population. This variability is crucial for adaptation and evolution, enabling species to survive changing environmental conditions.
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How does fertilization usually happen in animals?
Fertilization in animals typically involves the union of a sperm nucleus with an egg nucleus, leading to the formation of an embryo. In essence, fertilization is the fusion of genetic material from two different sex cells or gametes.
In animals, contact between eggs and sperm relies on random collisions. The spermatozoa undergo an acrosome reaction, which involves structural changes that allow it to fuse with the egg's plasma membranes. For fertilization to occur, the sperm must pass through the egg's outer layers to reach the interior of the egg.
Cross-fertilization, the fusion of male and female gametes from different individuals of the same species, is a common method of fertilization in animals.
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What is the difference between internal and external fertilization?
Fertilization is the fusion of a sperm and an egg, leading to the formation of an embryo. Fertilization can occur either externally or internally, with key differences:
- Internal fertilization In terrestrial animals, fertilization is internal, with the sperm being introduced into the body of the female.
- External fertilization Among most species that breed in aquatic habitats, the males and females each shed their sex cells into the water and external fertilization takes place.
Internal fertilization isn't always related to viviparity (live birth) or the presence of intromittent organs. Cross-fertilization, which combines genetic material from two parents, helps species adapt to environmental changes.
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How does fertilization work in plants?
Plant fertilization relies on pollination, where pollen grains move from the stamen to the ovule-bearing organs. In flowering plants, pollen lands on the stigma, grows a pollen tube through the pistil towards the ovule, and leads to double fertilization.
During double fertilization, one sperm cell fuses with the egg cell to form a zygote, which develops into an embryo. The other sperm cell fuses with two polar nuclei, forming the endosperm, a food-storage tissue. The ovule then develops into a seed.
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What happens to the fertilized cell after fertilization occurs?
After fertilization, the resulting zygote, or fertilized egg, embarks on a journey of cell division and development. This process, known as cleavage, involves the zygote dividing into smaller cells called blastomeres through mitosis.
As the blastomeres multiply, they form a cluster that enters the uterus around the fourth day after fertilization, becoming a morula. Fluid accumulation leads to the formation of a blastocyst, characterized by an inner cell mass and an outer layer called the trophoblast. The blastocyst implants in the uterine lining, where the trophoblast contributes to the development of the placenta, facilitating nutrient transfer from the mother to the developing embryo.
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Fertilization, union of a sperm 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 microorganisms 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.
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In plants, certain chemicals produced by the egg may attract spermatozoa. In animals, with the possible exception of some cnidarians (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.
Maturation of the egg
Maturation is the final step in the production of functional eggs (oogenesis) that can associate with a spermatozoon and develop a reaction that prevents the entry of more than one spermatozoon. In addition, the cytoplasm of a mature egg can support the changes that lead to fusion of spermatozoal and egg nuclei and initiate embryonic development.
Egg surface
Certain components of an egg’s surface, especially the cortical granules, are associated with a mature condition. Cortical granules of sea urchin eggs, aligned beneath the plasma membrane (thin, soft, pliable layer) of mature eggs, have a diameter of 0.8–1.0 micron (0.0008–0.001 millimetre) and are surrounded by a membrane similar in structure to the plasma membrane surrounding the egg. Cortical granules are formed in a cell component known as a Golgi complex, from which they migrate to the surface of the maturing egg.
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The surface of a sea urchin egg has the ability to affect the passage of light unequally in different directions; this property, called birefringence, is an indication that the molecules comprising the surface layers are arranged in a definite way. Since birefringence appears as an egg matures, it is likely that the properties of a mature egg membrane are associated with specific molecular arrangements. A mature egg is able to support the formation of a zygote nucleus; i.e., the result of fusion of spermatozoal and egg nuclei. In most eggs the process of reduction of chromosomal number (meiosis) is not completed prior to fertilization. In such cases the fertilizing spermatozoon remains beneath the egg surface until meiosis in the egg has been completed, after which changes and movements that lead to fusion and the formation of a zygote occur.
Egg coats
The surfaces of most animal eggs are surrounded by envelopes, which may be soft gelatinous coats (as in echinoderms and some amphibians) or thick membranes (as in fishes, insects, and mammals). In order to reach the egg surface, therefore, spermatozoa must penetrate these envelopes; indeed, spermatozoa contain enzymes (organic catalysts) that break them down. In some cases (e.g., fishes and insects) there is a channel, or micropyle, in the envelope, through which a spermatozoon can reach the egg.
The jelly coats of echinoderm and amphibian eggs consist of complex carbohydrates called sulfated mucopolysaccharides. The envelope of a mammalian egg is more complex. The egg is surrounded by a thick coat composed of a carbohydrate protein complex called zona pellucida. The zona is surrounded by an outer envelope, the corona radiata, which is many cell layers thick and formed by follicle cells adhering to the oocyte before it leaves the ovarian follicle.
Although it once was postulated that the jelly coat of an echinoderm egg contains a substance (fertilizin) thought to have an important role not only in the establishment of sperm-egg interaction but also in egg activation, fertilizin now has been shown identical with jelly-coat material, rather than a substance continuously secreted from it. Yet there is evidence that the egg envelopes do play a role in fertilization; i.e., contact with the egg coat elicits the acrosome reaction (described below) in spermatozoa.