FERTILIZATION
The essence of the fertilization process consists in the fusion of the female and male gametes into one cell - the zygote, which is not only a cell, but also becomes a unicellular embryo.
The fusion of gametes can occur only under the condition of the combined action of many biological factors. The developmental cycles of germ cells must proceed synchronously so that sperm and eggs mature and are released at the same time.
Fertilization is preceded by insemination is a process that leads to contact between male and female germ cells. Sperm cells become mobile only after they
enter the fluid, which is the secretion of the seminal vesicles, prostate and Cooper glands. In humans, sperm are inserted into the upper part of the vagina, from where they have to pass to the fallopian tubes, in the upper part of which the gametes meet and merge. One ejaculate in a person contains up to 250-300 million sperm, but only a small fraction of them manage to get close to the egg. The first obstacle the sperm encounters is the natural acidity of the upper part of the vagina, which creates a bactericidal environment. Semen, however, acts as a buffer against acidity and changes the pH of the medium very quickly (within a few seconds) from 4.3 to 7.2. Some spermatozoa reach the mouth of the fallopian tube within 30 minutes after insemination. Such a rapid advancement of spermatozoa cannot be explained only by their own active movement, since their speed is only 2-4 mm per minute. In this case, the promotion is facilitated by spasmodic contractions of the smooth muscles of the uterus, the action of the junction of the uterus with the fallopian tube as a valve, as well as the ability of sperm to move against a weak flow of fluid (positive rheotaxis).
It should also be borne in mind that usually the cervical canal is filled with thick mucus that predominates during most of the menstrual cycle. It has an alkaline reaction and promotes the penetration of sperm into the uterus due to the ability to change the state (physicochemical characteristics) of the micellar network-like structure of mucus under the influence of female sex hormones.
As the sperm moves forward, they are exposed to a special effect from the tissues of the female genital organs, which ensures their further penetration through the egg membranes (capsitation). Its nature remains not entirely clear.
The ovulated egg, surrounded by cells of the radiant crown, enters the fallopian tube due to the increased muscle activity of the fringed edge of the mouth of the funnel of the tube, as well as the flow of fluid created by the cilia of the ciliated epithelium, and moves towards its wide part (ampulla). The follicular cells that form the radiant crown also play an important role in the directional movement of the egg. Nevertheless, it has been established that the movement of the egg is still largely dependent on its mass, since other bodies of the same size are able to move just as effectively down the fallopian tube.
2.4.1. Viability of gametes
The viability of sperm and eggs in the female genital organs is limited in time. Immediately after ovulation, changes occur in the egg that can be characterized as aging. So, granularity appears in the ovoplasm, which quickly becomes coarse, the level of general metabolism decreases, which sharply increases only if fertilization occurs. Fertilization must take place within the first days after ovulation, otherwise the egg cell will lose its viability.
The preservation of sperm motility and their lifespan cannot be equated with their ability to fertilize. Sperm motility lasts much longer. It has been proven that in the genital tract, spermatozoa retain the ability to fertilize during the first 1-2 days, while their mobility is twice as long.
2.4.2. Fusion of gametes
The fusion of gametes occurs in a wide part of the fallopian tube. The meeting of the egg and the sperm is a matter of chance. Fertilization becomes possible only after the male and female gametes come into contact, but before that, the sperm must pass through the egg membranes - first of all through the cells of the radiant crown, then through the zona pellucida, and only after that through the plasma membrane of the egg. However, first, in spermatozoa, as mentioned earlier, an acrosomal reaction occurs, a necessary prerequisite for which is capacitation.
The sperm acrosomal response causes the membranes surrounding the egg to dissolve (Figure 2.7). Although its development is not completely clear, there is information that the onset of the reaction is initiated by an increase in the concentration of Ca +2 ions in the sperm during fertilization. An increase in the concentration of Ca +2 ions activates membrane phospholipases and acrosomal proteases. These enzymes, respectively, destroy the membranes of the acrosome and the apical plasmolemma of the sperm, and also activate the acrosomal enzymes. The acrosome is believed to contain at least three enzymes:
- enzyme, dissolving cells of the radiant crown(CPE);
- acrosomin - trypsin-like enzyme that loosens the oocyte zona pellucida;
- hyaluronidase - dissolving glittering shell.
After going through zona pellucida, The spermatozoon enters the perivi-tellin space, which separates the zona pellucida from the ovum plasmolemma (Fig. 2.8). At the site of the formation of contact between the sperm and the egg, the establishment of which is facilitated by the microvilli of the egg, a protrusion of the plasmolemma, known as the fertilization tubercle, is formed. After the fusion of the plasma membranes of the egg and
sperm, the fertilization tubercle is retracted, bringing the sperm head into the ovoplasm. In this case, the sperm cell is deprived of most of the cytoplasmic structures, namely: the tail, mitochondria of the insertion part, and the remnants of the acrosome. It remains not entirely clear whether he retains centrioles.
After the penetration of the sperm into the egg, DNA molecules are prepared for the resumption of synthetic processes, morphologically expressed in the swelling of the highly condensed nucleus, accompanied by unwinding of chromatin filaments. Already during the first 12 hours, swelling of the nuclei of the male and female gametes (pronuclei) is observed, their migration to the center of the egg and the appearance of well-visible nucleoli. Then, after the disappearance of the nuclear membranes that surrounded the pro-nuclei, they come closer to each other and there is a mixing of the maternal and paternal chromosomes (syngamia), which is the last stage of the fertilization process.
The unification of the genetic material of the sperm and the egg with the formation of a new unicellular organism - the zygote, is marked by the following important events:
Haploid sets of gamete chromosomes are combined into a diploid set of zygotes;
Paternal inheritance is combined with maternal inheritance;
The sperm, most likely, introduces centrioles into the ovoplasm, which become the cell center of the zygote;
The sperm cell activates the egg, and therefore the zygote acquires a high level of metabolic processes.
2.4.3. Fertilization reactions
In the process of evolution, animals have developed a variety of biomechanisms that characterize the features of various aspects of fertilization. A striking confirmation of this is the results of studying the ways in which the egg prevents polyspermia, that is, the mixing of its genetic material with the genetic material of more than one sperm. For example, some species of animals (tailed amphibians, reptiles and birds) have developed special mechanisms that allow eggs to inactivate the nuclei of excess spermatozoa. The oocytes of most vertebrates have learned to prevent polyspermia with the help of the surface layer of cortical granules.
In animals with monospermic fertilization, the first reaction that occurs in response to the fusion of a sperm with an egg is consists in a rapid change in the electrical properties of the ovum plasmolemma. For example, Cross and Elinson (1980) found that in a frog the membrane potential of the oocyte cell changes from -28 to +8 mV within a few seconds after fusion with the sperm and remains positive for 20 minutes, preventing the occurrence of polyspermia, while its decrease in a fertilized egg makes it possible.
Within a few minutes after the penetration of the sperm into the egg, another superficial reaction occurs that prevents polyspermy - cortical. Its essence lies in the fact that cortical granules, starting from the point at which the egg and sperm merge, move to the inner surface of the plasmolemma, merging with it, and then release their contents into the perivitelline space.
In addition, polyspermia is blocked by the zona pellucida, which becomes impermeable to spermatozoa within a few minutes after the development of a cortical reaction. And although the nature of the surface reactions remains not entirely clear, it has been suggested that the polysaccharide complexes that make up the cortical granules cause the zona pellucida to thicken or destroy sperm receptors on it, or even directly inactivate the sperm lysine zone. The plasma membrane of the egg also becomes impermeable to sperm, but this occurs only a few hours after the change in the zona pellucida.
The oldest form of reproduction. Vegetative propagation of plants. Spore formation. Asexual reproduction. Cloning. Yeast organisms. Assess your knowledge. Right answers. The process of reproducing their own kind. Budding. Vegetative reproduction. Division in two. Reproduction. What is reproduction. Reproduction of fruit trees. Houseplant propagation technology. Clone. Rosehip budding.
"Methods of reproduction of organisms" - Is everything shown correctly in the picture. Goose. Cat. Sex cells. Reproduction forms. Bisexual organisms. How many parents do these animals have. Gametes. Animal reproduction. Division. Living organisms. Dissolved organisms. Animals. Zygote. Sexual reproduction scheme. Forms of asexual reproduction. New animals. Flounder. Parents. Process. How living organisms multiply. New generation. Sexual reproduction.
"Cytological bases of reproduction" - Structures. Division of meiosis. Amitosis. Mitotic cycle. Mitosis diagram. Kidneys. Phases of mitosis. The growth of the organism. Parthenogenesis. Polities. Preparation for division. Stem cells. A renewing population. Meiosis. Cell life cycle. Epidermis. The essence of asexual reproduction. Cell reproduction. Life expectancy. Meaning. Self-renewal of the body. Asexual reproduction. The development of organisms. Mitosis. Cell division processes.
"Reproduction and development of organisms" - Remember. Meiosis. Postembryonic. Compare. Determine the breeding method. Breeding types. Fertilization. Chromosome sets. Pair up. Ontogenesis. Embryonic development. Independent work. Terms related to the topic. Reproduction of organisms. Division method. Reproduction forms. Laboratory work.
"Reproduction of living organisms" - Sexual reproduction. Reproduction, or self-reproduction, is a property of all living organisms. Real woman. Human reproduction and development. Coelenterates. Stamen. The diversity of the living world. The form of the sexual process in protozoa. Why did nature need two sexes? Fertilization scheme. Breeding evolution. Men and women. Parthenogenesis. The variability required for natural selection. Reproduction of living organisms.
"The difference between asexual and sexual reproduction" - Isogamy. New cells are formed by dividing existing ones. Gametes. Reproduction is necessary. What is the advantage of sexual reproduction over asexual. Laboratory work. Parthenogenesis. Asexual and sexual reproduction of organisms. Cloning. Binary fission. Assignment: finish the sentence. Gametogenesis. Forms of sexual reproduction. What is fertilization. Asexual reproduction. Forms of asexual reproduction.
What happens before the gametes merge?
To answer this question, you need to figure out what is "gamete fusion"?
What is "gamete fusion"
Fusion of gametes (syngamia) or in other words fertilization is a type of sexual reproduction in which the germ cells of individuals of both sexes (gametes) fusion. For example, in humans, sperm (male) and egg (female) are sex cells. All gametes contain chromosomes - there are 23 of them; when the sex cells merge, a zygote with a set of chromosomes 46 is formed. The zygote begins to divide, due to which organs and systems of a new person appear.
What happens before the fusion of gametes
In order for the fusion of gametes to occur, it is necessary for the formation of sex cells in the male and female individuals. This process is called gametogenesis, which takes place in the gonads. For example, in the female, the sex cells are formed in the ovaries (this process is called oogenesis), and the male in the testes (spermatogenesis). Cell formation takes place in several stages: reproduction, growth, maturation, formation (in males).
- Stage 1 - reproduction. At this stage, cells are actively dividing and germ cells are formed, which still have a diploid set of chromosomes (that is, they have a complete set of chromosomes, like all cells in the body). This stage in males occurs during puberty and continues throughout life. In females, the total number of germ cells that will form in the process of her reproductive activity are laid even in the embryo.
- Stage 2 - growth. At this stage, the cytoplasm of the cells increases. Nutrients accumulate and the set of chromosomes doubles.
- Stage 3 - maturation. At this stage, meiosis occurs - that is, this is cell division, in which the set of chromosomes is halved. Cells become with a haploid set of chromosomes (that is, with a single set of chromosomes).
- Stage 4 - formation. This stage is distinguished in the process of spermatogenesis formation.
Thus, for the fusion of gametes to occur, it is necessary that the sex cells of the male and female individuals are formed. The process of their formation is very complex and surprisingly interesting.
Developing into an individual (or a group of individuals) with hereditary characteristics of both parental organisms that produced gametes.
In some species, the development of a single gamete (unfertilized egg) in the body is also possible - parthenogenesis.
Gamete morphology and types of gametogamy
Isogamy, heterogamy and oogamy
The morphology of gametes of different species is quite diverse, while the produced gametes can differ both in the chromosomal set (with the heterogamety of the species) and in the size and mobility (the ability to move independently), while the gamete dimorphism in different species varies widely - from the absence dimorphism in the form of isogamy to its extreme manifestation in the form of oogamy.
Isogamy
If merging gametes do not differ morphologically from each other in size, structure and chromosome set, then they are called isogametes, or asexual gametes. Such gametes are mobile, can carry flagella or be amoeba-like. Isogamy is typical of many algae.
Anisogamy (heterogamy)
Gametes capable of fusion differ in size, mobile microgametes carry flagella, macrogametes can be both mobile (many algae) and immobile (macrogametes of many protists lacking flagella).
Oogamy
Gametes of one biological species capable of fusion differ sharply in size and mobility into two types: small mobile male gametes - spermatozoa - and large immobile female gametes - eggs. The difference in the size of gametes is due to the fact that the eggs contain a supply of nutrients sufficient to ensure the first few divisions of the zygote during its development into the embryo.
Parker's disruptive selection theory... If the size of the zygote is important enough for its survival (in organisms with external fertilization), then anisogamy would be an evolutionarily stable strategy. In such cases, a population of males (breeders of small gametes) and females (breeders of large gametes) will be stable. The theory of disruptive selection makes it possible to explain the emergence and maintenance of dioeciousness in many plants and some animals with external fertilization.
Gamete type and gender
The concept of sex is associated with differentiation by the size of gametes, that is, we refer to the male sex as individuals that produce small mobile gametes, and to the female sex - those that produce large ones. At the same time, differentiation by the type of gamety (homo - XX or heterogametic constitution - XY) in some species may not coincide with the differentiation in size.
In the process of evolution, in most species, small gametes and a heterogametic constitution XY were found in males, and large gametes and a homogametic constitution XX were found in females. These are species with gamete type Drosophila... On the contrary, in species with gametes of the type Abraxas (English)Russian(birds, butterflies, moths, some fish species, etc.) the directions of these differentiations did not coincide. The ova in the female are heterogametic, and the spermatozoa in the male are homogametic.
Zygote is a cell formed by the fusion of two gametes - female (egg) and male (sperm) as a result of the sexual process. Contains a double (diploid) set of chromosomes.
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"Formation of germ cells" - Reduction of the number of chromosomes by half. (Prophase I) 2. Spiralization of chromosomes. Phases of mitosis. "Reproduction and individual development of organisms." Breeding types. Ovogenesis, centromere, telophase, prophase, gametes, metaphase, anaphase, meiosis, zygote. Finding the sequence relation. Determine if the following judgments are correct.
"Embryonic period of development" - Changes in the hereditary apparatus under the influence of various mutagens. Bird development. The postembryonic period is indirect (with transformation), direct (without transformation). Viruses, bacteria, fungi, animals, plants. From parts of the mother's body. Learning new material. During the classes. Historical information.
"Forms of asexual reproduction" - Class Ciliated ciliates. The most ancient and simplest form of asexual reproduction. 5. Vegetative propagation. 1. Division. Reproduction by means of spores - specialized cells of fungi and plants. Forms of asexual reproduction. When gametes merge, a four-flagellate zygote is formed. Conjugation Parthenogenesis Heterogamy Oogamy Isogamy.
"Reproduction of organisms biology" - Hermaphrodites. Two organisms are involved. Sexual reproduction. Layers. 1. Female gender. 6. Yeast reproduces by cell division. Internal. One organism is involved. 5. Sexual reproduction involves two individuals that form gametes. Reproduction of living organisms. Male. Tubers. The ovaries. Bulbs. 8. Only mosses and fungi reproduce by spores.
"Asexual reproduction" - The problematic question of the lesson: Why does asexual reproduction ensure the constancy of the set of chromosomes in generations? Methods of vegetative propagation: Brood buds Stalk Leaf Tuber Bulb Rhizome Whiskers Layers. Lesson type: combined. Interdisciplinary relations: botany - zoology - genetics. For example, plants reproduce by roots, shoots.
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