Fertilization. The physiological process of fusion of egg and sperm, as a result of which a new cell is formed - a zygote, is called fertilization. The zygote has double heredity and gives rise to a new organism.
Fertilization occurs in the upper third of the oviduct and goes through four stages. The first stage is the release of the egg from the follicular cells of the corona radiata and loosening of the transparent membrane under the influence of the enzyme hyaluronidase secreted by sperm. This requires the participation of a large number of sperm (thousands). The ability to secrete hyaluronidase is obtained by sperm that have undergone capacitation in the uterus for several hours. The meaning of capacitation is to unblock the acrosomal reaction by eliminating the sperm plasma factor that prevents the release of enzymes. The dispersal of corona radiata cells is not a specific feature, and it can occur under the influence of sperm from males of another species.
This process is facilitated by the villi of the mucous membrane of the oviduct. Not necessary for fertilization complete liberation eggs from cells of the corona radiata. Enough small area so that sperm penetrate through the transparent membrane of the egg into the vitelline space. An excessive number of sperm not only leads to very rapid destruction of the corona radiata, severe loosening of the transparent membrane, but also to complete dissolution of the egg (cytolysis).
The second stage is the penetration of sperm through the transparent membrane into the vitelline space. This process is more species-specific and occurs with the participation of a trypsin-like enzyme. Up to 100 sperm cells are implanted into the transparent membrane of the egg in cows and sheep, from 200 to 1000 sperm cells in pigs, and up to 10 sperm cells in mares; a third of them penetrate into the vitelline space.
The third stage is the penetration of one, or less often several sperm cells through the vitelline membrane into the plasma of the egg. This stage is highly selective. Sperm of other species, as well as those with low viability, do not pass through the vitelline membrane. Having penetrated the cytoplasm, the sperm undergoes great changes. The head is separated from the tail, and pronuclei with half the set of chromosomes are formed.
The fourth stage is the fusion of the pronuclei of the egg and sperm; the stage lasts 4-7 hours. A zygote is formed, the nucleus contains the full set of chromosomes characteristic this species. In male mammals and female birds, one pair of chromosomes consists of one short and one long chromosome. These two chromosomes are directly related to sex determination.
To obtain a viable fetus, the biological usefulness and age of the germ cells, the period of stay of gametes in the female genital tract, etc. are important. The biological usefulness of gametes is determined by the metabolic processes occurring in them and depends on the age of the animals, the conditions of their keeping and feeding.
In old animals, the generative function of the gonads and the quality of germ cells decrease. Complete, balanced feeding of animals, active walking and keeping in bright clean rooms help prevent infertility and increase the fertilizing ability of gametes.
The ability of eggs to fertilize remains for 2-6 hours after ovulation. In cows and sheep best place to preserve (48 hours) sperm in the genital tract is the cervix; in the mare and pig - areas of the apex of the uterine horns at the transition to the oviducts. The lifespan of sperm in this area is 3-5 days in a mare, 48-60 hours in a pig. The maximum lifespan of sperm in the vagina in cows and sheep is 6 hours, in mares - 5 hours. Inflammatory processes in the vagina, cervix, uterus and oviducts negatively affect the lifespan of sperm in the genital tract of females of all animal species, reducing it several times.
The speed of sperm movement in the female reproductive tract depends on the stage of heat. Saturation of the oviducts with sperm, necessary for fertilization, occurs in cows after 2 hours, in sheep - after 6-7 hours, in a mare, pig, bitch - 30-60 minutes after insemination. Spermine move to the site of fertilization due to the suction force (vacuum) of the uterus, oviducts and their own movement.
Remember from the textbook “Animals” how this happens. reproduction in animals. How are eggs fertilized and where do the embryos of insects, fish, amphibians, birds and mammals develop?
The process of formation of male and female germ cells precedes sexual reproduction, that is, reproduction with the participation of sperm and eggs. Sexual reproduction can occur with or without fertilization.
Fertilization. The process of fusion of the nuclei of male and female germ cells is called fertilization. As a result of fertilization, a zygote is formed (from the Greek zygote - united together) - a fertilized egg. Its nucleus always has a double set of chromosomes. From the zygote an embryo develops, which gives rise to a new organism.
The fertilization process begins with the penetration of the sperm into the egg. Under the action of enzymes in the sperm vesicle, the shell of the egg at the point of contact dissolves. The sperm nucleus enters the egg (Fig. 87). In this case, the shell of the egg becomes impermeable to other sperm. After this, the gamete nuclei fuse and the zygote nucleus is formed.
Rice. 87. Fertilization in animals
There are two methods of fertilization - external and internal. During external fertilization, the female releases eggs (spawn), and the male releases sperm into the external environment, where fertilization occurs. This method is typical for aquatic animals, such as fish and amphibians. During internal fertilization, the fusion of gametes occurs in the female genital tract. This is how terrestrial and some aquatic inhabitants, such as insects, reptiles, birds and mammals, reproduce.
A fertilized egg can develop both in the female body, for example in mammals, and in the external environment. In the latter case, the eggs are covered with a special membrane or shell, and the female lays them in a safe place, for example in a nest (in birds).
Parthenogenesis. A type of sexual reproduction when development adult comes from an unfertilized egg, called parthenogenesis (from the Greek parthenos - virginity). Parthenogenesis occurs in crustaceans (daphnia), insects (bees, aphids), and some birds (turkeys) (Fig. 88). Development without fertilization most often alternates with normal sexual reproduction. From unfertilized eggs, cells begin to develop in which, in the first division of mitosis, the chromosomes do not diverge and a double set of chromosomes is restored.
Rice. 88. Animals capable of parthenogenesis: 1 - daphnia; 2 - bees
Ontogenesis of the organism and embryonic development. The individual development of an organism - ontogenesis (from the Greek ontos - existence and genesis - birth) covers the entire period of its life. During this time, the body goes through several successive stages: an embryo is formed, a new organism is born, it grows, develops, reproduces, ages and dies. Ontogenesis is divided into two periods - embryonic and postembryonic.
The embryonic period, or embryogenesis (from the Greek embryo - embryo and genesis) lasts from the moment of formation of the embryo from the zygote until its exit from the egg or birth. It occurs in several stages.
After fertilization, the embryo begins to develop from the zygote. A fertilized egg is divided by mitosis into 2, then 4, 8, 16, etc. cells. This process is called fragmentation, since, unlike ordinary division, the cells do not enlarge, that is, they do not grow (Fig. 89). Cleavage cells are used nutrients, accumulated in the egg. The process goes very quickly. For example, within 4 hours from the moment of fertilization, 64 cells arise from one cell of the zygote.
Rice. 89. Stages of development of the embryo of a chordate animal: 1 - crushing; 2 - blastula; 3 - invagination and formation of two layers of cells; 4 - gastrula: 5 - organogenesis (a - neural tube; b - notochord; c - digestive tube)
Fragmentation ends with the formation of a blastula (from the Greek blastos - sprout), an embryonic vesicle with a cavity inside. The walls of the vesicle consist of a single layer of cells (Fig. 89).
After the formation of the blastula, the second stage of development of the embryo begins - gastrula (from the Greek gaster - stomach). It is a two-layer bag. Its formation begins with the invagination of the lower wall of the blastula into the cavity. As a result, two germ layers are formed: the outer one - ectoderm (from the Greek ectos - outside and dermis - skin) and the inner one - endoderm (from the Greek entos - inside).
Rice. 90. Crushing frog eggs
At the gastrula stage, development ends in sponges and coelenterates. In more highly organized multicellular animals, the formation of the third germ layer further occurs. Between the ectoderm and endoderm is laid the mesoderm (from the Greek mesos - middle, intermediate and dermis). It is formed due to the movement of some cells from the outer and inner layers. As a result, a three-layer embryo is formed. At the same time, at this stage, axial organs are formed, for example, in chordates, the neural tube, notochord and digestive tube.
The subsequent development of the embryo in chordates is associated with the interaction of three germ layers, from which all the tissues and organs of the future organism develop. The stage of organ formation in the embryo is called organogenesis.
From the ectoderm, epithelial and nervous tissues, the epidermis of the skin and its derivatives (nails, hair), as well as the nervous system and sensory organs develop. The mucous epithelium and digestive organs are formed from the endoderm. From the mesoderm muscle and all types of connective tissue. From the notochord, in most chordates, a cartilaginous or bony skeleton is subsequently formed, and from the lateral sections of the mesoderm - muscles, blood vessels, heart, kidneys and organs of the reproductive system.
The influence of various factors on the development of the embryo. All cells of the embryo develop from one initial cell - the zygote (Fig. 90), have the same set of chromosomes and genetic information. However, in the cells of different germ layers it is realized hereditary information different genes, which leads to the biosynthesis of different proteins and, consequently, to the formation of different tissues and organs from them.
The specificity of cell functioning does not arise immediately, but at a certain stage of embryogenesis. It has been established that at the stage of 2-16 cells (depending on the type of animal), each cell can develop into normal body. If these cells are separated, then an independent organism is formed from each - identical twins arise. They look alike and are always the same sex (Fig. 91).
Rice. 91. Formation of identical twins
Research has shown that there are critical periods in the development of animal embryos when disruption may occur. normal development. For example, such periods are the middle of cleavage, the beginning of gastrulation and the stage of formation of axial organs. At this time, the embryo is especially sensitive to lack of oxygen, temperature changes, mechanical impact. The better the egg is protected, the less susceptible it is to external influences. Some viral diseases, such as measles in humans, have a negative effect on the development of the embryo. A number of medications have the same effect, for example, antibiotics, hormonal drugs, drugs and alcohol. Powerful factors causing disturbances in the embryonic development of animals and humans are X-rays and radioactive radiation. Their impact can lead to the death of the embryo or the birth of organisms with deformities (Fig. 92).
Rice. 92. Violation of embryonic development can lead to the development of deformities: 1 - two-headed snake; 2 - Siamese twins Chang and Eng
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Fertilization called the union of two gametes, resulting in the formation of a fertilized egg or zygote (Greek zygota - united in a pair), - initial stage development of a new organism.
Fertilization entails two important consequences: 1) activation of the egg, i.e. motivation for development, and 2) synkaryogamy, i.e. the formation of a diploid zygote nucleus as a result of the fusion of the haploid nuclei of germ cells carrying the genetic information of two parent organisms.
The meeting of gametes is facilitated by the fact that the eggs of plants and animals are released into the environment chemical substances- hormones that activate sperm. It is possible that activating substances are secreted by cells of the female reproductive tract of mammals. It has been established that mammalian sperm can penetrate an egg only if they have been in the female reproductive tract for at least one hour.
Sperm cells of a number of lower plants have positive chemotaxis to substances secreted by the egg cell. There is no convincing evidence of chemotaxis in animal sperm. Sperm move randomly and collide with eggs randomly.
In the egg shell of some animals there are tiny holes - micropyle, through which the sperm penetrates. In most species there is no micropyle; sperm penetration occurs through the acrosomal reaction, detected using electron microscopy. Located at the anterior end of the sperm, the acrosomal region is surrounded by a membrane. Upon contact with the egg, the acrosome membrane is destroyed. The acrosomal filament is released from it, an enzyme is released that dissolves the egg membrane, and the enzyme hyaluronidase, which destroys the follicular cells surrounding the egg. The acrosomal filament penetrates the dissolved zone of the egg membranes and fuses with the egg membrane. At this point, a receptive tubercle is formed from the cytoplasm of the egg. It captures the nucleus, centrioles and mitochondria of the sperm and carries them deep into the egg. The plasma membrane of the sperm is embedded in the surface membrane of the egg, forming a mosaic outer membrane of the zygote.
The penetration of a sperm into an egg changes its metabolism, which is indicated by a number of morphological and physiological transformations. The permeability of the cell membrane increases, absorption from environment phosphorus and potassium, calcium is released, carbohydrate metabolism increases, protein synthesis is activated. Some animals have a need for oxygen. Yes, y sea urchin in the first minute after fertilization, oxygen absorption increases 80 times. The colloidal properties of protoplasm change. Viscosity increases 6-8 times. In the outer layer of the egg, elasticity and optical properties change. The fertilization membrane peels off on the surface; A free, liquid-filled space is formed between it and the surface of the egg. A shell is formed under it, which provides attachment for the cells resulting from the crushing of the egg. Once the fertilization membrane is formed, other sperm can no longer penetrate the egg.
An indicator of changes in metabolism is the fact that in a number of animal species, egg maturation ends after sperm penetrates it. U roundworms and mollusks, only in fertilized eggs the second reduction body is released. In humans, sperm penetrate eggs that are still in the period of maturation. The first reduction body is released after 10 hours, the second - only 1 day after sperm penetration.
The culmination of the fertilization process is nuclear fusion. The sperm nucleus (male pronucleus) in the cytoplasm of the egg swells and reaches the size of the egg nucleus (female pronucleus). At the same time, the male pronucleus rotates 180 degrees and moves forward with the centrosome towards the female pronucleus; the latter also moves to meet him. After the meeting, the nuclei merge.
As a result of synkaryogamy, i.e. fusion of two nuclei with a haploid set, the diploid set of chromosomes is restored. After the formation of the synkarion, the egg begins to crush.
The study of the physiology of fertilization allows us to understand the role of the large number of sperm involved in fertilization. It has been established that if during artificial insemination of rabbits the seminal fluid contains less than 1000 sperm, fertilization does not occur. In the same way, fertilization does not occur when a very large number of sperm are introduced (more than 100 million). This is explained in the first case by an insufficient, and in the second by an excessive amount of enzymes necessary for the penetration of sperm into the egg.
