The ontogeny of individuals of different species varies in duration, rate, and character of differentiation (see below). It is usually divided into proembryonic, embryonic and postembryonic periods. In animals, the embryonic period is usually rich in differentiation, in plants, the postembryonic period. Each of these periods of ontogeny can be subdivided into successive qualitative stages. Ontogeny can be characterized by direct development or development by metamorphosis.
Features of ontogeny in different groups. The forms of manifestation of individuality in wildlife are diverse, the process of ontogenesis is unequal in content and the process of ontogenesis in different representatives of prokaryotes, fungi, plants and animals.
Rice. 14.1. Scheme of successive complication of the ontogeny of multicellular organisms in the course of evolution. A - reproduction of free-living unicellular; B - ontogenesis of a colony of unicellular Volvox type [cells differentiate into sex (black) and somatic]; B - ontogeny of a multicellular organism such as hydra (the stages of blastula and gastrula are added); D - ontogenesis of the primary bilaterally symmetrical animal (mesoderm is added); D - ontogeny of a higher bilaterally symmetrical animal (according to A.N. Severtsov, 1935)
With the transition to multicellularity (Metazoa), ontogenesis becomes more complicated in form and lengthens in time (Fig. 14.1), but in the process of ontogenesis evolution, there are also cases of simplification of development associated with the emergence of more advanced ways of implementing hereditary information. In the course of evolution, plants and animals develop complex cycles of development, each phase of which is adapted to certain environmental conditions. Sometimes in the process of evolution there is a secondary simplification of life cycles.
With the simplification of the life cycle, the whole process of ontogenetic development changes qualitatively. One of the consequences of simplifying the life cycle is the transition from the haploid phase of development to the diploid one and from development with metamorphosis (for example, in amphibians) to direct development (in reptiles and other higher vertebrates). With direct development, the newborn animal possesses all the basic features of the organization of an adult being. Development with metamorphosis goes through a series of larval stages; a larva emerges from the egg, which acquires the features of an adult animal through a complex transformation. The transition from development through metamorphosis to direct development is one of the most important outcomes of the last stages of the evolution of life on Earth.
Despite the complex dissection of the individual in trees, shrubs and perennial grasses, in terms of the level of organization of ontogenesis, they are inferior to one-, two-year-old and ephemeral flowering plants. In the latter, ontogenesis proceeds with strict coordination of the vital activity of a certain number of organs. The processes of differentiation and morphogenesis in their ontogeny are "explosive" in nature.
In plants, ontogenesis is characterized by greater lability due to the weak development of the regulatory system (see below). Ontogeny in plants as a whole is more dependent on environmental conditions than in animals.
Common features of ontogenesis in different organisms are its programming, the direction of its differentiation, the sequence of changes in development programs under the influence of environmental factors (epigenetic factors).
The diversity of ontogeny in different groups of organisms (even in representatives of the same species) testifies to the special role of environmental factors in the stabilization of differentiation and life cycles. Although the selection proceeds along a holistic ontogenesis, its individual stages act as necessary prerequisites for the implementation of the entire program and the flow of information between generations.
In representatives of different kingdoms, types, classes, ontogeny also differs in terms of the scale of differentiation. In unicellular organisms, it is primitive in terms of the complexity of differentiation processes. In plants, the processes of differentiation are extended and are not limited to the period of embryonic development (laying of metameric organs in plants occurs during the entire ontogenesis). In animals, the processes of differentiation and organ formation are limited mainly to the embryonic period. The processes of histo- and morphogenesis in plants are less complex and involve fewer organs and structures than animals.
duration of ontogeny. In representatives of different types, classes, and orders, the duration of ontogeny is an important specific feature. The limitation of life expectancy by the onset of natural death, even in the presence of favorable external conditions, is an important result of evolution, which makes it possible to carry out generational change. In unicellular organisms, ontogeny ends with the formation of daughter cells; death is not fixed morphologically (and they are, in a certain sense, immortal). In fungi and plants, the aging of different organs proceeds unevenly. In mushrooms, the “mycelium” itself lives in the substrate for a long time (in the meadow honey agaric (Marasmius oreades) - up to 500 years!). On the other hand, among fungi there are ephemeral organisms that live for weeks and months (Clavaria gyromitra). In table. 14.1 shows some data on the life span of a number of plants. Plants are as varied in individual lifespan as are animals.
| Table 14.1. The duration of ontogenesis of some species | |
| Kinds | Duration of ontogeny |
| 1. The realm of pre-nuclear | |
| cyanes | Few hours |
| II. mushroom kingdom | |
| Penicillum (Penicillium notatum) | Few weeks |
| Polypore (Fomes fomentarius) | Up to 25 years old |
| White mushroom (Botulus botulus) | Some years |
| III. plant kingdom | |
| Rezushka (Arabidopsis thaliana) | 60-70 days |
| Wheat (Triticum bulgare) | About 1 year |
| Grapes (Vitis vinifera) | 80-100 years old |
| Apple tree (Malus domestica) | 200 years |
| Walnut (Juglans regia) | 300-400 years |
| Linden (Tilia grandifolia) | 1000 years |
| Oak (Quercus robur) | 1200 years |
| Cypress (Cupressus fastigiata) | 3000 years |
| Mammoth tree (Sequoia gigantea) | 5000 years |
| IV. animal kingdom | |
| Wide tapeworm (Diphyllobothrium latum) | Up to 29 years old |
| Ant (Formica fusca) | Up to 7 years |
| Honey bee (Apis mellifera) | Up to 5 years |
| Sea urchin (Ehinus esculentus) | Up to 8 years |
| Com (Silurus glanis) | Up to 60 years old |
| Goby (Aphya pellucida) | 1 year |
| Common toad (Bufo bufo) | Up to 36 years old |
| Turtle (Testudo sumelri) | Up to 150 years old |
| Common Eagle Owl (Bubo bubo) | Up to 68 years old |
| Blue dove (Columba livid) | Up to 30 years old |
| African Elephant (Elephas maximus) | Up to 60 years old |
| Gibbon (Hylobates lar) | Up to 32 years old |
Periodization of the ontogeny of multicellular organisms
Embryonic (embryonic) stage and its periods in animals.
4. Embryonic stage- this is the time when a new organism develops inside the mother's organism or inside the egg. Embryogenesis ends with birth (hatching, germination). The embryonic period begins after fertilization or activation of the egg during parthenogenesis and is carried out inside the mother's body, egg, seed. Embryonic development ends with birth (mammals), exit from the shells of the egg (birds, reptiles), germination (seed plants). The main stages of the embryonic period are cleavage, gastrulation, histogenesis and organogenesis.
Splitting up- a series of successive mitotic divisions of the zygote, which ends with the formation of a single-layer stage - the blastula. The number of cells increases as a result of mitosis, but the interphase is very short and blastomeres do not grow. Features of crushing in different groups of organisms depend on the nature of the location and amount of the yolk, in connection with this, two types of crushing are distinguished.
Gastrulation - this is the process of formation of a two-layer embryo - gastrula. Cell growth does not occur during gastrulation. At this stage, two or three layers of the body of the embryo are formed - germ layers. In the process of gastrulation, it is extremely important to distinguish between two stages: a) the formation of ecto- and endoderm (an early gastrula is formed - a two-layer embryo) b) the formation of mesoderm (a late gastrula is formed - a three-layer embryo). At the stage of gastrulation, the embryogenesis of two-layer animals (sponges, coelenterates) is completed, the mesoderm is laid down in the embryonic development of three-layer animals (starting with flatworms).
In different organisms, the gastrula is formed in different ways. The following types of gastrula formation are distinguished: invagination (vagination), delamination (stratification), epiboly (fouling), immigration (creeping).
Histogenesis and organogenesis - the formation of tissues and organs. These processes are carried out as a result of differentiation (the appearance of differences in the structure and functions of cells, tissues, organs). Initial cells of educational tissues participate in plant histogenesis, and stem, semi-stem and mature cells participate in animal histogenesis. Of great importance in organogenesis are intercellular interactions, the influence of biologically active substances. The phases of histogenesis and organogenesis (using the lancelet as an example) are neurulation - the formation of an axial complex of organs (neural tube, chorda), the formation of other organs - organs acquire structural features inherent in adults. Organogenesis ends mainly at the end of the embryonic period of development, however, the differentiation and complication of organs continues in postembryogenesis.
Periodization of the ontogeny of multicellular organisms - the concept and types. Classification and features of the category "Periodization of the ontogeny of multicellular organisms" 2017, 2018.
Introduction
Individual development of organisms or ontogenesis- this is a long and complex process of the formation of organisms from the moment of formation of germ cells and fertilization (during sexual reproduction) or individual groups of cells (during asexual reproduction) until the end of life.
From the Greek "ontos" - existing and genesis - occurrence. Ontogeny is a chain of strictly defined complex processes at all levels of the organism, as a result of which structural features, life processes, and the ability to reproduce that are inherent only to individuals of a given species are formed. Ontogenesis ends with processes that naturally lead to aging and death.
With the genes of the parents, the new individual receives some kind of instructions about when and what changes should occur in the body so that it can successfully go through its entire life path. Thus, ontogenesis is the realization of hereditary information.
1. Historical information
The process of the appearance and development of living organisms has been of interest to people for a long time, but embryological knowledge accumulated gradually and slowly. The great Aristotle, observing the development of a chicken, suggested that the embryo is formed as a result of mixing fluids belonging to both parents. This opinion held for 200 years. In the 17th century, the English physician and biologist W. Harvey did some experiments to test Aristotle's theory. As the court physician of Charles I, Harvey received permission to use deer living in the royal lands for experiments. Harvey examined 12 female deer that died at different times after mating.
The first embryo, taken from a female deer a few weeks after mating, was very small and did not look like an adult animal at all. In deer that died at a later date, the embryos were larger, they had a great resemblance to small, newly born deer. This is how the knowledge of embryology accumulated.
The following scientists made significant contributions to embryology.
· Anthony van Leeuwenhoek (1632–1723) discovered spermatozoa in 1677, he was the first to study parthenogenesis in aphids.
Jan Swammerdam (1637–1680) pioneered the study of insect metamorphosis.
· Marcello Malpighi (1628–1694) was the first to study the microscopic anatomy of the development of the organs of the chicken embryo.
· Caspar Wolf (1734-1794) is considered the founder of modern embryology; more precisely and in more detail than all his predecessors investigated the development of a chicken in an egg.
· The true creator of embryology as a science is the Russian scientist Karl Baer (1792–1876), a native of the Estland province. He was the first to prove that during the development of all vertebrates, the embryo is first laid down from two primary cell layers, or layers. Baer saw, described, and then demonstrated at the congress of natural scientists the egg cell of mammals in a dog he had opened. He discovered a method for the development of the axial skeleton in vertebrates (from the so-called dorsal string-chord). Baer was the first to establish that the development of any animal is a process of unfolding something previous, or, as they would now say, the gradual differentiation of more and more complex formations from simpler rudiments (the law of differentiation). Finally, Baer was the first to appreciate the importance of the significance of embryology as a science and put it at the basis of the classification of the animal kingdom.
A.O. Kovalevsky (1840–1901) is known for his famous work The History of the Development of the Lancelet. Of particular interest are his works on the development of ascidians, ctenophores and holothurians, on the postembryonic development of insects, etc. By studying the development of the lancelet and extending the data obtained to vertebrates, Kovalevsky once again confirmed the correctness of the idea of the unity of development in the entire animal kingdom.
I.I. Mechnikov (1845–1916) became especially famous for his studies of sponges and jellyfish, i.e. lower multicellular. A prominent idea of Mechnikov was his theory of the origin of multicellular organisms.
A.N. Severtsov (1866-1936) is the largest modern embryologist and comparative anatomist, the creator of the theory of phylembryogenesis.
2. Individual development of unicellular organisms
ontogenesis embryology unicellular organism
In the simplest organisms, whose body consists of one cell, ontogeny coincides with the cell cycle, i.e. from the moment of appearance, by division of the mother cell, until the next division or death.
The ontogenesis of unicellular organisms consists of two periods:
- maturation (synthesis of cellular structures, growth).
- maturity (preparation for division).
- the division process itself.
Ontogenesis is much more complicated in multicellular organisms.
For example, in various divisions of the plant kingdom, ontogenesis is represented by complex development cycles with a change in sexual and asexual generations.
In multicellular animals, ontogenesis is also a very complex process and much more interesting than in plants.
In animals, three types of ontogeny are distinguished: larval, ovipositor, and intrauterine. The larval type of development is found, for example, in insects, fish, and amphibians. There is little yolk in their eggs, and the zygote quickly develops into a larva, which feeds and grows on its own. Then, after some time, metamorphosis occurs - the transformation of the larva into an adult. In some species, there is even a whole chain of transformations from one larva to another and only then to an adult. The meaning of the existence of larvae may lie in the fact that they feed on different food than adults, and thus the food base of the species is expanding. Compare, for example, the nutrition of caterpillars (leaves) and butterflies (nectar), or tadpoles (zooplankton) and frogs (insects). In addition, in the larval stage, many species actively colonize new territories. For example, bivalve larvae are capable of swimming, while adults are practically immobile. The oviparous type of ontogenesis is observed in reptiles, birds, and oviparous mammals, whose eggs are rich in yolk. The embryo of such species develops inside the egg; the larval stage is absent. The intrauterine type of ontogenesis is observed in most mammals, including humans. At the same time, the developing embryo lingers in the mother's body, a temporary organ is formed - the placenta, through which the mother's body provides all the needs of the growing embryo: respiration, nutrition, excretion, etc. Intrauterine development ends with the process of childbearing.
I. Embryonic period
The individual development of multicellular organisms can be divided into two stages:
the embryonic period.
post-embryonic period.
The embryonic or germinal period of the individual development of a multicellular organism covers the processes occurring in the zygote from the moment of the first division to the exit from the egg or birth.
The science that studies the laws of individual development of organisms at the stage of the embryo is called embryology (from the Greek embryo - embryo).
Embryonic development can proceed in two ways: in utero and end with birth (in most mammals), as well as outside the mother's body and end with the exit from the egg membranes (in birds, fish, reptiles, amphibians, echinoderms, molluscs and some mammals)
Multicellular animals have different levels of organization complexity; can develop in the womb and outside the mother's body, but in the vast majority the embryonic period proceeds in a similar way and consists of three periods: crushing, gastrulation and organogenesis.
1) Crushing.
The initial stage of development of a fertilized egg is called crushing . A few minutes or several hours (in different species in different ways) after the introduction of the sperm into the egg, the resulting zygote begins to divide by mitosis into cells called blastomeres. This process is called cleavage, since in the course of it the number of blastomeres increases exponentially, but they do not grow to the size of the original cell, but become smaller with each division. Blastomeres formed during crushing are early germ cells. During cleavage, mitoses follow one after another, and by the end of the period, the entire embryo is not much larger than the zygote.
The type of egg crushing depends on the amount of yolk and the nature of its distribution. Distinguish between complete and incomplete crushing. In yolk-poor eggs, uniform crushing is observed. The zygotes of the lancelet and mammals undergo complete crushing, since they contain little yolk and it is relatively evenly distributed.
In eggs rich in yolk, crushing can be complete (uniform and uneven) and incomplete. Due to the abundance of yolk, the blastomeres of one pole always lag behind the blastomeres of the other pole in the rate of cleavage. Complete but uneven fragmentation is characteristic of amphibians. In fish and birds, only the part of the egg located at one of the poles is crushed; incomplete occurs. splitting up. Part of the yolk remains outside the blastomeres, which are located on the yolk in the form of a disc.
Let us consider in more detail the crushing of the lancelet zygote. Cleavage covers the entire zygote. Furrows of the first and second crushing pass through the poles of the zygote in mutually perpendicular directions, resulting in the formation of an embryo consisting of four blastomeres.
Subsequent crushing takes place alternately in the longitudinal and transverse directions. At the stage of 32 blastomeres, the embryo resembles a mulberry or raspberry. It's called morula. With further crushing (at about 128 blastomeres), the embryo expands and the cells, located in a single layer, form a hollow ball. This stage is called blastula. The wall of a single-layer embryo is called the blastoderm, and the cavity inside is called the blastocoel (primary body cavity).
Rice. Fig. 1. Initial stages of lancelet development: a – cleavage (stage of two, four, eight, sixteen blastomeres); b - blastula; in - gastr. chiation; d – schematic cross section through the lancelet embryo; 2 - vegetative pole of the blastula; 3 - endoderm; 4 - blastogel; 5 - gastrula mouth (blastopore); 6,7 - dorsal and ventral lips of the blastopore; 8 - formation of the neural tube; 9 - formation of a chord; 10 - formation of mesoderm
2) Gastrulation
The next stage of embryonic development is the formation of a two-layer embryo - gastrulation. After the lancelet blastula is fully formed, further cell fragmentation occurs especially intensively at one of the poles. As a result, they are, as it were, drawn in (pushed) inward. As a result, a two-layer embryo is formed. At this stage, the embryo looks like a cup and is called a gastrula. The outer layer of cells of the gastrula is called the ectoderm or outer germ layer, and the inner layer lining the cavity of the gastrula - the gastric cavity (the cavity of the primary intestine), is called the endoderm or inner germ layer. The cavity of the gastrula, or primary intestine, in most animals at further stages of development turns into a digestive tract, opens outward through the primary mouth, or blastopore. In worms, mollusks, and arthropods, the blastonor develops into the mouth of an adult organism. Therefore, they are called primary. In echinoderms and chordates, the mouth erupts on the opposite side, and the blastonor turns into an anus. They are called secondary.
At the stage of two germ layers, the development of sponges and intestinal cavities ends. In all other animals, a third is formed - the middle germ layer, located between the ectoderm and endoderm. It's called the mesoderm.
After gastrulation, the next stage in the development of the embryo begins - the differentiation of the germ layers and the laying of organs (organogenesis). Initially, the formation of axial organs occurs - the nervous system, chord and digestive tube. The stage at which the laying of axial organs is carried out is called non-rule.
The nervous system in vertebrates is formed from the ectoderm in the form of a neural tube. In chordates, it initially looks like a neural plate. This plate grows more intensively than all other parts of the ectoderm and then bends, forming a groove. The edges of the groove close, a neural tube appears, which stretches from the anterior end to the posterior. At the anterior end of the tube, the brain is then formed. Simultaneously with the formation of the neural tube, the formation of the notochord occurs. The chordal material of the endoderm is bent, so that the chord separates from the common plate and turns into a separate strand in the form of a continuous cylinder. The neural tube, intestine, and notochord form a complex of axial organs of the embryo, which determines the bilateral symmetry of the body. Subsequently, the notochord in vertebrates is replaced by the spine, and only in some lower vertebrates its remnants are preserved between the vertebrae even in the adult state.
Simultaneously with the formation of the notochord, the third germ layer, the mesoderm, separates. There are several ways of mesoderm formation. In the lancelet, for example, the mesoderm, like all the main organs, is formed as a result of increased cell division on both sides of the primary intestine. As a result, two endodermal pockets are formed. These pockets increase, filling the primary body cavity, their edges break away from the endoderm and close together, forming two tubes consisting of separate segments, or somites. This is the third germ layer - the mesoderm. In the middle of the tubes is the secondary body cavity, or coelom.
3) Organogenesis.
Further differentiation of the cells of each germ layer leads to the formation of tissues (histogenesis) and the formation of organs (organogenesis). In addition to the nervous system, the outer cover of the skin develops from the ectoderm - the epidermis, and its derivatives (nails, hair, sebaceous and sweat glands), the epithelium of the mouth, nose, anus, the lining of the rectum, tooth enamel, perceiving cells of the organs of hearing, smell, vision and etc.
From the endoderm, epithelial tissues develop that line the esophagus, stomach, intestines, respiratory tract, lungs or gills, liver, pancreas, epithelium of the gallbladder and bladder, urethra, thyroid and parathyroid glands.
Derivatives of the mesoderm are the connective tissue base of the skin (dermis), the entire connective tissue itself, the bones of the skeleton, cartilage, the circulatory and lymphatic systems, the dentin of the teeth, the mesentery, the kidneys, the gonads, and the muscles.
The animal embryo develops as a single organism in which all cells, tissues and organs are in close interaction. At the same time, one germ influences the other, to a large extent determining the path of its development. In addition, the rate of growth and development of the embryo is influenced by external and internal conditions.
The embryonic development of organisms proceeds differently in different types of animals, but in all cases the necessary connection between the embryo and the environment is provided by special extra-embryonic organs that function temporarily and are called provisional. Examples of such temporary organs are the yolk sac in fish larvae and the placenta in mammals.
The development of the embryos of higher vertebrates, including humans, in the early stages of development is very similar to the development of the lancelet, but in them, starting from the blastula stage, the appearance of special germinal organs - additional embryonic membranes (chorion, amnion and allantois), providing protection of the developing embryo from drying out and various kinds of environmental influences.
The outer part of the spherical formation that develops around the blastula is called the chorion. This shell is covered with villi. In placental mammals, the chorion, together with the uterine mucosa, forms a child's place, or placenta, which provides a link between the fetus and the mother's body.
Rice. 2.5. Scheme of embryonic membranes: 1 - embryo; 2 – amnion and its cavity (3) filled with amniotic fluid; 4 - chorion with villi forming a child's place (5); 6 - umbilical or yolk bladder; 7 - allantois; 8 - umbilical cord
The second germinal membrane is the amnion (Latin amnion - periembryonic bladder). So in ancient times they called the bowl into which the blood of animals sacrificed to the gods was poured. The amnion of the embryo is filled with fluid. Amniotic fluid is an aqueous solution of proteins, sugars, mineral salts, which also contains hormones. The amount of this fluid in a six-month-old human embryo reaches 2 liters, and by the time of birth - 1 liter. The wall of the amniotic membrane is a derivative of the ecto- and mesoderm.
Allantois (lat. alios - sausage, oidos - view) - the third embryonic membrane. This is the rudiment of the urinary sac. Appearing as a small sac-like outgrowth on the abdominal wall of the posterior intestine, it exits through the umbilical opening and grows very quickly and covers the amnion and the yolk sac. In different vertebrates, its functions are different. In reptiles and birds, the waste products of the embryo accumulate in it before hatching from the egg. In the human embryo, it does not reach large sizes and disappears in the third month of embryonic development.
Organogenesis is completed mainly by the end of the embryonic period of development. However, the differentiation and complication of organs continues in the postembryonic period.
The impact of environmental factors on the developing embryo.
The developing embryo (especially the human one) has periods called critical periods, when it is most sensitive to the damaging effects of environmental factors. This is the period of implantation on the 6-7th day after fertilization, the period of placentation - the end of the second week and the period of childbirth. During these periods, there is a restructuring in all body systems.
The development of an organism from the moment of its birth or exit from the egg membranes to death is called the postembryonic period. In different organisms, it has a different duration: from several hours (for bacteria) to 5000 years (for sequoias).
There are two main types of postembryonic development:
indirect.
direct development in which an individual emerges from the mother's body or egg shells, which differs from the adult organism only in a smaller size (birds, mammals). There are: non-larval (oviparous) type, in which the embryo develops inside the egg (fish, birds), and intrauterine type, in which the embryo develops inside the mother's body - and is connected with it through the placenta (placental mammals).
Conclusion
The individual development of living organisms ends with aging and death.
The duration of the embryonic period can last from several tens of hours to several months.
The duration of the postembryonic period in different multicellular organisms is different. For example: turtles - 100-150 years, vulture - 117 years, beluga - 80-100 years, parrot - 70-95 years, elephant - 77 years, goose - 50-100 years, human - 70 years, crocodile - 60 years, carp - 50-100 years old, sea anemones - 50-70 years old, eagle owl - 68 years old, rhinoceros - 45 years old, lobster - 50 years old, horse - 40 years old, seagull - 30-45 years old, monkey - 35-40 years old, lion - 35 years old, already - 30 years old, a cow - 20-30 years old, a cat - 27 years old, a frog - 12-20 years old, a swallow - 9 years old, a mouse - 3-4 years old.
1. What is ontogeny?
Answer. Ontogenesis is the process of individual development of an organism, from the formation of a zygote to the death of the organism.
2. What is the set of chromosomes in a zygote?
Answer. The zygote contains a diploid set of chromosomes.
Questions after § 35
1. How does the ontogenesis of unicellular organisms differ from the ontogenesis of multicellular organisms?
Answer. Ontogeny is the individual development of an organism. In multicellular organisms, ontogenesis begins with the formation of a zygote (during sexual reproduction) or from the moment the offspring is separated from the mother (during asexual reproduction) and continues until the end of life. In unicellular organisms, it begins from the moment the organism is formed in the process of division of the maternal individual and ends with division or death.
In animals, two periods of ontogenesis are distinguished - embryonic (embryonic) and postembryonic (post-embryonic).
2. What types of ontogenesis are distinguished in animals? What are their features?
Answer. In animals, three types of ontogeny are distinguished: larval, ovipositor, and intrauterine.
The larval type of development is found, for example, in insects, fish, and amphibians. There is little yolk in their eggs, and the zygote quickly develops into a larva, which feeds and grows on its own. Then, after some time, metamorphosis occurs - the transformation of the larva into an adult. In some species, there is even a whole chain of transformations from one larva to another and only then to an adult. The meaning of the existence of larvae may lie in the fact that they feed on different food than adults, and thus the food base of the species is expanding. Compare, for example, the food of caterpillars (leaves) and butterflies (nectar) or tadpoles (zooplankton) and frogs (insects). In addition, in the larval stage, many species actively colonize new territories. For example, bivalve larvae are capable of swimming, while adults are practically immobile.
The oviparous type of ontogenesis is observed in reptiles, birds, and oviparous mammals, whose eggs are rich in yolk. The embryo of such species develops inside the egg; the larval stage is absent.
The intrauterine type of ontogenesis is observed in most mammals, including humans. At the same time, the developing embryo lingers in the mother's body, a temporary organ is formed - the placenta, through which the mother's body provides all the needs of the growing embryo: respiration, nutrition, excretion, etc. Intrauterine development ends with the process of childbearing.
3. How does the embryonic period of embryogenesis in a crocodile end?
Answer. The embryonic period of embryogenesis in a crocodile ends with the release of an individual from an egg.
4. What are the functions of the placenta?
Answer. Functions of the placenta:
Gas exchange occurs through the placenta: oxygen penetrates from the maternal blood to the fetus, and carbon dioxide is transported in the opposite direction.
The fetus receives through the placenta the nutrients necessary for its growth and development. In addition, with its help, the fetus gets rid of the products of its vital activity.
3. The placenta provides the immunological protection of the fetus, delaying the cells of the mother's immune system, which, having penetrated to the fetus and recognizing a foreign object in it, could trigger its rejection reactions. At the same time, the placenta passes maternal antibodies that protect the fetus from infections.
4. The placenta plays the role of an endocrine gland and synthesizes necessary to maintain pregnancy, growth and development of the fetus.
Ontogenesis- the individual development of an individual, the totality of its interrelated transformations, naturally occurring in the process of implementing the life cycle from the moment of formation of a zygote to death.
In multicellular animals that reproduce sexually, ontogeny is divided into embryonic(from the formation of a zygote to birth or exit from the egg membranes) and postembryonic(from the exit from the egg membranes or birth to the death of the organism) periods. The zygote is formed as a result of the fusion of male and female germ cells - gametes. Gametes are formed in the gonads, depending on the organism, male or female. The development of gametes is called gametogenesis. The process of sperm formation is called spermatogenesis and the formation of oocytes ovogenesis.
spermatogenesis
1 - phase of reproduction; 2 - growth phase; 3 - maturation phase; 4 - formation phase.
Spermatogenesis is carried out in the testes and is divided into four phases: 1) reproduction, 2) growth, 3) maturation, 4) formation. During the reproduction phase, diploid spermatogonia divide repeatedly by mitosis. Part of the formed spermatogonia may undergo repeated mitotic divisions, resulting in the formation of the same spermatogonia cells. The other part stops dividing and increases in size, entering the next phase of spermatogenesis - the growth phase.
The growth phase corresponds to interphase 1 of meiosis, i.e. during it, the cells prepare for meiosis. The main event of the growth phase is DNA replication. During the maturation phase, cells divide by meiosis; during the first division of meiosis they are called spermatocytes of the 1st order, during the second spermatocytes of the 2nd order. From one spermatocyte of the 1st order, four haploid spermatids arise. The formation phase is characterized by the fact that initially spherical spermatids undergo a series of complex transformations, as a result of which spermatozoa are formed. It involves all elements of the nucleus and cytoplasm.
In humans, spermatogenesis begins at puberty; the period of sperm formation is three months, i.e. every three months, spermatozoa are renewed. Spermatogenesis occurs continuously and synchronously in millions of cells.
1 - "head"; 2 - "neck"; 3 - middle part; 4 - flagellum; 5 - acrosome; 6 - core; 7 - centrioles; 8 - mitochondria.
Mammalian sperm is shaped like a long filament. The length of a human spermatozoon is 50-60 microns. In the structure of the spermatozoon, one can distinguish the “head”, “neck”, intermediate section and tail. The head contains the nucleus and acrosome. The nucleus contains a haploid set of chromosomes. The acrosome is a membrane organelle containing enzymes used to dissolve the membranes of the egg. There are two centrioles in the neck, and mitochondria in the intermediate section. The tail is represented by one, in some species - two or more flagella. The flagellum is an organelle of movement and is similar in structure to the flagella and cilia of protozoa. For the movement of flagella, the energy of macroergic bonds of ATP is used, ATP synthesis occurs in mitochondria.
The spermatozoon was discovered in 1677 by A. Leeuwenhoek.
Ovogenesis
It is carried out in the ovaries, is divided into three phases: 1) reproduction, 2) growth, 3) maturation.
During the reproductive phase, diploid ovogonia divide repeatedly by mitosis. The growth phase corresponds to interphase 1 of meiosis, i.e. during it, the preparation of cells for meiosis occurs: the cells increase significantly in size due to the accumulation of nutrients. The main event of the growth phase is DNA replication. During the maturation phase, cells divide by meiosis. During the first division of meiosis they are called oocytes of the 1st order. As a result of the first meiotic division, two daughter cells arise: a small one, called the first polar body, and the larger oocyte 2nd order. During the second meiotic division, the 2nd order oocyte divides to form the egg and the second polar body, and the first polar body divides to form the third and fourth polar bodies. Thus, as a result of meiosis, one egg and three polar bodies are formed from one oocyte of the 1st order.
1 - phase of reproduction; 2 - growth phase; 3 - phase of maturation.
Unlike the formation of spermatozoa, which occurs only after reaching puberty, the process of formation of eggs in humans begins even in the embryonic period and flows intermittently. In the embryo, the phases of reproduction and growth are fully realized, and the maturation phase begins. By the time the girl is born, hundreds of thousands of oocytes of the 1st order are in her ovaries, stopped, “frozen” at the diplotene stage of prophase 1 of meiosis - first block of oogenesis.
During puberty, meiosis will resume: approximately every month, under the influence of sex hormones, one of the oocytes (rarely two) will reach metaphase 2 of meiosis - second block of oogenesis. Meiosis can go to the end only under the condition of fertilization; if fertilization does not occur, the 2nd order oocyte dies and is excreted from the body.
The structure of the eggs
The shape of the eggs is usually round. The size of the eggs varies widely - from several tens of micrometers to several centimeters (a human egg is about 120 microns). The structural features of the egg cells include: the presence of membranes located on top of the plasma membrane and the presence in the cytoplasm of a more or less large amount of reserve nutrients.
1 — pronucleus at the stage of metaphase 2; 2 - shiny shell; 3 - radiant shell; 4 - the first polar body.
In most animals, the eggs have additional membranes located on top of the cytoplasmic membrane. Depending on the origin, they are distinguished: primary, secondary and tertiary shells. Primary shells formed from substances secreted by the oocyte. A layer is formed in contact with the cytoplasmic membrane of the egg. It performs a protective function, provides species-specific penetration of the spermatozoon, i.e. prevents sperm from other species from entering the egg. In mammals, this membrane is called the zona pellucida. Secondary shells are formed by secretions of ovarian follicular cells, are not present in all eggs. The secondary membrane of insect eggs contains a canal - micropyle, through which the sperm enters the egg. Tertiary shells are formed due to the activity of special glands of the oviducts. For example, from the secrets of special glands, protein, undershell parchment, shell and suprashell membranes are formed in birds and reptiles.
Secondary and tertiary membranes are formed in the eggs of animals, the embryos of which develop in the external environment. Since mammals have intrauterine development, their eggs have only a primary membrane, on top of which there is a radiant crown - a layer of follicular cells that deliver nutrients to the egg.
In the eggs, there is an accumulation of a supply of nutrients, which are called yolk. It contains fats, carbohydrates, RNA, minerals, proteins, and its bulk is made up of lipoproteins and glycoproteins. The yolk is present in the cytoplasm as yolk granules. The amount of nutrients accumulated in the egg cell depends on the conditions in which the embryo develops. If the development of the egg occurs outside the mother's body and leads to the formation of large animals, then the yolk can make up more than 95% of the volume of the egg. Mammalian eggs that develop inside the mother's body contain a small amount of yolk - less than 5%, since the embryos receive the nutrients necessary for development from the mother.
1 - alecithal; 2 - isolecital; 3 — moderately telolecital; 4 - sharply body-lecital.
Depending on the amount of yolk contained, the following are distinguished egg types: alecithal(do not contain yolk or have a small amount of yolk inclusions - mammals, flatworms); isolecithal(with evenly distributed yolk - lancelet, sea urchin); moderately telolecithal(with an unevenly distributed yolk - fish, amphibians); sharply telolecithal(the yolk occupies a large part, and only a small area of the cytoplasm on the animal pole is free from it - birds).
Due to the accumulation of nutrients, polarity appears in the eggs. Opposite poles are called vegetative and animal. Polarization is manifested in the fact that there is a change in the location of the nucleus in the cell (it shifts towards the animal pole), as well as in the distribution of cytoplasmic inclusions (in many eggs, the amount of yolk increases from the animal to the vegetative pole).
The human egg was discovered in 1827 by K.M. Baer.
Fertilization
The process of fusion of male and female germ cells, leading to the formation of a zygote, which gives rise to a new organism, is called fertilization. The actual process of fertilization begins with the moment of contact between the sperm and the egg. At the moment of such contact, the plasma membrane of the acrosomal outgrowth and the part of the membrane of the acrosomal vesicle adjacent to it dissolve, the hyaluronidase enzyme and other biologically active substances contained in the acrosome are released to the outside and dissolve the portion of the egg membrane. Most often, the spermatozoon is completely drawn into the egg, sometimes the flagellum remains outside and is discarded. From the moment the sperm enters the egg, the gametes cease to exist, as they form a single cell - the zygote. The sperm nucleus swells, its chromatin loosens, the nuclear membrane dissolves, and it turns into male pronucleus. This occurs simultaneously with the completion of the second division of meiosis of the egg nucleus, which was resumed due to fertilization. Gradually, the nucleus of the egg turns into female pronucleus. The pronuclei move to the center of the egg, DNA replication occurs, and after their fusion, the set of chromosomes and the DNA of the zygote becomes "2 n 4c". The union of pronuclei is actual fertilization. Thus, fertilization ends with the formation of a zygote with a diploid nucleus.
Fertilization is an irreversible process, that is, an egg once fertilized cannot be fertilized again.
Depending on the number of individuals participating in sexual reproduction, there are: cross fertilization- fertilization, in which gametes formed by different organisms take part; self-fertilization- fertilization, in which gametes formed by the same organism merge (tapeworms).
Embryonic period
Splitting up
Splitting up- This is a series of successive mitotic divisions of the zygote, as a result of which the huge volume of the cytoplasm of the egg is divided into numerous smaller cells containing nuclei. As a result of crushing, cells are formed, which are called blastomeres. Fragmentation differs from ordinary division in that the newly formed blastomeres do not increase in size. This becomes possible due to the loss of the presynthetic period of the interphase. In this case, the synthetic period of interphase begins in the telophase of the preceding mitosis. Thus, the number of blastomeres gradually increases, while their total volume practically does not change. The cytoplasm of cells during crushing is divided by the appearance of invaginations of the cell membrane ( crushing furrows).
Cleavage of an amphibian egg (frog): 1 - two-cell stage; 2 - four-cell stage; 3 - eight-cell stage;
4 - transition from the eight to sixteen cell stage (the cells of the animal pole have already divided, and the cells of the vegetative pole
just starting to crumble). 5 - later stage of crushing; 6 - blastula; 7 - blastula in section.
The biological significance of the crushing process: due to repeated cycles of reproduction, the genotype of the zygote reproduces; there is an accumulation of cell mass for further transformations, the embryo turns from a unicellular into a multicellular one.
Division of blastomeres occurs synchronous and out of sync. In most species, it is asynchronous from the very beginning of development, in others it becomes so after the first divisions.
The nature of crushing is determined, first of all, by the structure of the egg, mainly by the amount of yolk and the features of its distribution in the cytoplasm. In this regard, according to the method of crushing, two main types of eggs are distinguished: fully crushed and partially crushed. Complete crushing is called when the cytoplasm of the egg is completely divided into blastomeres. It may be uniform- all formed blastomeres have the same size and shape (typical for alecital and isolecithal eggs) and uneven- blastomeres of unequal size are formed (typical of telolecithal eggs with a moderate content of yolk). Small blastomeres arise at the animal pole, large ones - in the region of the vegetative pole of the embryo.
A - complete; B - partial; B - discoidal.
Partial crushing- a type of crushing, in which the cytoplasm of the egg is not completely divided into blastomeres. One type of partial cleavage is discoidal, in which only the yolk-free portion of the cytoplasm at the animal pole, where the nucleus is located, undergoes cleavage. The section of the cytoplasm that has undergone fragmentation is called germinal disc. This type of crushing is typical for sharply telolecithal eggs with a large amount of yolk (reptiles, birds, fish).
Cleavage in representatives of different groups of animals has its own characteristics, but it ends with the formation of a structure similar in structure - blastula.
Blastula
Blastula- single-layer embryo. It consists of a layer of cells - the blastoderm, which limits the cavity - the blastocoel. Blastula begins to form in the early stages of cleavage due to the divergence of blastomeres. The resulting cavity is filled with liquid. The structure of the blastula largely depends on the type of crushing.
coeloblastula(typical blastula) is formed with uniform crushing. It has the appearance of a single-layer vesicle with a large blastocoel (lancelet).
Amphiblastula formed during the crushing of telolecithal eggs; The blastoderm is built from blastomeres of different sizes: micromeres at the animal and macromeres at the vegetative poles. At the same time, the blastocoel shifts towards the animal pole (amphibians).
1 - coeloblastula; 2 - amphiblastula; 3 - discoblastula; 4 - blastocyst; 5 - embryoblast; 6 - trophoblast.
Discoblastula formed during discoidal crushing. The cavity of the blastula has the appearance of a narrow slit located under the embryonic disk (of a bird).
Blastocyst It is a single-layer vesicle filled with liquid, in which an embryoblast is distinguished (an embryo develops from it) and a trophoblast that provides nutrition to the embryo (mammals).
1 - ectoderm; 2 - endoderm; 3 - blastopore; 4 - gastrocoel.
After the blastula has formed, the next stage of embryogenesis begins - gastrulation(formation of germ layers). As a result of gastrulation, a two-layer, and then a three-layer embryo (in most animals) is formed - gastrula. Initially, the outer (ectoderm) and inner (endoderm) layers are formed. Later, between the ecto- and endoderm, a third germ layer, the mesoderm, is laid.
germ layers- separate layers of cells that occupy a certain position in the embryo and give rise to the corresponding organs and organ systems. The germ layers arise not only as a result of the movement of cell masses, but also as a result of the differentiation of similar, relatively homogeneous blastula cells. In the process of gastrulation, the germ layers take up a position corresponding to the structural plan of the adult organism. Differentiation- the process of the appearance and growth of morphological and functional differences between individual cells and parts of the embryo. Depending on the type of blastula and on the characteristics of cell movement, the following main methods of gastrulation are distinguished: invagination, immigration, delamination, epiboly.
1 - invagination; 2 - epibolic; 3 - immigration; 4 - delamination;
a - ectoderm; b - endoderm; c — gastrocoel.
At invaginations one of the sections of the blastoderm begins to bulge into the blastocoel (in the lancelet). In this case, the blastocoel is almost completely replaced. A two-layer sac is formed, the outer wall of which is the primary ectoderm, and the inner wall is the primary endoderm lining the cavity of the primary intestine, or gastrocoel. The opening through which the cavity communicates with the environment is called blastopore, or primary mouth. In representatives of different groups of animals, the fate of the blastopore is different. In protostomes, it turns into a mouth opening. In deuterostomes, the blastopore overgrows, and an anus often appears in its place, and the oral opening erupts at the opposite pole (anterior end of the body).
Immigration- “violation” of a part of the blastoderm cells into the cavity of the blastocoel (higher vertebrates). These cells form the endoderm.
Delamination occurs in animals that have a blastula without a blastocoel (birds). With this method of gastrulation, cellular movements are minimal or completely absent, since stratification occurs - the outer cells of the blastula are transformed into the ectoderm, and the inner cells form the endoderm.
epiboly occurs when the smaller blastomeres of the animal pole cleave faster and grow over the larger blastomeres of the vegetative pole, forming the ectoderm (amphibians). The cells of the vegetative pole give rise to the inner germ layer - the endoderm.
The described methods of gastrulation are rarely found in their pure form and their combinations are usually observed (invagination with epiboly in amphibians or delamination with immigration in echinoderms).
Most often, the cellular material of the mesoderm is part of the endoderm. It invaginates into the blastocoel in the form of pocket-like outgrowths, which are then laced off. With the formation of the mesoderm, the formation of a secondary body cavity, or coelom, occurs.
The process of organ formation in embryonic development is called organogenesis. Organogenesis can be divided into two phases: neurulation- formation of a complex of axial organs (neural tube, notochord, intestinal tube and somite mesoderm), which involves almost the entire embryo, and construction of other organs, the acquisition by various parts of the body of their typical form and features of internal organization, the establishment of certain proportions (spatially limited processes).
By the theory of germ layers by Karl Baer, the emergence of organs is due to the transformation of one or another germ layer - ecto-, meso- or endoderm. Some organs may be of mixed origin, that is, they are formed with the participation of several germ layers at once. For example, the musculature of the digestive tract is a derivative of the mesoderm, and its inner lining is a derivative of the endoderm. However, simplifying somewhat, the origin of the main organs and their systems can still be associated with certain germ layers. An embryo at the stage of neurulation is called neurula. The material used to build the nervous system in vertebrates - neuroectoderm, is part of the dorsal part of the ectoderm. It is located above the rudiment of the chord.
1 - ecto-dermis; 2 - chord; 3 - secondary cavity of the body; 4 - meso-dermis; 5 - endo-dermis; 6 - intestinal cavity; 7 - neural tube.
First, in the region of the neuroectoderm, a flattening of the cell layer occurs, which leads to the formation of the neural plate. Then the edges of the neural plate thicken and rise, forming neural folds. In the center of the plate, due to the movement of cells along the midline, a neural groove appears, dividing the embryo into future right and left halves. The neural plate begins to fold along the midline. Its edges touch and then close. As a result of these processes, a neural tube with a cavity arises - neuroceleme.
The closure of the ridges occurs first in the middle and then in the posterior part of the neural groove. Last of all, this happens in the head part, which is wider than the others. The anterior expanded section later forms the brain, the rest of the neural tube - the dorsal. As a result, the neural plate turns into a neural tube lying under the ectoderm.
During neurulation, part of the neural plate cells are not included in the neural tube. They form the ganglion plate, or neural crest, a collection of cells along the neural tube. Later, these cells migrate throughout the embryo, forming cells of nerve ganglions, adrenal medulla, pigment cells, etc.
From the ectoderm material, in addition to the neural tube, the epidermis and its derivatives (feather, hair, nails, claws, skin glands, etc.), components of the organs of vision, hearing, smell, oral cavity epithelium, and tooth enamel develop.
Mesodermal and endodermal organs are not formed after the formation of the neural tube, but simultaneously with it. Pockets or folds are formed along the side walls of the primary intestine by protrusion of the endoderm. The area of the endoderm located between these folds thickens, bends, folds and laces off from the main mass of the endoderm. So it appears chord. The resulting pocket-like protrusions of the endoderm are laced from the primary intestine and turn into a series of segmentally located closed sacs, also called coelomic sacs. Their walls are formed by the mesoderm, and the cavity inside is a secondary body cavity (or in general).
All types of connective tissue, dermis, skeleton, striated and smooth muscles, circulatory and lymphatic systems, and the reproductive system develop from the mesoderm.
From the endoderm, the epithelium of the intestine and stomach, liver cells, secreting cells of the pancreas, intestinal and gastric glands develop. The anterior part of the embryonic intestine forms the epithelium of the lungs and airways, secreting the anterior and middle lobe of the pituitary, thyroid and parathyroid glands.
1 - the beginning of the chordo-meso-dermis; 2 — a cavity of a blas-tula; 3 - induced neural tube; 4 - induced chord; 5 - primary neural tube; 6 - primary chord; 7 - formation of a secondary embryo connected to the host embryo.
- this is the interaction between the parts of the embryo, during which one part of it - the inductor, - in contact with the other part - the reacting system, determines the direction of development of the latter.
The phenomenon of induction was discovered by H. Spemann in 1901 while studying the formation of the eye lens from the ectodermal epithelium in amphibian embryos. In 1924, the results of the experiments of H. Spemann and G. Mangold were published, which are considered classical proof of the existence of embryonic induction. At the early gastrula stage, the rudiment of ectoderm, which under normal conditions should have developed into the structures of the nervous system, was transplanted from the embryo of the crested (non-pigmented) newt under the ectoderm of the ventral side, giving rise to the epidermis of the skin, the embryo of the common (pigmented) newt. As a result, the neural tube and other components of the complex of axial organs first appeared on the ventral side of the recipient embryo, and then an additional embryo formed. Moreover, observations have shown that the tissues of the additional embryo are formed almost exclusively from the cellular material of the recipient.
If at the stage of early gastrula the chord rudiment is completely removed, then the neural tube does not develop. The ectoderm on the dorsal side of the embryo, from which the neural tube normally forms, forms the skin epithelium. Upon further study of the development of the embryos, it turned out that the chordomesoderm anlage, being an inducer of the neural tube, needs an inducing influence from the anlage of the nervous system for differentiation.
Postembryonic period of development
The postembryonic period of development begins at the moment of birth or release of the organism from the egg membranes and continues until its death. Postembryonic development includes: the growth of the organism; establishing the final proportions of the body; the transition of organ systems to the mode of an adult organism (in particular, puberty). Distinguish two main types of postembryonic development: 1) direct, 2) with transformation.
At direct development an individual emerges from the mother's body or egg membranes, which differs from the adult organism only in a smaller size (birds, mammals).
Distinguish: non-larval(oviparous) type, in which the embryo develops inside the egg (fish, birds); intrauterine type in which the embryo develops inside the mother's body and is associated with it through the placenta (placental mammals).
During development with transformation (metamorphosis), a larva emerges from the egg, arranged more simply than an adult animal (sometimes very different from it); as a rule, it has special larval organs, often leads a different way of life than an adult animal (insects, some arachnids, amphibians).
For example, in tailless amphibians, a larva emerges from the egg shells - a tadpole. It has a streamlined body shape, a tail fin, gill slits and gills, lateral line organs, a two-chambered heart, and one circulation. Over time, under the influence of thyroid hormone, the tadpole undergoes metamorphosis. His tail resolves, limbs appear, the lateral line disappears, lungs and the second circle of blood circulation develop, i.e. gradually it acquires features characteristic of amphibians.
Parthenogenesis
Parthenogenesis is called the development of an organism from an unfertilized egg. It occurs in a number of plant species, invertebrates and vertebrates, except for mammals, in which parthenogenetic embryos die in the early stages of embryogenesis. Parthenogenesis can be artificial and natural.
artificial parthenogenesis caused by a person by activating the egg by exposing it to various substances, mechanical irritation, fever, etc.
At natural parthenogenesis the egg begins to break up and develop into an embryo without the participation of a spermatozoon, only under the influence of internal or external causes. Distinguish somatic and generative parthenogenesis. In generative, or haploid, parthenogenesis, the embryo begins to develop from a haploid egg (drones of bees). In somatic, or diploid parthenogenesis, the embryo begins to develop from a diploid cell: 1) either from a diploid oocyte (meiosis does not occur), 2) or from a cell formed as a result of the fusion of two haploid nuclei (meiosis occurs) (aphids, daphnia, dandelions).
If the development of the egg occurs without the participation of the sperm nucleus (some fish, roundworms), then this type of parthenogenesis is called gynogenesis. However, it is the spermatozoon that stimulates the beginning of the crushing of the egg, although it does not fertilize it.
If the development of the egg occurs only due to the genetic material of the spermatozoa and the cytoplasm of the egg, then in this case they speak of androgenesis. This type of development can be carried out if the nucleus of the egg cell dies even before fertilization, and not one, but several spermatozoa (silkworm) enter the egg cell.
Go to lectures №15"Sexual reproduction in angiosperms"
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