Question 1. What is called the individual development of an organism?
Individual development of an organism or ontogenesis refers to the entire set of transformations of an individual from origin to the end of life. The cell with which ontogenesis begins contains the program for the development of the organism. It is realized through the interaction of the nucleus (genetic information) and cytoplasm of each cell, as well as cells and tissues with each other.
In bacteria and unicellular eukaryotes, ontogenesis begins at the moment of formation of a new cell as a result of division and ends with death or a new division.
In multicellular organisms that reproduce asexually, ontogenesis begins from the moment of separation of a cell or group of cells of the mother organism.
In organisms that reproduce sexually, ontogenesis begins from the moment of fertilization and the formation of the zygote.

Question 2. List the periods of ontogenesis.
Periods of ontogenesis:
In ontogenesis there are 3 periods: proembryonic, embryonic And postembryonic. For higher animals and humans, the division into prenatal (before birth), intranatal (birth) and postnatal (after birth) periods of development is accepted.
Proembryonic period . Proembryonic period, preceding the formation of the zygote, is associated with the formation of gametes. Otherwise, this is gametogenesis (ovogenesis and spermatogenesis).
Embryonic period . Embryonic period(Greek embryon - embryo) begins with fertilization and the formation of a zygote. The end of this period for different types of ontogenesis is associated with different moments of development. The embryonic period is divided into the following stages:
1) fertilization - the formation of a zygote;
2) crushing – formation of a blastula;
3) gastrulation – formation of germ layers;
4) histo- and organogenesis - the formation of organs and tissues of the embryo. Postembryonic period of animal development.
Postembryonic period The development of animals begins after their birth and is divided into three periods:
Period of growth and morphogenesis (pre-reproductive);
Period of maturity (reproductive);
The period of old age (post-reproductive).
Postembryonic period human development.
Postembryonic postnatal) period of human development, otherwise called postnatal, is also divided into three periods:
1) Juvenile (before puberty);
2) Mature (adults, sexually mature state);
3) The period of old age ending in death.
In other words, we can say that for humans it is also possible to distinguish pre-reproductive, reproductive and post-reproductive periods of post-embryonic development. It should be borne in mind that any scheme is conditional, since the actual state of two people of the same age may differ significantly.

Question 3. Which development is called embryonic and which is postembryonic?
Ontogenesis is divided into two periods. The first of these is the embryonic period (embryogenesis) lasts from the moment of fertilization until exit from the egg or birth. Let us describe its stages using the example of the lancelet.
Crushing: the egg divides repeatedly and quickly by mitosis, interphases are very short;
blastula: a hollow ball is formed, consisting of a single layer of cells; at one of the poles of the ball, cells begin to divide more actively, preparing the next stage;
gastrula: formed as a result of invagination of the more actively dividing pole of the blastula; the early gastrula is a two-layer embryo; its outer layer (germ layer) is called ectoderm, the inner layer is endoderm; the gastrula cavity represents the future intestinal cavity of the body; late gastrula - a three-layer embryo: formed in all organisms (except coelenterates and sponges) during the formation of the third germ layer - mesoderm, which arises between the ectoderm and endoderm;
histo- and organogenesis: the development of tissues and organ systems of the embryo occurs. The second stage of ontogenesis is the postembryonic period. It lasts from the moment of exit from the egg (or birth) until death.

Question 4. What types of postembryonic development of the body are there? Give examples.
There are two types of postembryonic development.
Indirect development, or development with metamorphosis. This type of development is characterized by the fact that the born individual (larva) is often completely different from the adult organism. After some time, she undergoes metamorphosis - transformation into an adult form. Indirect development is characteristic of amphibians, insects and many other organisms.
Direct development. With this type of development, the born baby is similar to an adult. Direct development is oviparous and intrauterine. During oviparous development, the embryo spends the first stage of ontogenesis in an egg, supplied with nutrients and protected by a shell (shell) from the environment. This is how, for example, the young of birds, reptiles and egg-laying mammals develop. During intrauterine development, the growth of the embryo occurs inside the mother's body. All vital functions (nutrition, breathing, excretion, etc.) are carried out through interaction with the mother through a special organ - the placenta, formed by the tissues of the uterus and the embryonic membranes of the baby. The intrauterine type of development is characteristic of all higher mammals, including humans.

Question 5. What is the biological significance of metamorphosis?
Metamorphosis makes it possible for individuals of different ages not to compete for food. For example, tadpoles and frogs, butterflies and caterpillars have different food sources. Also, the presence of a larval stage often increases the possibility of dispersal of organisms. This is especially important if the adults are sedentary (eg, many marine molluscs, worms and arthropods).

Question 6. Tell us about the germ layers.
The first two germ layers - ectoderm and endoderm - are formed at the stage of formation of the gastrula from the blastula. Later, in all (except coelenterates and sponges) the third germ layer develops - the mesoderm, which is located between the ectoderm and endoderm. Next, all organs of the embryo develop from the three germ layers. For example, in humans, the nervous system, skin glands, tooth enamel, hair, nails, and outer epithelium are formed from the ectoderm. From the endoderm - the tissues lining the intestines and respiratory tract, lungs, liver, pancreas. From the mesoderm, muscles, cartilage and bone skeleton, organs of the excretory, endocrine, reproductive and circulatory systems are formed.

Question 7. What is cell differentiation? How is it carried out during embryonic development?
Differentiation is the process of transformation of unspecialized germ cells into various cells of the body, differing in structure and performing specific functions. Differentiation does not begin immediately, but at a certain stage of development and is carried out through the interaction of germ layers (at an early stage) and organ rudiments (at a later stage).
Some cells, even in an adult organism, remain not fully differentiated. Such cells are called stem cells. In humans they are found, for example, in the red bone marrow. Currently, the possibility of using stem cells to treat many diseases, restore organs after injuries, etc. is being actively studied.

Question 8. Describe the concept of “growth”. What is a certain height? Uncertain growth?
The growth of the body is accompanied by an increase in cells and accumulation of body weight. A distinction is made between definite and indefinite growth.
Indefinite growth is characteristic of mollusks, crustaceans, fish, amphibians, reptiles and other animals that do not stop growing throughout their lives.
A certain amount of growth is characteristic of organisms that grow only for a limited period of time, such as insects, birds and mammals. In humans, intensive growth stops at the age of 13-15 years, corresponding to the period of puberty.
The growth and development of the organism is controlled genetically and also depends on the environmental conditions in which development occurs.
With a type of growth that is called definite, the organism, having reached a certain level of maturity, ceases to increase in size. This type of growth is characteristic of most animals. If an organism grows throughout its life, then it is called an indefinite type of growth. It is characteristic of plants, fish, mollusks, and amphibians.

ONTOGENESIS

Ontogenesis is the individual development of an organism, which is based on the implementation of hereditary information at all stages of existence in certain environmental conditions; it begins with the formation of a zygote (during sexual reproduction) and ends with death.

The ontogeny of multicellular animals that reproduce sexually is divided into two periods: embryonic (fetal) and postembryonic.

EMBRYONAL PERIOD

The embryonic period begins with the formation of the zygote and ends with the release of the egg membranes or the birth of the organism.

The embryonic development of most multicellular animals includes three main stages:

1. crushing;

2. gastrulation;

3. histo- and organogenesis.

1. Crushing

Crushing stage characterized by the formation of a multicellular single-layer embryo - the blastula stage.

The type of egg crushing depends on the amount of yolk and the nature of its distribution.

There are three main types of eggs:

- isolecithal eggs - contain little yolk, and it is evenly distributed, such eggs are found in lancelets and mammals.

- telolecithal eggs are typical for amphibians, reptiles, and birds; they contain a large amount of yolk, concentrated at one of the poles - the vegetative one. The opposite pole, containing the nucleus and cytoplasm without the yolk, is called the animal pole.

- centrolecithal eggs are characterized by the fact that the yolk is in the center of the cell, and the cytoplasm is located on the periphery (insect eggs).

CRUSHING TYPE

Full Incomplete

(the whole egg is crushed) (part of the egg is crushed)

uniform uneven discoidal

(forming blasto- (forming blastomeres (only the disc is crushed

measures equal in size), not equal in size), cytoplasm with nucleus)

characteristic of zygotes with characteristic of eggs with characteristic only of eggs

yolk - lancelet yolk (frogs) yolk - reptiles,

After fertilization occurs fragmentation of a diploid zygote - mitotic divisions without cell growth. During the crushing process, the volume of the embryo does not change, and the size of the cells decreases each time. The cells formed as a result of fragmentation of the zygote are called blastomeres.

With complete fragmentation (in the lancelet) at the stage of 32 blastomeres, the embryo has the appearance of a raspberry and is called morula (embryo has no cavity). At the stage of 64 blastomeres, a cavity is formed in it, and the blastomeres are located in one layer around it. This stage is called blastula (multicellular single-layer embryo). The cavity inside is called blastocoel - primary body cavity. All cells of the embryo have a diploid (2n) set of chromosomes.

2. Gastrulation

Gastrulation is the next stage of embryonic development - the formation of a two-layer embryo. In the lancelet, a 2-layer embryo is formed by invagination (invagination) of the blastoderm into the cavity of the blastocoel. The gastrula has two layers of cells: the outer ectoderm and the inner endoderm. They are called the first and second germ layers. The cavity is called the gastrocoel or cavity of the primary gut, and the entrance to it is the primary mouth, or blastopore. In invertebrates, the blastopore turns into a final mouth (protostomes), in deuterostomes (chordates), the anus is formed from the blastopore, and the mouth is formed on the opposite side of the body.

At the stage of two germ layers, the development of coelenterates (hydra, jellyfish) ends; in all other types of animals, a third germ layer is formed between the ecto- and endoderm - mesoderm (formed from endoderm cells).

Germ layers are separate layers of cells that occupy a separate position in the embryo, from which all organ systems subsequently develop.

3. Histo and organogenesis– the process of formation of tissues and organs is the next stage of embryonic development.

Ectoderm on the dorsal side of the embryo it bends, forming a groove, the edges of which meet. The resulting neural tube sinks under the ectoderm. The brain forms at the anterior end of the neural tube. The process of formation of an embryo with a complex of axial organs (neural tube, notochord, intestinal tube) is called neurulation, and the resulting embryo is called neurula. The processes of nerve cells of the central nervous system form peripheral nerves. In addition, integuments and their derivatives develop from the ectoderm (nails, hair, sebaceous and sweat glands, tooth enamel, sensory cells (receptors) of the analyzers, the adrenal medulla.

Endoderm, located under the neural tube, separates and forms

elastic cord – chord. The rest of the endoderm forms the epithelium

intestinal tube, digestive glands (liver, pancreas), respiratory organs.

From mesoderm All types of connective tissue develop: bones, cartilage, tendons, subcutaneous tissue, etc.), muscles, circulatory, excretory and reproductive systems.

PROVISORAL (TEMPORARY BODIES)

During embryogenesis, the necessary connection of the embryo with the environment is provided by special extra-embryonic organs that function temporarily and are called provisional. The purpose of provisional organs is to ensure the vital functions of the embryo in a variety of environmental conditions.

Thus, in truly terrestrial animals (reptiles, birds, mammals), which have lost contact with the aquatic environment, the embryos develop in a special membrane filled with liquid - the amnion. Vertebrates that have an amnion are united in the group of higher vertebrates - amniotes.

Amniotes, in addition to the amnion, also have other embryonic membranes, the allantais and the yolk sac (reptiles, birds). In addition to the amnion, allantois and yolk sac, mammals also have a chorion.

1. Chorion (choroid) formed from the ectoderm of the embryo, covered with villi that grow into the mucous membrane of the uterus. Later, part of the chorion loses villi and is called smooth, and the place of greatest branching of the chorion villi, which most closely contacts the uterus, is called the baby's place, or placenta. Through the placenta, the fetus is supplied with nutrients, oxygen and is freed from waste products (CO 2, etc.), the placenta performs barrier functions, trapping many harmful substances and microorganisms, but alcohol, nicotine and some medications can pass through it.

2. Amnion - inner germinal membrane(aqueous membrane - amniotic sac). The function of its epithelium is the secretion of amniotic fluid, which determines the most important conditions for the development of the fetus, as well as the excretion of its metabolic products into the amniotic fluid, prevents the embryo from losing water, serves as a protective cushion and creates the opportunity for the embryo to have some mobility.

3. Yolk sac in mammals it is reduced, filled with liquid containing proteins and salts. In the early stages of development, it plays the role of a hematopoietic organ; the first blood cells and vessels of the embryo are formed from special blood islands; the germ cells of the embryo are also formed here; the yolk sac is part of the placenta. Later, the umbilical cord is formed from the yolk sac.

4. Allantois (urinary membrane) grows from the hindgut of the embryo until it comes into contact with the chorion, forming the chorioallantois structure, rich in blood vessels. The allantois, together with the yolk sac, participates in the formation of the umbilical cord.

POSTEMBRYONAL DEVELOPMENT

The postembryonic period of ontogenesis begins with the moment of birth or exit from the egg membranes and ends with the death of the organism. This period is characterized by growth and puberty. There are direct and indirect (with metamorphoses) postembryonic development.

Postembryonic development

Direct – Indirect –

Characterized by growth, development with transformations (with metamorphosis)

and puberty

(reptiles, birds, with complete with incomplete

mammals) transformation: transformation:

Egg - egg

Larva (caterpillar) - larva

Pupa (tadpole)

Imago - adult individual

With direct development An organism similar to an adult individual is born, but differs from it only in size, underdevelopment of the genital organs, and also in body proportions. Postembryonic development, in this case, comes down to growth and puberty. Characteristic of reptiles, birds and mammals.

With indirect development(development with metamorphosis) - transformations, a larva emerges from the egg shells, differing from the adult organism (morphologically and physiologically). It has specialized larval organs, but lacks some adult organs. The larva feeds, grows, the larval organs are destroyed, and the organs of the adult animal are formed. Biological significance indirect development is that the organism at the larval stage grows and develops not due to the reserve nutrients of the egg, but due to independent nutrition. Consequently, this type of development is typical for organisms whose eggs contain a small amount of yolk (amphibians, many arthropods, etc.)

Thus, with indirect development, competition for food and habitat between adults and their offspring decreases. For example, a frog larva - a tadpole - feeds on plants, and an adult frog - insects. Also, in a number of species, for example corals, adult individuals lead an attached lifestyle; they cannot move. But their larvae are mobile, which contributes to the spread of the species.

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 (with sexual reproduction) or individual groups of cells (with asexual reproduction) until the end of life.

From the Greek “ontos” - existing and genesis - emergence. Ontogenesis is a chain of strictly defined complex processes at all levels of the body, as a result of which the structural features, life processes, and 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 its parents, the new individual receives a kind of instructions about when and what changes should occur in the body so that it can successfully go through its entire life course. Thus, ontogeny represents the implementation of hereditary information.

1. Historical information

The process of the appearance and development of living organisms has interested 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 the mixing of fluids belonging to both parents. This opinion lasted for 200 years. In the 17th century, the English physician and biologist W. Harvey carried out some experiments to test Aristotle's theory. As court physician to Charles I, Harvey received permission to use deer living on royal lands for experiments. Harvey studied 12 female deer that died at different times after mating.

The first embryo, removed from a female deer a few weeks after mating, was very small and did not look at all like an adult animal. In deer that died at a later date, the embryos were larger, they were very similar to small, newly born fawns. This is how knowledge in embryology accumulated.

The following scientists made significant contributions to embryology.

· Anthony van Leeuwenhoek (1632-1723) discovered sperm in 1677 and was the first to study parthenogenesis in aphids.

· Jan Swammerdam (1637-1680) pioneered the study of insect metamorphosis.

· Marcello Malpighi (1628-1694) made the first studies on the microscopic anatomy of the development of organs in the chicken embryo.

· Kaspar Wolf (1734-1794) is considered the founder of modern embryology; more precisely and in more detail than all his predecessors, he studied 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 Estonian province. He was the first to prove that during the development of all vertebrate animals, the embryo is first formed from two primary cell layers, or layers. Baer saw, described, and then demonstrated at a congress of naturalists a mammalian egg cell from a dog he had opened. He discovered a method for the development of the axial skeleton in vertebrates (from the so-called dorsal chordae). Baer was the first to establish that the development of any animal is a process of unfolding something preceding, or, as they would now say, the gradual differentiation of increasingly complex formations from simpler rudiments (the law of differentiation). Finally, Baer was the first to appreciate the importance of embryology as a science and based it on 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 throughout the animal kingdom.

· I.I. Mechnikov (1845-1916) gained particular fame for his studies of sponges and jellyfish, i.e. lower multicellular organisms. Mechnikov's prominent idea was his theory of the origin of multicellular organisms.

· A.N. Severtsov (1866-1936) is the largest of modern embryologists and comparative anatomists, the creator of the theory of phylembryogenesis.

2. Individual development of unicellular organisms

ontogenesis embryology single-celled organism

In the simplest organisms, whose body consists of one cell, ontogenesis coincides with the cell cycle, i.e. from the moment of appearance, through the division of the mother cell, until the next division or death.

The ontogeny of unicellular organisms consists of two periods:

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 alternation of sexual and asexual generations.

In multicellular animals, ontogeny is also a very complex process and much more interesting than in plants.

In animals, there are three types of ontogenesis: larval, oviparous 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 independently. 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 reason for the existence of larvae may lie in the fact that they feed on different foods than adults, and thus the food base of the species expands. Compare, for example, the nutrition of caterpillars (leaves) and butterflies (nectar), or tadpoles (zooplankton) and frogs (insects). In addition, during the larval stage, many species actively colonize new territories. For example, the larvae of bivalve mollusks are capable of swimming, while adults are practically motionless. 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; there is no larval stage. The intrauterine type of ontogenesis is observed in most mammals, including humans. In this case, the developing embryo is retained 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: breathing, nutrition, excretion, etc. Intrauterine development ends with the process of childbirth.

I. Embryonic period

The individual development of multicellular organisms can be divided into two stages:

· embryonic period.

· postembryonic period.

The embryonic or embryonic period of the individual development of a multicellular organism covers the processes occurring in the zygote from the moment of the first division until exit from the egg or birth.

The science that studies the laws of individual development of organisms at the embryonic stage is called embryology (from the Greek embryo - embryo).

Embryonic development can occur in two ways: in utero and ending with birth (in most mammals), as well as outside the mother’s body and ending with the release of the egg membranes (in birds, fish, reptiles, amphibians, echinoderms, mollusks and some mammals)

Multicellular animals have varying levels of organizational complexity; can develop in the womb and outside the mother’s body, but for the vast majority, the embryonic period proceeds in a similar way and consists of three periods: cleavage, gastrulation and organogenesis.

) Crushing.

The initial stage of development of a fertilized egg is called cleavage . A few minutes or a few hours (different species vary) after the sperm is implanted into the egg, the resulting zygote begins to divide by mitosis into cells called blastomeres. This process is called cleavage, since during 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. Blastomers formed during cleavage 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. A distinction is made between complete and incomplete crushing. In yolk-poor eggs, uniform crushing is observed. Lancelet and mammal zygotes undergo complete crushing, since they contain little yolk and it is distributed relatively evenly.

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 fragmentation. 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. crushing. Part of the yolk remains outside the blastomeres, which are located on the yolk in the form of a disk.

Let us consider in more detail the fragmentation of the lancelet zygote. Cleavage covers the entire zygote. The furrows of the first and second cleavage 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 a morula. With further fragmentation (at approximately the stage of 128 blastomeres), the embryo expands and the cells, arranged in a single layer, form a hollow ball. This stage is called blastula. The wall of a single-layer embryo is called blastoderm, and the cavity inside is called blastocoel (primary body cavity).

Rice. 1. Initial stages of lancelet development: a - crushing (stage of two, four, eight, sixteen blastomeres); b - blastula; in - gastra. cation; 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 - chord formation; 10 - formation of mesoderm

) Gastrulation

The next stage of embryonic development is the formation of a two-layer embryo - gastrulation. After the lancelet blastula has fully formed, further cell fragmentation occurs especially intensively at one of the poles. As a result, they seem to be drawn in (bulge) inward. As a result, a two-layer embryo is formed. At this stage, the embryo is cup-shaped and is called a gastrula. The outer layer of gastrula cells is called the ectoderm or outer germ layer, and the inner layer lining the gastrula cavity - the gastric cavity (the cavity of the primary intestine) is called the endoderm or inner germ layer. The gastrula cavity, or primary intestine, turns into the digestive tract in most animals at further stages of development and opens outwards into the primary mouth, or blastopore. In worms, mollusks and arthropods, the blastonore develops into the mouth of an adult organism. That's why they are called protostomes. In echinoderms and chordates, the mouth breaks through on the opposite side, and the blastonore turns into an anus. They are called deuterostomes.

At the stage of two germ layers, the development of sponges and coelenterates ends. In all other animals, a third is formed - the middle germ layer, located between the ectoderm and endoderm. It's called mesoderm.

After gastrulation, the next stage in the development of the embryo begins - differentiation of the germ layers and the laying of organs (organogenesis). First, the formation of axial organs occurs - the nervous system, notochord and digestive tube. The stage at which the formation of axial organs occurs is called neirula.

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. The brain then forms at the anterior end of the tube. Simultaneously with the formation of the neural tube, the formation of the notochord occurs. The notochordal material of the endoderm is bent, so that the notochord is separated from the common plate and turns into a separate cord in the form of a solid 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 remains are preserved between the vertebrae even in adulthood.

Simultaneously with the formation of the notochord, the separation of the third germ layer, the mesoderm, occurs. There are several ways to form mesoderm. In the lancelet, for example, the mesoderm, like all major organs, is formed as a result of increased cell division on both sides of the primary gut. As a result, two endodermal pockets are formed. These pockets enlarge, 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.

) 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 covering 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, lining of the rectum, tooth enamel, sensory cells of the organs of hearing, smell, vision and etc.

From the endoderm develop epithelial tissues lining the esophagus, stomach, intestines, respiratory tract, lungs or gills, liver, pancreas, epithelium of the gall and bladder, urethra, thyroid and parathyroid glands.

Derivatives of mesoderm are the connective tissue base of the skin (dermis), all connective tissue itself, skeletal bones, cartilage, circulatory and lymphatic systems, dental dentin, mesentery, kidneys, gonads, and muscles.

The animal embryo develops as a single organism in which all cells, tissues and organs are in close interaction. In this case, one rudiment influences the other, largely 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 of the embryo with the environment is ensured 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, already starting from the blastula stage, the appearance of special embryonic organs is observed - additional embryonic membranes (chorion, amnion and allantois), providing protection of the developing embryo from drying out and various environmental influences.

The outer part of the spherical formation developing around the blastula is called the chorion. This shell is covered with villi. In placental mammals, the chorion, together with the mucous membrane of the uterus, forms the baby's place, or placenta, which provides a connection between the fetus and the maternal 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 the baby's place (5); 6 - umbilical or yolk vesicle; 7 - allantois; 8 - umbilical cord

The second embryonic membrane is the amnion (lat. amnion - peri-embryonic vesicle). This is the name given in ancient times to the cup 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, also containing hormones. The amount of this fluid in a six-month human embryo reaches 2 liters, and by the time of birth - 1 liter. The wall of the amniotic membrane is a derivative of ecto- and mesoderm.

Allantois (lat. alios - sausage, oidos - species) is 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 hindgut, it exits through the umbilical opening and grows very quickly to cover the amnion and yolk sac. Its functions vary in different vertebrates. In reptiles and birds, 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, differentiation and complication of organs continues in the postembryonic period.

A developing embryo (especially a human embryo) has periods called critical periods, when it is most sensitive to the damaging effects of environmental factors. This is the implantation period on days 6-7 after fertilization, the placentation period - the end of the second week and the period of childbirth. During these periods, restructuring occurs in all body systems.

The development of an organism from the moment of its birth or emergence from the egg shells until death is called the postembryonic period. In different organisms it has different durations: from several hours (in bacteria) to 5000 years (in sequoia).

There are two main types of postembryonic development:

· indirect.

Direct development, in which an individual emerges from the mother’s body or egg shells, differing from the adult organism only in 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 to 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 varies among different multicellular organisms. For example: turtle - 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, sea anemone - 50-70 years, eagle owl - 68 years, rhinoceros - 45 years, lobster - 50 years, horse - 40 years, seagull - 30-45 years, monkey - 35-40 years, lion - 35 years old, already - 30 years old, cow - 20-30 years old, cat - 27 years old, frog - 12-20 years old, swallow - 9 years old, mouse - 3-4 years old.

2. Embryonic development of the embryo in animals:

a) crushing; types of crushing;

b) gastrulation; methods of gastrulation;

c) primary organogenesis (laying down the axial complex of organs);

d) embryonic induction.

3. Postembryonic development:

a) types of postembryonic development;

b) direct development – ​​non-larval and intrauterine;

c) indirect development - with complete and incomplete metamorphosis.

4. The influence of environmental factors on the individual development of the organism.

1. Ontogenesis. Types of ontogeny. Periodization of ontogeny.

Ontogenesis – the process of individual development of an individual, i.e. the entire set of transformations from the moment of formation of the zygote until the death of the organism.

In species that reproduce asexually, ontogenesis begins with the separation of one or a group of cells of the maternal organism. In species with sexual reproduction, it begins with fertilization of the egg. In prokaryotes and single-celled eukaryotic organisms, ontogeny is essentially a cell cycle, usually ending with cell division or cell death.

During individual development, multicellular organisms undergo a number of regular processes:

The formation of morphofunctional traits inherent in a particular biological species;

Implementation of specific functions;

Reaching puberty;

Reproduction;

Aging;

All these processes, as components of ontogenesis, occur on the basis of hereditary information received by descendants from their parents. This information is a kind of instruction about the time, place and nature of the individual’s private development mechanisms. Therefore, ontogeny can be defined as the process of implementing genetic information received from parents under certain environmental conditions.

The following types of ontogenesis are distinguished: direct and indirect. Indirect development occurs in larval form, and direct development– in non-larval and intrauterine (Fig...)

TYPES OF ONTOGENESIS

Direct development Indirect development

(with metamorphosis)

Non-larval type with incomplete metamorphosis:

(laying eggs with a large amount of yolk) egg - larva - adult - individual

Intrauterine with complete metamorphosis

Egg – larva – pupa – adult – individual

Ontogenesis is a continuous process of development of an individual. However, its stages differ in the content and mechanisms of the processes occurring. For this reason, the ontogenesis of multicellular organisms is divided into periods: embryonic– from the moment of fertilization of the egg until the release of the egg membranes or birth and postembryonic– from exit from the egg shells or birth to death. For placental animals and humans, a division into prenatal (before birth) and postnatal (after birth) periods is accepted. Often, a proembryonic, or prezygotic, period is also distinguished, which includes the processes of formation of germ cells (spermato- and oogenesis).

1. Embryonic development in animals.

Embryonic (embryogenesis) development begins from the moment of formation of the zygote and is the process of transforming the latter into a multicellular organism.

Embryonic development consists of the following main stages:

crushing, as a result of which a multicellular embryo is formed;

gastrulation, during which the first tissues arise - ectoderm, endoderm And mesoderm, and the embryo becomes two- or three-layered;

primary organogenesis - the formation of a complex of axial organs of the embryo (neural tube, notochord, intestinal tube);

exit from egg or embryonic membranes (with larval and non-larval types of development) or by birth (with intrauterine development).

Crushing - a process of multiple rapidly successive mitotic divisions of the zygote, leading to the formation of a multicellular embryo. Cleavage divisions, unlike ordinary cell divisions, occur without a postmitotic period. The resulting cells ( blastomeres) do not grow. During the cleavage process, the total volume of the embryo does not change, but

the size of its constituent cells decreases, i.e. the embryo is fragmented.

The type of fragmentation of a fertilized egg depends on the amount of yolk and the nature of its distribution in the cytoplasm of the egg, i.e., on the type of egg. In this regard, crushing is distinguished complete when the entire egg is crushed, and incomplete, when part of it is crushed. This, in turn, is due to the fact that the yolk prevents the formation of a constriction during division of the cell body.

Complete crushing happens uniform, if the cells formed as a result of division are approximately equal in size, and uneven if they differ in size.

Incomplete crushing may be partial superficial, or discoidal.

Crushing happens synchronous(simultaneous division of all cells) and asynchronous(non-simultaneous cell division).

Isolecithal Moderate lecithal Telolecithal Alecithal

Complete, Complete, Incomplete, Complete,

Uniform Uneven Discoidal Uniform

(lancelet) (frog) (birds) asynchronous

(Human)

Complete uniform crushing .

There is little yolk in the lancelet egg and it is evenly distributed in the cytoplasm, so the fragmentation of the fertilized egg is complete and uniform.

I – zygote; II – stages 2, 4 and 32 blastomeres; III - blastula; IV – gastrula; V – anlage of the axial complex of organs (1 – neural tube; 2 – notochord; 3 – ectoderm; 4 – intestinal tube).

The 1st groove runs in the meridional plane in the direction from the animal pole to the vegetative pole; The zygote divides into two equal cells - blastomeres.

The 2nd groove runs perpendicular to the first, also in the meridional plane; 4 blastomeres are formed.

The 3rd furrow is latitudinal - it runs slightly above the equator and immediately divides 4 blastomeres into 8 cells.

Further, the meridional and latitudinal furrows alternate correctly. As the number of cells increases, division becomes asynchronous. At the stage of 32 blastomeres, the embryo has the appearance of a raspberry and is called Morula. The blastomeres diverge further and further, forming a cavity at the stage of 64 blastomeres - blastocoel and the embryo takes the form of a vesicle with a wall formed by one layer of cells closely adjacent to each other, inside which there is primary body cavity, i.e. it is formed blastula (coeloblastula).

Complete uneven crushing.

Characteristic of moderately telolecithal eggs. There is more yolk in the egg of a frog than in a lancelet, and it is concentrated mainly at the vegetative pole.

The first two meridional grooves divide the egg into 4 equal blastomeres.

3rd – latitudinal groove is strongly shifted towards the animal pole, where there is less yolk. As a result, 4 micro- and 4 macroblastomeres are formed, sharply differing in size.

As a result of ongoing fragmentation, the cells of the animal pole, less overloaded with yolk, divide more often and are smaller in size than the cells of the vegetative pole. The blastula has a wall formed by several rows of cells; The blastocoel is small and displaced towards the animal pole ( amphiblastula).

Incomplete discoidal crushing.

Characteristic of telolecithal eggs of reptiles and birds, heavily overloaded with yolk. The yolk-free cytoplasm makes up about 1% of the volume. The yolk prevents fragmentation and therefore only a narrow strip of cytoplasm at the animal pole is fragmented. As a result, germinal disc (discoblastula).

Regardless of the characteristics of the fragmentation of fertilized eggs in different animals, due to differences in the quantity and nature of the distribution of the yolk in the cytoplasm, fragmentation, as a period of embryonic development, is characterized by the following features:

As a result of fragmentation, a multicellular embryo is formed (blastulation) - a blastula and cellular material accumulates for further development.

All cells in the blastula have a diploid set of chromosomes (2n), are identical in structure and differ from each other mainly in the amount of yolk, i.e. the blastula cells are not differentiated.

A characteristic feature of cleavage is a very short mitotic cycle compared to its duration in adult animals.

During the period of fragmentation, DNA and proteins are intensively synthesized, but RNA synthesis is absent. The genetic information contained in the blastomere nuclei is not used.

During cleavage, the cytoplasm does not move.

Gastrulation - this is the process of formation of a two- or three-layer embryo - gastrula, the basis of which is complex and diverse movements of cell masses and cell differentiation. The resulting layers are called germ layers. They are layers of cells that have a similar structure, occupy a certain position in the embryo and give rise to certain organs and organ systems.

There are external - ectoderm- and internal – endoderm- sheets between which in three-layered animals there is mesoderm.

During gastrulation, cell division is either weakly expressed or absent - the embryo does not grow.

1 – intussusception; 2 – epiboly; 3 – immigration; 4 – delamination.

Depending on the type of blastula, there are four main methods of gastrulation:

- intussusception– formation of a two-layer embryo by invagination of the wall of the blastula into the cavity of the blastocoel (lancelet);

- epiboly– the formation of a two-layer embryo as a result of the creep of small cells of the animal pole onto the vegetative pole, the cells of the animal pole grow over it and it ends up inside the embryo (amphibian);

- immigration– penetration by immersion of part of the blastula cells into the blastocoel (coelenterates);

- delamination– as a result of cell division, the germinal disc is split into two layers (reptiles and birds).

However, the listed methods of gastrulation are almost never found in nature in their pure form, which gives reason to single out a fifth method - mixed, or combined.

Gastrula is a two-layer sac, the cavity of which (gastrocoel) communicates with the external environment through an opening - blastopore(primary mouth). The outer layer of the gastrula is ectoderm, the inner layer is endoderm. The structure of the gastrula depends on the type of egg and the lifestyle of the embryo at this stage. Thus, in coelenterates, the gastrula is a free-living larva - a planula; in other species, the gastrula develops in the egg membranes or in the mother’s body.

In some animals (sponges, coelenterates), the gastrulation process ends with the formation of two germ layers - ecto- and endoderm. The remaining representatives of the animal world are characterized by the formation of the third germ layer - mesoderm. The laying and formation of mesoderm is carried out in two ways: teloblastic And enterocoelous. With the teloblastic method of anlage, 2 large cells are formed in the region of the blastopore lips ( teloblasts); By multiplying, they give rise to two mesodermal stripes, from which (with the appearance inside the cavity) coelomic vesicles are formed. With the enterocoel method of anlage, the primary intestine forms symmetrical protrusions into the blastocoel, which then detach and turn into coelomic vesicles. In both cases, the anlage vesicles grow and fill the primary body cavity. The layer of mesoderm adjacent to the ectoderm is called wall, or parietal layer, and adjacent to the endoderm – visceral, or visceral layer. The cavity formed in the mesodermic vesicles and replacing the primary one is called secondary body cavity, or whole. With the teloblastic method of laying down the mesoderm, the blastopore turns into the mouth of an adult animal ( protostomes). With the enterocoel method, the blastopore closes, and the mouth of the adult is formed a second time ( deuterostomes).

The formation of germ layers is the result of differentiation of relatively homogeneous blastula cells that are similar to each other.

Differentiation is the process of the appearance and increase of morphological and functional differences between individual cells and parts of the embryo.

Morphological differentiation manifests itself in the formation of several hundred types of cells of a specific structure.

Biochemical differentiation– specialization of cells in the synthesis of specific proteins characteristic only of a given cell type. Keratin is synthesized in the epidermis, insulin is synthesized in the islet tissue of the pancreas, etc. The biochemical specialization of cells is ensured by the differential activity of genes, i.e., different genes begin to function in different primordia. Genetic information is realized through the synthesis of mRNA at the gastrula stage, which increases sharply during the formation of the axial organ complex.

With further differentiation of the cells of the germ layers in the process of histo- and organogenesis, the same tissues and organs are formed in different animal species, which means the homology of the germ layers. The homology of the germ layers of the vast majority of animals is one of the proofs of the unity of the animal world.

Histo- and organogenesis.

After gastrulation is completed, the embryo develops a complex of axial organs: the neural tube, notochord and intestinal tube. Let's consider this process using the example of the lancelet

The ectoderm, located on the dorsal side of the embryo, bends along the midline, forming a longitudinal groove. The areas of ectoderm located to the right and left of the groove begin to grow on its edges. The groove - the rudiment of the nervous system - sinks under the ectoderm and its edges close (the process is called neurulation, and the stage of development is neurula). A neural tube is formed. The rest of the ectoderm represents the rudiments of the skin epithelium, sensory organs..

The dorsal part of the endoderm, located under the neural tube, gradually separates (separates) from the rest of the endoderm and folds into a dense elastic cord - chord. From the remaining part of the endoderm, the intestinal epithelium, digestive glands, and respiratory organs develop.

Further differentiation of the embryonic cells leads to the emergence of numerous derivatives of the germ layers - organs and tissues.

Embryonic induction.

The process of cell differentiation is largely determined by the influence of parts of the developing embryo on each other. Observations of the development of a fertilized frog egg allow us to trace the path of development of cells in different parts of the embryo. It turns out that strictly defined cells, occupying a strictly defined place in the blastula, give rise to strictly defined organ rudiments. With the onset of gastrulation, cell movement begins. If at this moment (at the early gastrula stage) a part of the cells on the dorsal side - the rudiment of the axial complex - is cut out and transplanted under the skin ectoderm of another embryo on the ventral side, then it is possible to obtain the development of an additional complex of axial organs in the second embryo. In this case, the embryo, deprived of its organizing cells, dies. Consequently, during the development process, one rudiment influences another, determining the path of its development. This phenomenon is called embryonic induction, and the parts of the embryo that direct the development of associated structures are called inductors(or organizational centers). The phenomenon of induction is also observed with the emergence of other organs: contact of the protrusion of the neural tube - the optic vesicle - with the ectoderm leads to the development of the lens of the eye; the lens, in turn, induces the transformation of the ectoderm into the cornea.

The development of the embryo is greatly influenced by unfavorable environmental factors in which the future organism is formed (temperature, light, humidity, alcohol, nicotine, pesticides, a number of medications, drugs, etc.). They can disrupt the normal course of embryogenesis and lead to the formation of various deformities or complete cessation of development.

Germ layer derivatives

ECTODERM	ENDODERM	MESODERM
The neural plate, which gives rise to the central and peripheral nervous systems; Ganglion plate, from which the ganglia of the autonomic nervous system, cells of the adrenal medulla, and pigment cells are formed; Components of the organs of vision, hearing, smell; Epidermis of the skin, hair, nails, sweat, sebaceous and mammary glands; Tooth enamel; Epithelium of the oral cavity and rectum.	Epithelium of the intestinal tube (midgut); Liver, pancreas; Lungs; Epithelium of the gills.	All types of connective tissue (bones, cartilage, tendons, dermis); Skeletal muscles; Circulatory system; Excretory system; Reproductive system.

Lecture No. 3 Individual development of organisms. Ontogenesis

1 General concept of ontogenesis. Embryonic development

The first ideas about growth and development date back to the ancient world. Hippocrates assumed that the eggs already contained a fully formed organism, but in a very reduced form. This idea was developed into a doctrine called preformationism(Latin preformatio - transformation). This teaching was popular in the 11th-11th centuries. His supporters were W. Harvey, M. Malpighi and other prominent scientists. Preformationism is a metaphysical doctrine from beginning to end; it recognized only growth, not development. However, this doctrine was refuted by M. Bonnet (1720-1793), who discovered parthenogenesis using the example of the development of aphids from unfertilized eggs.

In the ancient world there was another doctrine called epigenesis (Greek epi - after, genesis - development). A prominent supporter of this doctrine was K.F. Wolf, who believed that the egg contains neither a preformed organism nor its parts, and the egg consists of an initially homogeneous mass. However, later new facts appeared that made it possible to reconsider the provisions of epigenesis. In particular, in the 18th century, K. Blair published his work “The Theory of Animal Development,” in which he proved that the contents of an egg are heterogeneous. Moreover, the degree of heterogeneity increases with the development of the embryo. Within the framework of modern concepts, the development of an organism is understood as a process in which structures formed earlier stimulate the formation of subsequent structures. The development process is determined genetically and is closely related to the environment.

In general terms, growth can be represented as a gradual increase in the number of cells, mass and size of the organism, resulting in changes in shape, the formation of new structures, differentiation of cells, tissues and organs, and biochemical changes in cells and tissues. There is a unity between growth and development. Qualitative changes occurring in cells give rise to tissues and organs.

Cell differentiation is the process by which cells become morphologically, biochemically, and functionally different from others. Growth and differentiation determine the development of the organism.

Ontogenesis and its types. Ontogenesis (Greek ontos - being, genesis - development) is the history (cycle) of the development of an individual, beginning with the formation of the germ cells that gave rise to it and ending with death.

Depending on the nature of the individual development of organisms, indirect and direct ontogenesis are distinguished. The former is observed in the form of larval development, while the latter occurs in nature in the form of intrauterine development.

Larval development – organisms with such development pass through one or several larval stages, which is typical for insects, amphibians and echinoderms. Their larvae lead an independent lifestyle, then undergoing transformations.

Direct (non-larval and intrauterine) development. Non-larval development is typical for invertebrates, fish, reptiles and birds, whose eggs are rich in yolk (nutrient material). Due to this, a significant part of ontogeny occurs in eggs laid in the external environment. The metabolism of the embryo is ensured by the developing so-called provisional organs, which are embryonic membranes (yolk sac, amnion, allantoin).

Intrauterine development typical for mammals, including humans. Since the eggs of these organisms are very poor in nutrients, all vital functions of the embryos are provided by the maternal body through the formation of provisional organs from the tissues of the mother and the embryo, the main one of which is the placenta. Evolutionarily, intrauterine development is the latest form, but this form is the most beneficial for the embryos, since it most effectively ensures their survival.

Embryonic development

It occurs as a continuous process of change in the embryo, but for ease of study it is divided into periods:

Unicellular;

Cleavage and formation of gastrula;

The period of formation of tissues and organs, ending with the formation of an organism ready for birth.

Fertilization . Its result is the emergence of a one-cell embryo. The most important during fertilization are 2 points. The first is the fusion of the haploid nuclei of male and female gametes to form a diploid zygote. The second is activating the egg and encouraging it to develop.

Crushing . At its core, it is a series of continuously following one after another mitotic divisions of the egg.

P.I. Balinsky characterizes this process this way.

1 One cell - a fertilized egg - through a series of mitotic divisions turns into a multicellular complex.

2 There is no growth.

3 The general shape of the embryo does not change during crushing, but an internal (primary) body cavity is formed.

4 During the process of fragmentation, the transformation of substances stored in the cytoplasm into nuclear substances takes place.

5 The relative position of the parts of the cytoplasm of the egg during the crushing process for the most part does not change, and they remain in their places.

6 The nuclear-plasma ratio, low at the beginning of fragmentation, reaches at the end a level characteristic of ordinary somatic cells.

Depending on the type of eggs, crushing occurs differently.

In lower chordates, small primitive animals, the eggs are small, with a small amount of yolk. Their crushing is complete and uniform. In lower vertebrates, which are more complexly structured (fish, amphibians), the eggs are larger and contain an unevenly distributed supply of yolk, so fragmentation, although complete, is not uniform. In higher vertebrates (reptiles, birds), the eggs are large and rich in yolk. Crushing in such animals is incomplete and superficial. It occurs only in the germinal disc, i.e. area free of yolk.

Mammals and humans occupy a special place among higher vertebrates. The development of the embryo occurs inside the mother's body, which provides nutrition to the embryo. In this regard, the eggs are small, the yolk is a small amount. They are characterized by complete uniform crushing.

Gastrulation. With the completion of the fragmentation process and the formation of the embryo (blastula), the period of gastrulation begins. A characteristic feature of this period is the massive movement of cells, ordered in direction and sequence. As a result, the embryo changes from a single layer to a multilayer one. Germ layers are formed. These are layers of cells that occupy a certain place relative to each other. From these, certain organs and parts of the body naturally develop. The outer germ layer is called ectoderm, the inner layer is called endoderm, and the middle layer is called mesoderm. The embryo during gastrulation is called a gastrula.

Organogenesis . This is the process of organ formation. Tissues also develop in parallel. This process is called histogenesis. Complex spatial transformations of cells and parts of the embryo are called morphogenesis.

Each organ rudiment, and then the organ, develops from cells of certain germ layers.

The entire nervous system, certain parts of the sensory organs, for example, the cornea of ​​the eye, the skin epithelium and its derivatives (mammary, sweat and sebaceous glands, hair, nails, tooth enamel), the epithelium of the oral cavity and rectum are formed from the ectoderm.

From the mesoderm, the skeleton of the spine, skull and limbs, transversely striated skeletal muscles, smooth muscles of internal organs, the circulatory system and blood, the excretory system, the gonads, with the exception of the germ cells themselves, and all connective tissue develop.

The endoderm gives rise to the epithelium of the digestive and respiratory systems, liver, pancreas, thyroid and parathyroid glands.

Germ membranes. All higher vertebrates, starting with reptiles, form special temporary organs designed to provide the most important functions of the embryo. There are 4 such germ membranes. They are formed after gastrulation from the germ layers.

Amnion, or aqueous membrane, immediately surrounds the embryo. It contains liquid inside, which protects the embryo from drying out and mechanical damage. The amnion of a human embryo is called the amniotic sac and also contains fluid that is released when it ruptures during childbirth.

Chorion . It promotes respiration and nutrition of the embryo. In birds, the chorion is closest to the shell, and in mammals it is part of the placenta and is adjacent to the wall of the mother’s uterus.

Allantois , urinary sac, used in birds to excrete liquid nitrogen-containing metabolic products and for respiration; (in mammals, along with the chorion, it is part of the placenta).

Yolk sac. In birds it serves for nutrition, respiration and hematopoiesis. In mammals, it does not have a nutritional function due to the transition to the transplacental method of supplying nutrients, but serves as a source of germ cells and blood cells.

The embryonic membranes cease to exist after birth and come out along with the uterine mucosa.

Twins. Humans and some mammals, such as horses and cows, usually give birth to one young. If there are more of them, then they talk about twins.

There are 2 types of twins: identical (OB) and fraternal (RB). The main mechanism for the occurrence of OB is the division of the embryo into 2 or more parts during the period of fragmentation of embryogenesis. The reasons for this are not yet clear. The birth rate of RB is associated with the individual characteristics of the parents. Families are described in which twins were born in several generations. In the early stages, embryos are capable of correcting mistakes, as it were, and from each separated part a completely normal organism develops. The ability of the early stages of the embryo to develop into a normal whole after a change in their cellular composition is called embryonic regulation and is fully manifested in the development of OB. RBs develop from two different eggs fertilized by different sperm.

Sometimes there is incomplete separation of the cell mass of blastomeres in the early stages of embryogenesis. In such cases, undivided twins appear due to impaired development of the embryo in the 1st week after fertilization. They are always identical. Unseparated twins can be symmetrical and connected by different parts of the body, for example, in the chest, abdomen, and sacrum. Most often they are not viable and die during intrauterine development. However, exceptions are possible. Twins born in Siam (Thailand) who lived for 63 years gave the common name to such twins (Conjoined twins).

Congenital defects and critical periods of human development .

In addition to twin deformities, there are newborns with various other abnormalities. A congenital malformation is a permanent change in the structure of an organ, leading to a disorder of its function. The number of forms of vices is in the thousands. There are malformations of the face and neck, nervous system, eyes, musculoskeletal system, cardiovascular, respiratory, digestive, excretory and reproductive systems.

Embryogenesis disorders occur at the cellular, tissue, organ and organism levels. It follows that the main mechanisms of congenital defects are violations of the most important processes, such as reproduction, differentiation, movement and death of cells, growth and interaction of developing tissues and organs. In addition to hereditary defects, there are also defects that arise as a result of the action of exogenous factors. These factors include:

Physical (penetrating radiation);

Chemical (medicines, toxic substances, hypoxia of the fetus, endocrine diseases of the mother);

Biological agents (viruses).

Critical periods are those during which the sensitivity of the embryo to the action of external factors is especially great. In humans, the 1st period occurs at the end of the 1st beginning of the 2nd week of pregnancy; 2nd from 3 to 6 weeks, although there are no stages in which the embryo would be resistant to all damaging influences.

2 Postembryonic development

Postembryonic development begins from the moment the developing organism leaves the membranes of the egg or the mother’s body. Its main feature is that it comes into direct contact with the external environment. He begins to breathe, eat, and respond to various influences. After birth, the development of the body continues in the form of changes in growth, further specialization of cells and tissues, regeneration and aging.

Indirect postembryonic development

Many lower multicellular organisms (sponges, coelenterates, flatworms and annelids), lower chordates (ascidians, lancelets), and lower vertebrates (fish, amphibians) emerge from the egg shells in the form of larvae. They do not have developed gonads; they often have special larval organs that ensure adaptation to their mode of existence. The sizes are usually small, feeding independently, the larvae grow, change their shape and turn into a sexually mature individual.

It is believed that in insects development with metamorphosis is secondary; if the metamorphosis is not of a pronounced nature and the larva changes little in shape (cockroaches), then it is called incomplete.

During complete metamorphosis, the structure of the larva and the adult are very different; in development, pronounced processes of destruction of some organs and new formation of other organs (flies, frogs) occur; the progress of metamorphosis is regulated by hormones.

Direct postembryonic development

In the evolution of vertebrates, a straightening of postembryonic development is observed.

Due to an increase in the amount of yolk in the eggs or intrauterine development, the embryonic period lengthens, and by the time the organism emerges from the egg membranes, the main structural features resemble the adult stage. In this case, the postembryonic period is characterized mainly by growth and changes in body proportions, the acquisition of a state of functional maturity of organs and systems. The reproductive system matures and begins to function.

This type of embryonic development, devoid of the larval stage, is called direct. It is characteristic of the ontogenesis of higher vertebrates: reptiles, birds, mammals.

Growth and development in the postembryonic period are greatly influenced by environmental conditions. For plants, the decisive factors are light, humidity, temperature, quantity and quality of nutrients in the soil. For animals, adequate feeding is also of great importance (the presence of proteins, carbohydrates, lipids, minerals, vitamins, microelements in the feed). Oxygen, temperature, light (synthesis of vitamin D) are also important.

The growth and individual development of organisms are also controlled humoral and nervous regulatory mechanisms.

Animal cells synthesize chemically active substances that affect vital processes. Nerve cells of invertebrates and vertebrates produce so-called neurosecrets. Endocrine or internal secretion glands produce hormones. In vertebrates, the endocrine glands are the thyroid, parathyroid, pancreas, adrenal glands, pituitary gland, pineal gland, gonads, which are closely related to each other.

Pituitary produces gonadotropic hormone that stimulates the activity of the gonads. In humans, the pituitary hormone affects growth. With its deficiency, dwarfism develops, with excess, gigantism.

Pineal gland produces a hormone responsible for seasonal fluctuations in the sexual activity of animals.

Thyroid hormone influences the metamorphosis of insects and amphibians. In mammals, underdevelopment of the thyroid gland leads to growth retardation and underdevelopment of the genital organs. A person’s ossification and growth are delayed, puberty does not occur, and mental development stops.

Adrenal glands produce hormones that affect metabolism, growth and differentiation of cells.

Sex glands produce sex hormones that determine secondary sexual characteristics. For example, in castrated roosters, the growth of the comb stops and the sexual instinct is lost. A castrated man acquires an outward resemblance to a woman (a beard and hair do not grow on the skin, fat is deposited on the chest and pelvic area, castration at an early age preserves the childish timbre of the voice, etc.).

In all periods of ontogenesis, organisms are capable of restoring lost or damaged body parts. This property is called regeneration. It can be physiological and reparative.

Physiological is the replacement of lost body parts in the life of the body. Regeneration of this type is very widespread in the animal world. For example, in arthropods it is represented by molting, which is associated with growth, in reptiles - by the replacement of the tail and scales, in birds - by feathers, claws and spurs, in mammals - by the annual shedding of antlers by deer.

Regenerative regeneration is the restoration of a body part of an organism that has been violently torn away. Regeneration of this type is possible in many animals, but its manifestations vary. For example, it is often carried out in hydras, when the whole animal is restored from a part. In humans, epithelial, connective, muscle and bone tissues have reparative ability.

Old age. This is the penultimate period of ontogenesis and its duration is determined by the total life expectancy, which is a species characteristic. In accordance with WHO recommendations, old age is considered to be the age starting from 75 years, for the elderly - from 60 to 75 years.

In the case of humans, a distinction is made between physiological old age, associated with calendar age, and premature aging under the influence of social factors and diseases, and old age is characterized by a number of external and internal signs. Among the external signs are a decrease in the smoothness of movements, changes in posture, a decrease in the elasticity of the skin and muscles, the elasticity of the latter, the appearance of wrinkles, and tooth loss. The first (the acuity of the sensory organs is dulled) and the second (speech intonation changes, the voice becomes dull) signaling systems undergo changes.

Among the internal signs is the reverse development (involution) of organs. A decrease in the size of the liver and kidneys, the elasticity of blood vessels, the elasticity of ligaments, and the ability of organs and tissues to regenerate are noted. Inorganic salts accumulate in the bones, and cartilage becomes calcified. Significant changes occur in cells.

Modern genetic ideas about the mechanism of aging boil down to the fact that during the course of life, mutant genes accumulate in the cells of the body, as a result of which the synthesis of defective proteins occurs. Defective proteins play a disintegrating role in cellular metabolism, leading to aging. However, a comprehensive theory of aging has not yet been created.

Death. This is the final stage of ontogenesis. In single-celled organisms, death is their death, but the cessation of cell existence can be associated with division.

In the case of more organized animals, such as mammals, death in the full sense of the word ends the life of the organism.

In humans, there is a distinction between clinical and biological death. Clinical death is expressed in loss of consciousness, cessation of heartbeat and breathing. However, most cells and organs remain alive. Clinical death is reversible, since a person can be “returned” to life, but only within 6-7 minutes from the onset of clinical death.

Biological death is characterized by the fact that it is irreversible and is accompanied by the cessation of self-renewal processes, death, and decomposition of cells. However, cell death does not begin in all organs at the same time. First, the cerebral cortex dies, then the epithelium of the intestines, lungs, liver, muscle cells, and heart. That's it, it's over.

Lifespan. The lifespan of different organisms is not the same. Herbaceous plants live for one season. In contrast, woody plants live much longer. For example, cherry - 100 years, spruce - 1000, oak - 2000, pine -3000-4000. Fish live 35-80 years, frogs -16, crocodiles -50-60, birds of some species - up to 100 years. Mammals live shorter lives. For example, small cattle live 20-25 years, large cattle - more than 30 years, horses, dogs - more than 20, wolves -15, bears -50, elephants - 100 years. Among mammals, humans are the longest-lived. Many people live to be 115-120 years or more.

It is believed that actual life expectancy does not coincide with natural life expectancy. A.A. Bogomolets and I.I. Shmalgauzen calculated that the natural life expectancy of a person should be 120-150 years. However, only a few survive to this age.

In connection with the beginning of human history, social factors and medicine began to influence his life expectancy, which in our time have become very important. The main reasons for the decline in life expectancy are mortality from hunger, disease, and insufficient medical care. The average life expectancy on the planet in 1988 was 61 years, with 73 years in industrialized countries and 52 years in Africa.