Formation of mesoderm. Mesoderm is the precursor of many organs and tissues. Mesoderm is formed

Ontogenesis of any organism is characterized by the formation of germ layers. In primitive organisms such as coelenterates and sponges, the embryo consists of only two layers: endoderm and ectoderm. Over time, more progressive forms of organisms develop a third layer - the mesoderm.

What is mesoderm?

Ontogenesis is the sequential development of the embryo, which is accompanied by a number of changes in the morphology and anatomy of the future young organism. Mesoderm is a germ layer that plays an important role in the formation of many organs and tissues. It is not for nothing that such primitive multicellular animals as hydra, jellyfish, corals or sponges are called two-layer animals, because during the process of ontogenesis they formed only two germ layers.

Mesoderm formation

The process of initiation of the middle germ layer differs among different taxonomic groups. There are three most well-known ways in which mesoderm is formed: teloblastic, enterocoelous and ectodermal.

1. The teloblastic path of mesoderm development is characteristic of many protostomes and is based on the formation of blastomeres. Some of them specialize in laying the middle germ layer, which ultimately takes the form of two longitudinal parallel ribbons. These ribbons give rise to mesoderm.
2. The enterocelous method is fundamentally different in that the mesoderm precursor cells form an invagination (invagination) together with the endoderm. This invagination in the future forms the primary intestine. The border between the two layers remains indistinguishable for a long time, and only after a long period of time the mesoderm as an independent layer is disconnected from the endoderm. This method of development is characteristic of animals such as the lancelet or starfish.

3. The ectodermal method of mesoderm development is characteristic of such types of animals as reptiles, birds and mammals (including humans). The bottom line is that after invagination, only endoderm is formed. If you imagine a cross-section of the embryo, then after gastrulation (formation of invagination), free space will appear between the endo- and ectoderm. Cells of ectodermal origin “bud off” there, giving rise to a new germinal layer.

Morphology of mesoderm

The mesoderm plays a major role in the formation of the embryo. In biology, this is a good evolutionary sign, because the difference in the morphology of the middle germ layer in different ones is used in taxonomy.

If we consider two longitudinal ribbons that are formed during the teloblastic mode of development, then the mesoderm will be represented by metamerically repeating sections. The dorsal side of each such band is divided into somites, the lateral side into nephrotomes, and the ventral side into splanchnotomes.

What role does mesoderm play? Human organs formed from mesoderm

Each germinal layer is a kind of precursor to the organ systems and tissues of the future organism. The topology of the forming leaves largely determines their future fate. Since mesoderm is the middle germinal layer, it participates in the formation of tissues and organs that are located between the integument of a person and the innermost layers of the body. What structures are of mesodermal origin?

Conclusion

The mesoderm is a complex embryo that ultimately gives rise to many vital organs and tissues. The formation and development of the middle leaf differs in different animals, and this is one of the evolutionary characteristics. The presence of mesoderm indicates that the animal is three-layered, which is a significant sign of the advancement of the group.

Germ layers, or germ layers - layers of the body of the embryo of multicellular animals, formed in the process and giving rise to various organs and tissues.

They are formed in the process of differentiation of similar homogeneous cells

Gastrulation- education process two germ layers(ento- and ectoderm).

During gastrulation, all cells move and form gastrula- two-layer embryo sac, inside of which there is a cavity - gastrocel, connected by the primary mouth ( blastopore) with the external environment.

Gastrulation ends with the formation of the third germ layer - mesoderm, located between the ecto- and endoderm.

In most organisms (except coelenterates), three germ layers are formed:

- external - ectoderm,
- internal - endoderm And
- average - mesoderm.

After completion of gastrulation, the embryo forms a complex of axial organs: neural tube, notochord, and intestinal tube. This is the stage neurulae.

Education germ layers- the beginning of the transformation of a multicellular organism into an organism in which cells become differentiated and in the future form tissues and organs.

So, first the zygote begins to divide, increasing the number of cells. Having gained sufficient mass, the body begins the next stage - the cells begin to move - they shift to the periphery, forming blastodermic vesicle.

At one edge of this vesicle, cells are grouped and form an internal cavity - this is inner germ layer - endoderm.

Outer cells of the embryo (outermost layer) - ectoderm.

The layer of cells between these two germ layers is mesoderm, these cells are formed partly from ecto-, partly from endoderm.

• This division of leaves is typical for all higher animals;
• at simple animals- y and - only 2 germ layers(external and internal).

Here is an example question from Unified State Exam in Biology just on topic:

1. from the ectoderm are formed: the ear and the brain;

2. from the endoderm - liver, lungs, intestines, stomach, pancreas;

3. from the mesoderm - muscles, blood vessels, bones.

Germ layers were first described in the work of a Russian academician X. Pandera in 1817, who studied the embryonic development of the chicken embryo. A particularly important role in the study of the germ layers of vertebrates was played by the classical works of another Russian academician - Carla Bara, who showed that germ layers are also present in embryos of other vertebrates (fish, amphibians, reptiles).

Rice. 51. Rough diagram of the formation of mesoderm in protostomes (A) and deuterostomes (B) (according to V.M. Shimkevich, 1925, modified):
/ - ectoderm, 2 - mesenchyme, 3 - endoderm, 4 - teloblast (L) and coelomic mesoderm (5)
blasts), separating small cells from themselves (due to divisions) (Fig. 51, L, Fig. 69). Thus, the middle layer is formed - the mesoderm. Teloblasts, giving rise to new generations of mesoderm cells, remain at the posterior end of the embryo. For this reason, this method of mesoderm formation is called teloblastic (from the Greek telos - end).
With the enterocoel method, a set of cells of the developing mesoderm appears in the form of pocket-like protrusions of the primary intestine (protrusion of its walls into the blastocoel, Fig. 51, B, 4). These protrusions, into which parts of the primary intestinal cavity enter, are separated from the intestine and separated from it in the form of pouches. The cavity of the sacs turns into a whole, i.e., into a secondary body cavity; coelomic sacs can be divided into segments.
This description of the methods of origin of the middle germ layer does not reflect the entire variety of variations and deviations that are strictly natural for individual groups of animals. Similar to the teloblastic method, but only externally, is the method of formation of mesoderm not by dividing teloblasts, but by the appearance at the edges of the blastopore of an unpaired dense primordium (group of cells), subsequently dividing into two symmetrical stripes of cells. With the enterocoel method, the mesoderm rudiment can be paired or unpaired; in some cases, two symmetrical coelomic sacs are formed, and in others, one common coelomic sac is first formed, which is subsequently divided into two symmetrical halves.
It has already been said about the peculiar development processes of nematodes and other animals, in relation to which it would be artificial to apply the concept of “germ layers” - in them, bypassing the formation of cellular germ layers, the rudiments of future organs are isolated in the form of separate blastomeres.
Due to its importance for embryology in general and for understanding the processes of organ development, the next chapter will give a comparative embryological outline of the processes of gastrulation in various animals, making appropriate adjustments to the overly simplified classical ideas about the germ layers, in particular about the enterocoelous method of mesoderm formation.

Gastrulation is a stage in the development of the embryo. The germ layers are not something separate from each other; their emergence and further changes occur due to the mutual dependence of the parts of the embryo. The germ layers, as collections of cells, differ from each other not only in their position in the general system of the embryo, but also in some cytological features. At the same time, experiments convince that their fate can still be changed, forcing them to “build” cellular systems and organs that are unusual for them (see Chapters XI and XVII).
During the normal development of embryos, the germ layers, interacting with each other under the influence of the integrating influences of the embryo as a whole, continue to differentiate in a certain direction, and each of them takes part in the emergence of the rudiments of certain organs and organ systems. We can talk about a new stage in the development of embryos - organogenesis.
Throughout the animal kingdom, certain organs originate from the same germinal layer. Exceptions to this law, which will be discussed later, are due to changes in ontogenesis, in connection with certain unique paths of animal evolution. They are considered as homologous formations in the embryo. For germ layers, see Chap. VII-IX.
Derivatives of ectoderm. Most of the cells that make up the outer piaste, multiplying and differentiating accordingly, remain on the surface, taking part in the development of the integument of the body. From them are formed: outer epithelium, skin glands, surface layer of teeth, horny scales. d. Almost always, each organ develops from the cellular elements of two, or even all three germ layers. For example, mammalian skin develops from ecto- and mesoderm.
As a rule, a large part of the primary ectoderm (up to a third or more of the entire surface of amphibian embryos), due to special morphogenetic processes, “sinks” inside, under the outer epithelium, and gives rise to the entire nervous system. In many animals, the ectoderm at the anterior and posterior ends of the body is invaginated towards the anterior and posterior ends of the intestine developing from the endoderm (midgut). These invaginations break into the cavity of the midgut and form the stomodeum (foregut) and proctodeum (hindgut).
Endoderm derivatives. The internal germ layer, differentiating in conjunction with other parts of the embryo, develops into the epithelium of the midgut and its digestive glands. Development of the epithelium of the respiratory system (branch and

lungs) in different vertebrates has not been traced equally completely and is not yet so clear in detail. It is indisputable that this epithelium develops from the foregut. However, it cannot be said categorically that it is entirely of endodermal origin, since
how the cellular material of the prechordal plate undoubtedly participates in its origin (see p. 126, etc.).
Mesoderm derivatives. All other organs not previously listed develop from the mesoderm: all muscle tissue, wherever they are located (the wall of the body, intestines and other formations), all types of connective, cartilage and bone tissue, canals of excretory organs, peritoneum of the body cavity, circulatory system , part of the tissues of the ovaries and testes. With the development of the corresponding organs, specific differentiation of the cellular elements of the mesoderm occurs. In most animals, the middle layer appears not only in the form of a collection of cells forming a compact epithelium-like layer, i.e., the mesoderm itself, but also in the form of a loose complex of scattered, amoeba-like cells. This part of the mesoderm is called mesenchyme. Mesoderm and mesenchyme differ from each other in their origin, there is no direct connection between them, they are not homologous. Mesenchyme is mostly of ectodermal origin, while mesoderm begins with endoderm. In vertebrates, however, a smaller part of the mesenchyme is of ectodermal origin, while the bulk of the mesenchyme has a common origin with the rest of the mesoderm. In many animals with spiral cleavage, mesenchyme appears during cleavage. In echinoderms, the source of mesenchyme is micromeres and endoderm. Cells of the bottom of the developing primary intestine migrate into the blastocoel.
Despite its different origin from mesoderm, mesenchyme can be considered as part of the middle layer. It plays a large role in the formation of the larva and definitive organs.
To understand the issues discussed further, it is necessary to have an idea of ​​​​an important formation - the coelom, the secondary cavity of the body. In all animals that are characterized by a coelom, the hollow coelomic sacs give rise to the mesoderm. It has already been said that with the enterocoelic origin of the mesoderm, coelomic pouches are formed by changing, differentiating pocket-like protrusions of the primary intestine. In teloblastic and similar methods, when mesodermal cords are formed, a gap appears inside them, which eventually turns into a whole. Coelomic pouches form symmetrically on the sides of the intestine. The wall of each coelomic sac facing the intestine is called the splanchnopleura. The wall facing the ectoderm of the embryo is called somatopleura.
Thus, during the development of the embryo, different

Rice. 52. Scheme of organogenesis of embryos of higher vertebrates (according to K. Waddington; 1957):
/ - neural tube, 2 - somite, 3 - notochord, 4 - intestine, 5 - lateral mesoderm, as a whole, 7 - epidermis, c - pharynx, 9 - gill slits, 10 - optic vesicle, 11 - brain
personal cavities that have important morphogenetic or functional significance. First, Baer's cavity appears, turning into the primary body cavity - the blastocoel, then, in connection with the processes of gastrulation, the gastrocoel (or gastric cavity) appears, and finally, in many animals - the coelom. With the formation of the gastrocoel and coelom, the blastocoel becomes increasingly smaller, so that all that remains of the former primary body cavity are gaps in the spaces between the walls of the intestine and the coelom. In some animals these slits turn into cavities of the circulatory system. The gastrocoel eventually turns into the midgut cavity.
With the enterocoel method of separating the mesoderm and coelom at the expense of the gastrocoel, in addition, a secondary body cavity appears.
The processes of gastrulation directly lead to a period of organogenesis. In some animals, organs and organ systems are formed, which gradually acquire definitive significance, while in other animals, organs characteristic of the larva are first formed, then metamorphosis occurs (see Chapter X) and the processes of formation of the definitive organs of the adult organism occur.
Due to the lack of a unified plan in the structure of the embryos of all invertebrates, it is impossible to give a diagram of an abstract embryo of an invertebrate animal. A diagram of the late embryo of a vertebrate animal is shown in Fig. 52.

See also `Mesoderm` in other dictionaries

(mesoderma, LNE; meso- + Greek derma skin; synonym mesoblast) - the middle germ layer formed in mammals by the growth of the primary streak in the form of a layer of cells between the ecto- and endoderm.
extraembryonic mesoderm (m. extraembryonicum, LNE) - part of the mesoderm, part of the temporary (provisional) organs of the embryo - the embryonic membranes and vitelline
splanchnic mesoderm (m. splanchnicum, m. viscerale, LNE; synonym: splanchnomesoderm, splanchnopleura) - part of the lateral mesoderm, from which the visceral layers of the pleura, peritoneum and mesentery, the heart, the endothelium of blood vessels, connective and smooth muscle tissue of internal organs are formed .
dermal mesoderm (m. paraxiale, LNE; m. dermale) - part of the mesoderm, which subsequently forms the connective tissue part of the skin (dermis).
dorsal mesoderm (m. dorsale; syn. M. parachordal) - part of the mesoderm, which is paired thickenings on both sides of the notochord, forming somites.
larval mesoderm (m. larvale; lat. larva mask...

(from meso... and dermis), mesoblast, middle germ layer in multicellular animals (except sponges and coelenterates). Located between the ectoderm and endoderm. Different groups of animals develop different ways (see Gastrulation). In flatworms and nemerteans, M.'s stripes provide connective tissue that fills the space between the internal parts. organs, in annelids and most other invertebrates, the M. stripes are divided into paired somites with a secondary cavity - the coelom. In vertebrates, during the period of neurulation, from the sides of the notochord primordium, the M- is divided into dorsal (primary) segments - somites, nephrotomes, and unsegmented abdominal M. - lateral plates. Between the two leaves of each of them a coelom is formed. M. and its derivatives have an inducing effect ((see Induction) on the development of derivatives of the ectoderm and endoderm and, in turn, experience an inducing influence on their part (see Germ Layers).

the middle layer of the embryo from which many body tissues develop.

(Source: “Dictionary of foreign words included in the Russian language.” Pavlenkov F., 1907)

1. Type of germinal tissue.

(mesoderm) - the middle germ layer of an embryo in the early stages of development. It serves as a source of development of cartilage, muscles, bones, blood, kidneys, gonads and their ducts and connective tissue. The mesoderm is divided into two layers: the outer somatic and deep, visceral, separated by a cavity - the coelom (coelom), which becomes the body cavity. The dorsal somatic mesoderm becomes segmented into a number of somites. see also Mesenchyme. - Mesodermal (mesodermat).

MESODERM

(Sorry... And dermis)(mesoblast), the middle germ layer in multicellular animals (except sponges and coelenterates) and humans. From M. muscles, cartilage, bones, organs of blood and lymph formation, secretions, genitals, etc. develop. Ectoderm, Endoderm.

(mesoderma, LNE; meso- + Greek derma skin; synonym mesoblast) middle germ layer, formed in mammals by the growth of the primitive streak in the form of a layer of cells between the ecto- and endoderm. Encyclopedic Dictionary F.A. Brockhaus and I.A. Efron. - S.-Pb.: Brockhaus-Efron

(mesoderma, LNE; Meso- + Greek derma skin; synonym mesoblast)

the middle germ layer formed in mammals by the growth of the primitive streak in the form of a layer of cells between the ecto- and endoderm. Extraembryonic mesoderm

(m. extraembryonicum, LNE) - part of M., part of the temporary (provisional) organs of the embryo - the embryonic membranes and the yolk sac.(m. splanchnicum, m. viscerale, LNE; synonym: splanchnomesoderm, splanchpopleura) - part of the lateral M., from which the visceral layers of the pleura, peritoneum and mesentery, the heart, the endothelium of blood vessels, connective and smooth muscle tissue of internal organs are formed.

Dermal mesoderm (...

(mesoderma, LNE; meso- + Greek derma skin; synonym mesoblast) middle germ layer, formed in mammals by the growth of the primitive streak in the form of a layer of cells between the ecto- and endoderm. (from Meso... and Greek derma - skin)

mesoblast, middle germ layer (See Germ layers) in multicellular animals (except sponges and coelenterates) and humans. As a result of gastrulation (See Gastrulation) it is located between the outer germ layer - the ectoderm (See Ectoderm) and the inner one - the endoderm (See Endoderm). In protostomes (See Protostomes) animals (most invertebrates), the muscle is formed by the teloblastic method - from large cells - teloblasts, lying between the ectoderm and endoderm at the posterior end of the embryo and entering the primary body cavity during gastrulation, where they multiply and turn into two mesodermal stripes. In most deuterostomes (See Deuterostomes) animals - echinoderms, brachiopods ...

Embryogenesis is a complex process that is characterized by the gradual formation of organs and tissues. In most multicellular organisms, the embryonic rudiment consists of three layers: ectoderm, endoderm, mesoderm. What is mesoderm? Both the chitinous skeleton of arthropods, the epidermis of the skin, and the nervous system are of ectodermal origin. The digestive, endocrine and respiratory systems are formed from the endoderm. What organs and tissues does the mesoderm give rise to? How is it formed?

What is mesoderm. Definition

Any tissue or organ system is formed from a certain layer of embryonic cells. What is mesoderm? In biology, the definition is as follows: this is one of the germ layers from which a number of organs and tissues are formed during embryogenesis. The second name for mesoderm is mesoblast. The formation of this layer is characteristic of most multicellular animals (exception: type Sponges and type Coelenterates).

The mesoderm is located between the ectoderm and endoderm. Each of the nearby germ layers can take part in the formation of the mesoblast. Accordingly, according to their origin, two types of median germ layer are distinguished: endomesoderm, exomesoderm. There are also situations when both structures take part in the formation of the mesoblast at once.

The mesoderm is formed as an independent structure at the stage of gastrulation.

Formation of mesoderm. Features of formation

What is mesoderm? In biology, it is generally accepted that each organ of a multicellular animal in embryogenesis is formed by one of the germ layers. The formation of mesoderm is a characteristic aramorphosis since they form the true middle germinal layer for the first time. Type Sponges and are representatives of two-layer animals: during embryogenesis, only ectoderm and endoderm are formed.

How does mesoderm form?

There are three ways to form a mesoblast.

Mesoderm structure

What is mesoderm? This is not just an accumulation of identical cells, but a germinal layer differentiated into several functional sections. The division of the mesoblast occurs gradually, as a result of which the following areas are distinguished:

1. Somites are paired ribbon-like formations, between which a coelom is formed - a secondary body cavity. They are also preserved in arthropods.
2. The notochord primordium is a section of mesoderm that in the future develops into a notochord. Distinctive feature of vertebrates.
3. In vertebrates, each somite forms a sclerotome, a dermatome, and a myotome.
4. Splanchnotomes are lateral plates that are divided into two separate layers: internal and external. Between them, the coelom is formed in vertebrates.
5. Nephrotomes are paired structures connecting splanchnostomies.

By studying each section of the germ layer, scientists were able to determine what mesoderm is and understand what functions it performs.

Histogenesis

The mesoderm gives rise to several types of tissues.

1. Parenchyma of flatworms that fills the space between organs. Formed from mesoderm.
2. Some epithelial tissues covering the outside of organs. These include secretory cells, endocrine and exocrine glands.
3. Loose fibrous and dense fibrous connective tissues are formed from the mesoderm. This includes the formation of collagen and elastic fibers.
4. are also formed from mesoderm.
5. Bone and cartilaginous tissues and their constituent elements are of mesodermal origin.
6. By analogy with the formed elements of blood, mesoderm also takes part in the formation of cells of the immune system.
7. All types of muscle tissue. Smooth muscles are found in the walls of most organs. Striated fibers are the structural elements of skeletal muscles. Do not forget about the striated cardiac muscle tissue, which forms the muscles of the heart.

Organogenesis

Tissues form organs, so it is not difficult to guess which of them are of mesodermal origin. The classification is given according to the areas of the mesoderm:

• dermatomes - form the dermis of the skin (the skin contains sweat and sebaceous glands);
• the passive part of the musculoskeletal system (skeleton) is formed from sclerotomes;
• from the myotome, respectively, the active part of the musculoskeletal system (muscles);
• splanchnostomies give rise to mesothelium - a single-layer epithelium that lines the secondary body cavity;
• nephrostomal cells form the excretory and reproductive systems.

mesodermal origin

It is worth mentioning those organs that are lost at different stages of ontogenesis after performing their functions. They are called provisional. These include:

1. The amnion is one of the membranes of the embryo that performs several vital functions at once. The first is to create an aquatic environment for fetal development. This is explained by the fact that the formation of the organism must take place in water. For vertebrates living on land, water in this case is the limiting factor, which is why this shell was formed in the process of evolution. The Amnion also protects the fetus from mechanical damage, maintains a constant environment by maintaining the concentration of salts at a constant level, and also protects the embryo from exposure to toxic substances.
2. Allantois is another organ of the embryo that simultaneously performs the functions of nutrition and respiration. By origin, it is an outgrowth of the yolk sac, which means it is also formed by endoderm and mesoderm cells. In humans, the allantois is less developed than in other representatives of vertebrates, but blood vessels pass through it, which then enter the umbilical cord tissue.
3. Yolk sac. This temporary organ contains nutrients necessary for the development of the fetus. Both mesoderm and endoderm cells participate in the formation of the yolk sac. An interesting feature of the organ is the formation of the very first blood cells of the body in it.
4. Umbilical cord (umbilical cord) - connects the embryo and the placenta.
5. The chorion is the membrane of the embryo through which it attaches to the uterus and forms the placenta.
6. The placenta is the only human organ formed by the tissues of two organisms: the mother and the fetus. From the mother's blood, the fetus receives nutrients and oxygen through the placenta.

Functions of mesoderm

We looked at what mesoderm is. What are the functions of this germ layer?

The development of mesoderm allowed flatworms to fill the spaces between organs with parenchymal tissue. More advanced organisms do not have parenchyma, but the principle is similar: tissues of mesodermal origin form the boundary layers between organs. The most important function of the mesoblast is the formation of temporary organs in the embryo (allantois, umbilical cord, placenta, etc.) Mesoderm cells also form tissues of the internal environment: blood and lymph.

Conclusion

Now we can fully explain what mesoderm is. Its formation allowed animals to move to a new stage of development, as evidenced by the origin of many organs and tissues. In addition, the formation of the amniotic membrane led to a qualitative leap in the development of vertebrates. Therefore, mesoderm is an important evolutionary element.