Gastrulation is ... The process of formation of a multilayer embryo

Blastula, also called the embryonic bladder, is the final result of the process of crushing the egg. The next stage, occupying an intermediate position between crushing and organogenesis, is gastrulation in embryogenesis. Its main purpose is to form three germinal leaves: endoderm, ectoderm and mesoderm. In other words, it is with gastrulation that embryonic differentiation and morphogenesis of the organism begins.

The definition of the term "gastrulation"

Back in 1901 gastrulation was described as a pathway through which mesodermal, endodermal and ectodermal cells enter the embryo. This definition implies the presence of blastula special organ-forming spaces. Having understood this rather simple description, it is easy to move on to a more complex, modern meaning of the term. Gastrulation is a sequence of morphogenetic movements, the result of which is the movement of the rudiments of tissues to the places intended for them in accordance with the "plan" of the organization of the organism. The process is complex, the changes are accompanied by growth and multiplication, directed movement and differentiation of cells.

Considering gastrulation in a more general sense, it is possible to define it as an intermediate stage belonging to a single dynamic process during which the blastula parts are reconstructed, which considerably facilitates the transition to the organogenesis process.

Moving cells

If we give a general description of the process under consideration, then we can say that gastrulation is embolism and epiboly. Both terms reflect the morphogenetic movement of cells, which occurs absolutely at all stages of ontogenetic development of the organism. However, they are most pronounced when gastrulation. Epibolia is the process of moving cells around the surface of the embryo, and embolism is their movement inside it.

In embryology, the following main types of gastrulation or cell movement are distinguished: invagination, immigration, involution, delamination and epiboly. More details about them - later in the article.

Moving of cellular layers

Not only separate (freely migrating) cells can participate in the process of gastrulation, but also entire cellular layers. Direction determine the constant and remote interaction. The first forces were discovered by P. Weiss in the 1920s and apparently also occur in embryogenesis, the latter - rare and special, with ordinary morphogenesis occur with a small degree of probability.

When gastrulation, cell fragmentation does not occur. As already mentioned above, the movement of the cell masses begins and, as a result, the formation of a two-layered embryo called the gastrula. Becomes clearly visible endoderm and ectoderm. In all multicellular organisms (except for the coelenterates), parallel with gastrulation, or immediately after it, the third germinal leaf, called the mesoderm, is formed. It is a collection of cells located between the ectoderm and the endoderm. As a result, the embryo becomes three-layered.

The methods of gastrulation directly depend on the type of blastula.

Invagination gastrula

The name of the method speaks for itself. Invagination is the invagination of a single-layered blastula (balstoderm) wall into the blastocoel. The most primitive and most obvious is the example with a rubber ball. When you press it, some of the material is pressed inward. The invagination can be brought to the furthest wall or made insignificant. As a result, the blastula is transformed, and gastrula is obtained as a two-layer bag with archenterone. Its inner wall is the primary endoderm, and the outer wall is the primary ectoderm. The archenterone (primary intestine) thus formed communicates with the external environment by means of a hole called the blastopore. The second name is the primary mouth. Its further development depends on the type of organism. In many animals, the blastopore eventually transforms into a definitive mouth. In this regard, they are called primary moles (mollusks, worms, arthropods). In the second-averaged blastopores, it turns into a nerve channel located in the posterior part of the embryo (in chordates) or in the anus.

Immigration gastrula

Immigration gastrulation is a method of formation of a two-layered embryo, most characteristic of coelenterates. Gastrul is formed by actively evacuating a part of blastula cells into the blastocoel. Such immigration is unipolar. Cells move only from the vegetative pole. Later they form an endoderm, that is, an inner layer. It is in this way that gastrulation is performed in the hydroid polyp, jellyfish.

Cells of the blastoderm in the blastocoel can penetrate in no one area, but over the entire surface of the embryo. Such immigration is called multipolar, but it is rare.

In many coelenterates, which are characterized by the immigration method of gastrulation, there is a very active "eviction" of the blastula cells, and gastrula, formed as a result, completely loses blastocoel. In this case, the blastopore, which is characteristic of the previous invagination method, is absent.

Delamination gastrula

This rare species of gastrula was first described by Mechnikov II, and it is characteristic of coelenterates. The processes accompanying gastrulation are very peculiar, but when considering a typical case, they are perceived easier. For example, the eggs of some sciophedus have concentrically located and well-distinguishable parts of the cytoplasm: dense and granular (ectoplasm) and cellular (endoplasm). They are characterized by relatively synchronous and uniform division: 2, 4, 8, 16. Ultimately, the embryo contains 32 blastomeres. Further, the division is carried out parallel to the surface of the embryo. An outer layer of blastomeres consisting of ectoplasm forms and the inner layer is partly from ectoplasm and from endoplasm. In other words, the process of formation of a multilayer embryo proceeds by splitting one layer of cells into two. Then only internal blastomeres are crushed and again parallel to the surface of the embryo, which, as a result of such a peculiar gastrulation, acquires the shape of a ball. It consists of 64 ectoderm-forming flat cells and 32 more convex cells, which are the basis of the endoderm.

Epibolic gastrula

In animals with a pronounced body structure of eggs (displacement of the yolk to the vegetative pole), gastrulation occurs according to the epibolic method. Macromeres are large blastomeres that divide very slowly and contain a large amount of yolk. They lack the ability to move, in connection with this, they literally "crawl" more active micromers located on the surface of the cell. With such gastrulation, the blastopore is absent, and archenterone is not formed. Only later, when the macromeres still decrease in size, the cavity, the rudiment of the primary intestine begins to form.

Involution

Involutionary gastrulation is the process of "tucking" the outer layer of cells into the interior of the embryo. It increases in size, spreads on the inner surface. This method of gastrulation is characteristic of animals with mesolecitic ovules - amphibians (amphibians). The displacement of the leading deep cells of the marginal region inhibits the development of archenterone. It is in them that the driving force of involution lies.

Mixed mode of gastrulation

As is known, embryogenesis is the earliest period of development of each individual organism: from conception to birth. Gastrulation is one of its stages, the second in chronology after crushing. Its ways are so different that you can compare them with a large share of convention. Each of them requires detailed study and analysis. However, there are still definite lines of intersection between them. So, as a kind of invagination one can consider the process of epiboly, and delamination has similarities with immigration.

Note that many animals have gastrulation in a combined way. In such cases, epiboly and invagination occur simultaneously, as well as other morphogenetic processes. In particular, this is how gastrulation takes place in amphibians. In this regard, many authors distinguish a mixed method.

Gastrula

Literally in Latin, the term "gastrula" is translated as "the belly, the stomach." It denotes a specific stage in the development of the embryo of multicellular organisms. A distinctive feature of gastrula is the presence of two or three embryonic sheets. The process of its formation is the phase of gastrulation.

The simplest device is observed in representatives of coelenterates . They are characterized by an ellipsoidal gastrula with a unicellular outer layer (ectoderm) and an internal congestion of cells (endoderm), as well as a "primary" gut. A typical gastrula of the sea urchin, which is formed by intussusception, is considered. In humans, gastrulation takes place on the 8th to 9th day of development. Gastrula is a discoid flattened formation, formed from the inner cell mass.

As a rule, in most animals at the gastrula stage the embryo can not live freely and is located in the uterus or in the egg membranes. However, there are exceptions. So, the coelenterates larvae, the planules, represent a free-floating gastrula.