The blastula, also called the germinal bladder, is the final result of the egg crushing process. The next stage, which occupies an intermediate position between fragmentation and organogenesis, is gastrulation in embryogenesis. Its main meaning is the formation of three germ layers: the endoderm, ectoderm and mesoderm. In other words, it is with gastrulation that embryonic differentiation and morphogenesis of the body begins.
Definition of the term "gastrulation"
Back in 1901, gastrulation was described as the path through which mesoderm, endoderm and ectoderm cells enter the embryo. This definition implies the presence of special organ-forming spaces in the blastula. 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 tissue primordia to places intended for them in accordance with the "plan" of the organization of the body. The process is complex, changes are accompanied by growth and reproduction, directed by the movement and differentiation of cells.
Considering gastrulation in a more general sense, we can define it as an intermediate stage, belonging to a single dynamic process, during which there is a restructuring of the blastula sites, which significantly facilitates the transition to the process of organogenesis.
Cell movement
If we give a general description of the process under consideration, then we can say that gastrulation is an embolism and epibolism. Both terms reflect the morphogenetic movement of cells, which occurs at absolutely all stages of the ontogenetic development of the body. However, they are most pronounced precisely during gastrulation. Epibole is called the process of moving cells along 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: intussusception, immigration, involution, delamination and epibolism. In more detail about them - further in the article.
Cell bed movement
Not only individual (freely migrating) cells, but also entire cell layers can take part in the gastrulation process. The direction is determined by the constant and distant interactions. The first forces were discovered by P. Weiss in the 20s of the last century and apparently occur in embryogenesis, the second are rare and special, with ordinary morphogenesis they occur with little probability.
During gastrulation, cell crushing does not occur. As mentioned above, the movement of cell masses begins and as a result the formation of a two-layer embryo called gastrula. The endoderm and ectoderm become clearly visible. In all multicellular organisms (only gastrointestinal ones are the exception), a third germinal leaf, called the mesoderm, is formed in parallel with gastrulation or immediately after it. It is a collection of cells located between the ectoderm and endoderm. As a result, the embryo becomes three-layered.
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-layer blastula wall (balstoderm) into the blastocoel. The most primitive and most obvious will be an example with a rubber ball. When you click on it, a part of the material is pressed inward. The indentation can be brought to the farthest wall or make it insignificant. As a result, blastula is transformed, and gastrula is obtained in the form of a two-layer bag with argenteron. Its inner wall is the primary endoderm, and the outer is the primary ectoderm. The archeterone (primary intestine) formed in this way communicates with the external environment through an opening called a blastopore. Its second name is the primary mouth. Its further development depends on the type of organism. In many animals, blastopore is eventually converted to a definitive mouth. In this regard, they are called primary-legged (mollusks, worms, arthropods). In the secondary, the blastopore turns into the neuro-intestinal canal located in the posterior part of the embryo (in chordates), or into the anus.
Immigration gastrula
Immigration gastrulation is a method of forming a two-layer embryo, the most characteristic of intestinal. Gastrula is formed by the active eviction of part of the blastula cells inside the blastocoel. Such immigration is unipolar in nature. Cells move only from the vegetative pole. Later, they form the endoderm, i.e., the inner layer. In this way, gastrulation is carried out in a hydroid polyp, jellyfish.
Blastoderm cells in the blastocele can penetrate in any one area, but on the entire surface of the embryo. Such immigration is called multipolar, but it is quite rare.
Many intestinal cavities, which are characterized by an immigration method of gastrulation, have a very active “eviction” of blastula cells, and the gastrula, which is formed as a result, completely loses the blastocele. In this case, the blastopore characteristic of the previous invagination method is absent.
Delamination gastrula
This rare species of gastrula was first described by I. Mechnikov, and it is characteristic of the intestinal. The processes accompanying gastrulation are very peculiar, but when considering a typical case, they are perceived easier. For example, the eggs of some scyphomeduses have concentrically located and clearly distinguishable sections 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 is formed, consisting of ectoplasm, and an inner layer, partly of ectoplasm and endoplasm. In other words, the process of formation of a multilayer nucleus 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, takes on the shape of a ball. It consists of 64 flat cells forming the ectoderm and 32 more convex ones, which are the basis of the endoderm.
Epibolic gastrula
In animals that have a pronounced teloecital structure of eggs (yolk shift to the vegetative pole), gastrulation occurs by the epibolic method. Macromers are large blastomeres that divide very slowly and contain a large amount of yolk. They lack the ability to move, and therefore more active micromeres located on the surface of the cell literally "creep" onto them. With such gastrulation, there is no blastopore, but archeterone does not form. Only in the future, when macromers nevertheless decrease in size, a cavity begins to form, the primordium of the primordium.
Involution
Involutional gastrulation is the process of “tucking” the outer layer of cells into the embryo. It, increasing in size, spreads along the inner surface. This method of gastrulation is characteristic of animals with mesolecitic eggs - amphibians (amphibians). The movement of the leading deep cells of the marginal region inhibits the development of argenteron. It is in them that the driving force of involution lies.
Mixed gastrulation method
As you know, 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 methods are so varied that they can be compared with a great deal of conventionality. Each of them requires a detailed study and analysis. However, there are still certain intersection lines between them. So, as a peculiar variant of invagination, one can consider the process of epibolism, and delamination has features similar to immigration.
Note that in many animals gastrulation occurs in a combined way. In such cases, epibolism and invagination, as well as other morphogenetic processes, occur simultaneously. In particular, this is exactly how gastrulation occurs in amphibians. In this regard, many authors distinguish a mixed method.
Gastrul
Literally from Latin, the term "gastrula" is translated as "belly, stomach." It denotes a specific stage of development of the embryo of multicellular organisms. A distinctive feature of gastrula is the presence of two or three germ layers. The process of its formation is a phase of gastrulation.
The simplest device is observed in representatives of intestinal animals. They are characterized by gastrula of an ellipsoidal shape with a unicellular outer layer (ectoderm) and an internal accumulation of cells (endoderm), as well as the “primary gut”. The gastrula of a sea urchin, which is formed by intussusception, is considered typical. In humans, gastrulation takes place on the 8-9th day of development. Gastrul is a disc-shaped flattened formation formed from the inner cell mass.
As a rule, in most animals at the gastrula stage, the embryo cannot live freely and is located in the uterus or egg membranes. However, there are exceptions. So, the larvae of the gastrointestinal, planula, are a free-floating gastrula.