Investigation of the activities of all genes during the development of the embryo of the round worm Caenorhabditis Elegans. Revealed new ontogenetic details. It turns out that the inner germinal sheet - Entoderma - acquires its specific features before the external leaflet - Etoderma. The same procedure for the development of germinal leaves was found in other animal species. In addition, the genes active in Entoderme were on average "older" than genes working in other germinal sheets. This indicates an older origin of the Entoderma in the course of evolution compared to ectoderma.
In the embryonic development of even quite dissimilar organisms, similar stages can be distinguished. The fertilized egg is divided, forming a lump of cells in which the cavity is formed (the lump of cells with the cavity is called Blastula). A portion of the surface of the hollow ball begins to be vague inside (this process is called gastrojulation), and something like a two-layer bag is formed (which is called gastrole). Its outer layer becomes an embryonic leaflet ectoderm, from which body cover and sensitive cells are formed. From the inner layer, which is called an Entoderma, a digestive system and other internal organs are formed. Animals developing only by these two germinal sheets - Entoderms and Etodermas are called two-layer. These include intestinal and grains - quite simply arranged organisms with radial symmetry, as well as sponges - even easier arranged animals in general without symmetry and real tissues.
In more complexly organized animals, the third embryonic leaflet - Mesoderma arises during embryonic development. This is the protrusion of the Entoderma, forming an independent layer. Muscle cells and skeleton originate from the mesoderma. Animals that develop from three germinal leaves - bilateral-symmetrical: they have the right and left side of the body, in contrast to, for example, jellyfish. As for the fact that Mesoderma appeared in evolution later, Entoderma and Etoderma, no doubt - still two-layer animals are easier than three-layer, and they have arisen before. And in the article recently published in the journal Nature. , data from which should be said to be a revolutionary conclusion: Entoderma, it turns out, originated echtoderma . It would seem that these two layers - two sides of one gastraul (Fig. 1). Could it be that one of them arose during the evolution earlier?
Scientists decided to explore the processes of formation of germinal leaves, focusing on gene activity data. In order not to be fine and not to miss something important, the researchers rated the activity of all genes that work throughout the whole embryonic development of round worm (nematodes) from the stage of five cells to an adult larvae. This worm is - Caenorhabditis Elegans. - One of the most detailed studied organisms. For example, the fate of each of the five first nucleus cells is known. This became possible, among other things, thanks to the surprisingly accurate development of nematodes: any adult hermaphroditic part consists of exactly of 959 cells (for this, the worm is characterized by the ewelle). In addition, a small number of cells allows in detail to study the structure of a whole body. For example, a nervous worm system consisting of 302 neurons is described with an accuracy of each connection.
Using a relatively new method of studying RNA of individual cells (CEL-SEQ) *, scientists received the expression profiles of all genes of the worm, depending on the time and location in the developing embryo. The main component method was to highlight groups of genes specifically expressed in ectoderm, entoderm and mesoderm. Genes active in mesoderm, expected to be transcribed later than other groups. It was unexpected that the expression program of the Entoderma was launched earlier than the Etoderma genes were activated. It turns out that these germinal sheets acquire their characteristic features at the same time.
To check whether such an order of development of germinal sheets is not a feature of nematodes alone, scientists investigated gene expression profiles and in other organisms: sponges ( Amphimedon Queenslandica. ), Actinia ( Nematostella vectensis ) and frogs ( Xenopus Tropicalis ). In all these animals, the trend was the same: the orthologists of genes expressed in nematodes in Entoderma, "included" during embryonic development were first (Fig. 2). It turns out that embryos of different animals, Entoderma appears earlier than Etoderma. But does this mean that in the evolution of Entoderma originated earlier?
Since scientists have identified the genes active in different germinal sheets, then the bioinformatic methods it became possible to determine the evolutionary age of these genes. In general, this is done as follows: a phylogenetic tree is taken with quite a variety of organisms and for each gene studying is determined, who also has a sequence for the first time, sufficiently similar to this gene. So scientists found out that genes expressing in Entoderm worm Caenorhabditis Elegans. , on average, older genes active in other germinal sheets. This is a strong argument in favor of the fact that Entoderma arose during the evolution earlier than Etoderma.
Such a discovery is extremely important for Theories of the emergence of multicellularity . According to one of them, one day a cell with flagellas resembling modern Hoanoflagellyt, "decided" to unite into a spherical structure with the digestive cavity inside. Researchers compared the genes of the representative of this group of unicellular ( Monosiga Brevicolli. ) With genes, selectively expressed in three germinal sheets of nematodes. It turned out that the proportion of gene orthologists Monosiga. Above among genes, active in Entoderma. This is not surprising, given that the genes working in Entoderme are mainly responsible for nutritional functions: energy production, metabolism and transport, - that is, the questions of the priority importance for any body.
So, taking into account the procedure for the occurrence of the layers, one can submit such a model of formation of multicellular:
- At first there was a ball of cells with flames, which for some reason it was advantageous to form a single structure (for example, together they could catch and digest the production of larger), these cells became predecessors Entoderm ;
- The ball was covered with another layer of cells specializing in interaction with the external environment - appeared Etoderma ;
- Between the Entoderma and Etoderma, the third layer was formed - Mesoderma From which structures developed to make complex movements.
All three layers gave the body incredible opportunities, and three-layer design allowed the many variety of complex organisms. But everything began with Entoderma - the most important germinal sheet for life.
- HASHIMSHONY T., Feder M., Levin M., Hall B.K., Yanai I. (2015). Spatiotempor Transcriptomics Reveals The Evolutionary History of the Endoderm Germ Layer. Nature. . 519. , 219-222;
- Sequencing of single cells (version - Metazoa);
- A method has developed a method for analyzing gene expression at the level of individual cells.
Ontogenesis - Individual development of the body from the beginning of existence to the end of life.
In the ontogenesis of animals, two periods are distinguished - embryonic и Posthambrium .
Embryonic (germs) Development covers the processes from the first division of the zygotes before exiting an egg or birth and in most animals includes three main stages: crushing, gastration and organogenesis.
Splitting up - These are seven or eight consecutive mitotic fissionions of the zygota.
When crushing, subsidiaries ( Blastomeres ) Do not diverge and do not increase in size. With each following division, their dimensions decrease.
Eggs with a small supply of nutrients are fully divided, i.e., complete crushing occurs. If the egg contains a large amount of yolk, then a partial crushing is observed - only the cytoplasm disk with the kernel is divided, and the yolk itself remains unchanged (for example, in birds).
Ends crushing the formation of a single-layer multicellular embryo - Blastuly.
Blastula - this is a spherical embryo, whose wall ( blastoderm ) is formed by one cell layer, and inside - cavity ( blastocel ).
After crushing there is a process Gastralization which is characterized by the movement of a part of the cell material from the surface of Blastuly inside, to the places of future organs. As a result of these movements, gastrol is formed.
Gastrol - a two-layer embryo, consisting of two germinal leaflets: Outdoor ( Etoderma ) and internal ( Entoderm ).
The gastrolus lancer arises in the patch of blastoderma in the bustice cavity.
The internal cavity of the gastraul is called Primary intestine . It binds to the outer middle hole ( blastopor ) that becomes Primary mouth .
At the stage of two germinal leaves, the development of sponges and shepherds is ends.
All other animals have continued development, and the third germistic sheet is formed - Mesoderma . It is formed from the Entoderma and is always located between the ecto and entoderm in the primary body cavity.
Further differentiation of cells of each embryonic leaf leads to the formation of tissues and organs, i.e. histoom и Organogenesis .
From Entoderma Forms chord - the inner skeleton in the form of a flexible heavy strand, located on the dorsal side. Subsequently, chord the vertebrate is replaced by the spine, and only in some animals (for example, in cartilage fish), its residues are stored throughout life.
Of Etoderma located over the chord itself, stand out nerve plate . Then the edges of the plate rise and closed. Forms nervous tube - Concept of the central nervous system. Forming Neurula .
Nervous tube, chord and intestines create Axial complex organs The embryo that determines the bilateral symmetry of the body.
Fabrics and organs develop equally in all three-layer animals.
Of Etoderma The vertebrates are formed by the nervous system, the senses, cover epithelium with its glands and derivative structures (hair, feathers, hooves, claws, etc.).
Of Entoderm The organs of the digestive and respiratory system are formed: the middle intestine epithelium, liver and pancreas, gills, light, swimming bubble, as well as thyroid gland.
Of Mesoderm Muscle cloth are formed, all types of connective tissue (for example, the dermis of the skin, body of the vertebral), the blood system, the isolation organs, sex glands.
The embryo is developing as a single organism in which all cells, fabrics and organs are in close cooperation.
Biology's exam is often mentioned in the structure and germinal sheets. What it is? What information about you need to remember to answer these questions without difficulty? Let's deal with!
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After the spermatozoide fertilizes the egg, their genetic information merges and the diploid zygote is formed. After this, the cell event begins to unleasely share and in time forms a three-layer neurulu. Here are these layers and are called germinal sheets.
There is a simple way to remember the location of the leaves. Etoderma is an outer layer, and Entoderma is internal. The letter "K" in the alphabet goes earlier than the letter "H", so the ectoderma is located on the outside of the embryo. Mesoderma is an average embryonic leaf and, so it happened that the letter "M" is between "K" and "N".
There is another way for those who oppose chemistry: you can recall exothermic and endothermic chemicals. With exothermic reactions, the energy is highlighted out (from Latin "Exo" - outward), and with endothermic energy absorbed inside (from "Endo" - inside). And Mesoderma is a leaflet that is between Etoderm and Entoderma.
After we remembered exactly where each of the germinal leaves are located, it is important to realize what fabrics, organs and system systems are formed from each of them.
What is formed from Etoderma?
- The nervous system is a nervous fabric that forms a head and spinal cord, nerves and some cells of different analyzers (for example, a lens of the eye).
- Epithelium and its derivatives (leather, nails, claws, horns), as well as skin glands. All these structures are formed by epithelial cloth.
- tooth enamel
Lyfhak for memorization. Can you touch your eyes? Or do they feel touching the nerve endings on the skin? And touch the skin or hair? Yes. Spend a parallel, ectoderma - the outer layer and what you can touch, mainly laid from the ectoderma.
What is formed from the Entoderma?
- digestive system
- respiratory system
- Selective system
- endocrine glands
Lyfhak for memorization. These organ systems we call internal bodies. Entoderma - the inner layer and internal organs are formed from it.
What is formed from the Mesoderm?
- Skeleton - cartilage and bones
- Cardiovary - Vascular SSERDER system, blood vessels and cells
- Sexuals and ovaries
Lyfhak for memorization. All these structures consist of muscle and connective tissue, it is these types of fabrics that are laid out of the mesoderm.
Structures and germinal sheets: practical part
Let's solve the tasks where the structure and germinal sheets are mentioned. I draw your attention that this topic is found in the tasks for 2 and 3 points.
Example 1. The establishment between the human body structure and the germs leaflet, it was formed.
|The structure of the body||Germ leaf|
|A) pain receptors b) hair b) lymph g) blood d) nail plates||1) Mesoderma 2) Etoderma|
Now, after discussion of theoretical material, this task does not seem difficult to us. ECTODERMA -NAGE ENGLISH LETTLE, of it are formed by those structures that we can "touch". That is, from the options listed here - these are pain receptors, hair and nail plates. Mesoderma is an average embryonic leaflet, the connecting and muscle tissue is unplained. Blood and lymph are formed with a coniferous cloth and are formed from the mesoderm.
Example 2. Name the germs of the spinal animal, indicated in the figure of the number 2. What types of tissues and system systems are formed from it?
A drawing similar to this, we have already considered today. Figure 2 marked an embryonic leaf, which is between two others, and therefore - Mesoderma. The mesoderm is formed by connecting and muscle tissue, cardiovascular and musculoskeletal system.
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