Жануарлардың эмбриондық дамуында жақшарлардың қалыптасуы

Stage in embryonic development in which germ layers form

Gastrulation is the stage in the early embryonic development of most animals, during which the blastula (a single layered hollow sphere of cells), or in mammals the blastocyst, is reorganized into a two layered or three layered embryo known as the gastrula. Before gastrulation, the embryo is a continuous epithelial sheet of cells; by the end of gastrulation, the embryo has begun differentiation to establish distinct cell lineages, set up the basic axes of the body (e. g. dorsal–ventral, anterior–posterior), and internalized one or more cell types including the prospective gut.

In triploblastic organisms, the gastrula is trilaminar (three layered). These three germ layers are the ectoderm (outer layer), mesoderm (middle layer), and endoderm (inner layer). In diploblastic organisms, such as Cnidaria and Ctenophora, the gastrula has only ectoderm and endoderm. The two layers are also sometimes referred to as the hypoblast and epiblast. Sponges do not go through the gastrula stage. Gastrulation takes place after cleavage and the formation of the blastula, or blastocyst. Gastrulation is followed by organogenesis, when individual organs develop within the newly formed germ layers. Each layer gives rise to specific tissues and organs in the developing embryo. The ectoderm gives rise to epidermis, the nervous system, and to the neural crest in vertebrates. Following gastrulation, cells in the body are either organized into sheets of connected cells (as in epithelia), or as a mesh of isolated cells, such as mesenchyme.

The terms "gastrula" and "gastrulation" were coined by Ernst Haeckel, in his 1872 work "Biology of Calcareous Sponges". Gastrula (literally, "little belly") is a neo Latin diminutive based on the Ancient Greek γαστήρ gastḗr ("a belly").

Lewis Wolpert, pioneering developmental biologist in the field, has been credited for noting that "It is not birth, marriage, or death, but gastrulation which is truly the most important time in your life."

Gastrulation is highly variable across the animal kingdom but has underlying similarities. Gastrulation has been studied in many animals, but some models have been used for longer than others. Furthermore, it is easier to study development in animals that develop outside the mother. Model organisms whose gastrulation is understood in the greatest detail include the mollusc, sea urchin, frog, and chicken. A human model system is the gastruloid.

Sea urchins have been important model organisms in developmental biology since the 19th century. Their gastrulation is often considered the archetype for invertebrate deuterostomes. Experiments along with computer simulations have been used to gain knowledge about gastrulation in the sea urchin. Recent simulations found that planar cell polarity is sufficient to drive sea urchin gastrulation.

Sea urchins exhibit highly stereotyped cleavage patterns and cell fates. Maternally deposited mRNAs establish the organizing center of the sea urchin embryo. Canonical Wnt and Delta Notch signaling progressively segregate progressive endoderm and mesoderm.

In sea urchins the first cells to internalize are the primary mesenchyme cells (PMCs), which have a skeletogenic fate, which ingress during the blastula stage. Gastrulation – internalization of the prospective endoderm and non skeletogenic mesoderm – begins shortly thereafter with invagination and other cell rearrangements the vegetal pole, which contribute approximately 30% to the final archenteron length. The gut's final length depends on cell rearrangements within the archenteron.

The frog genus Xenopus has been used as a model organism for the study of gastrulation.

The sperm contributes one of the two mitotic asters needed to complete first cleavage. The sperm can enter anywhere in the animal half of the egg but its exact point of entry will break the egg's radial symmetry by organizing the cytoskeleton. Prior to first cleavage, the egg's cortex rotates relative to the internal cytoplasm by the coordinated action of microtubules, in a process known as cortical rotation. This displacement brings maternally loaded determinants of cell fate from the equatorial cytoplasm and vegetal cortex into contact, and together these determinants set up the organizer. Thus, the area on the vegetal side opposite the sperm entry point will become the organizer. Hilde Mangold, working in the lab of Hans Spemann, demonstrated that this special "organizer" of the embryo is necessary and sufficient to induce gastrulation.

Specification of endoderm depends on rearrangement of maternally deposited determinants, leading to nuclearization of Beta catenin. Mesoderm is induced by signaling from the presumptive endoderm to cells that would otherwise become ectoderm. RA signaling in this organism can affect the formation of the endoderm and depending on the timing of the signaling, it can determine the fate whether its pancreatic, intestinal, or respiratory. Other signals such as Wnt and BMP also play a role in respiratory fate of the Xenopus by activating cell lineage tracers. Formation of the primitive streak is reliant upon nodal signaling Furthermore, Cer1 and Lefty1 restrict the primitive streak to the appropriate location by antagonizing nodal signaling. The region defined as the primitive streak continues to grow towards the distal tip. To form the streak, reptiles, birds and mammals arrange mesenchymal cells along the prospective midline, establishing the first embryonic axis, as well as the place where cells will ingress and migrate during the process of gastrulation and germ layer formation. The primitive streak extends through this midline and creates the antero posterior body axis, becoming the first symmetry breaking event in the embryo, and marks the beginning of gastrulation. This process involves the ingression of mesoderm and endoderm progenitors and their migration to their ultimate position, where they will differentiate into the three germ layers. Fibroblast growth factors (FGF), canonical Wnt pathway, bone morphogenetic protein (BMP), and retinoic acid (RA) are all important in the formation and development of the endoderm. and 3D cell (Embryonic organoids) culture techniques using embryonic stem cells (ESCs) or induced pluripotent stem cells (iPSCs). These are associated with number of clear advantages in using tissue culture based protocols, some of which include reducing the cost of associated in vivo work (thereby reducing, replacing and refining the use of animals in experiments; the 3Rs), being able to accurately apply agonists/antagonists in spatially and temporally specific manner

In vitro fertilization occurs in a laboratory. The process of in vitro fertilization is when mature eggs are removed from the ovaries and are placed in a cultured medium where they are fertilized by sperm. In the culture the embryo will form. 14 days after fertilization the primitive streak forms. The formation of the primitive streak has been known to some countries as "human individuality". This means that the embryo is now a being itself, it is its own entity. The countries that believe this have created a 14 day rule in which it is illegal to study or experiment on a human embryo after the 14 day period in vitro. Research has been conducted on the first 14 days of an embryo, but no known studies have been done after the 14 days. With the rule in place, mice embryos are used understand the development after 14 days; however, there are differences in the development between mice and humans.