Gastrulation (from Greek gaster "belly of a vessel") describes a phase of embryogenesis in multicellular animals , to which humans also belong. The blastula turns inside out and the cotyledons develop . This was preceded by the formation of the bladder germ , the so-called blastulation . The British developmental biologist Lewis Wolpert is credited with the following quote about gastrulation: "It is not birth, marriage or death, but gastrulation, which is actually the most important point in your life."
Gastrulation is basically similar in all multicellular animals (more precisely in tissue animals , Eumetazoa ), but there are very clear differences both in the process itself and in the result of gastrulation. In the course of evolution, the processes have adapted to new requirements and changed from the basic pattern, so that the gastrulation of mammals can only be traced back to the common original form if one takes into account the evolutionary descent of egg-laying animals such as fish and reptiles and their embryology as considered evolutionary link .
Bilaterally symmetrical animals form three cotyledons ( triploblastic ), the outer ectoderm , the inner endoderm and the middle mesoderm . The cnidarians (Cnidaria) and rib jellyfish (Ctenophora ) assigned to the outdated, taxonomically incorrect phylum of the coelenterates ( Radiata ) form only two cotyledons ( diploblastic ), namely endoderm and ectoderm (does not apply to all).
The starting point is the blastula in multicellular and lower mammals or the blastocyst in higher mammals (including humans ), a hollow sphere made of a layer of cells, which - in principle and to put it simply - is first transformed into the two-layered "cup germ", the gastrula . The inside of the two primary cotyledons is the endoderm, the outside the ectoderm. An opening of the endoderm to the outside is called the primordial mouth ( blastopore ), the endoderm itself represents the primitive intestine ( archenteron ). The development of the mesoderm takes place at the same time or slightly offset in time to the formation of the two primary germ layers.
The further development of the primordial mouth divides the bilaterally symmetrical animals ( Bilateria ) into two main groups: In the primitive mouths (protostomians) the primordial mouth actually forms the later mouth, while in the new mouths (deuterostomians), which include vertebrates and humans, the original mouth develops more or less clearly into the anus, while the later mouth "breaks through" on the other side of the blastula when gastrulation is complete.
As already mentioned, the process of gastrulation shows clear differences between the species, but can be traced back to a number of basic movements:
- Invagination, invagination of the prospective ("expected") endoderm into the inner, fluid-filled cavity (the blastocoel ) of the blastula: By deforming cells of one pole of the blastula, part of the outer wall is invaginated; the result looks like a vacuum, dented soccer ball on one side. From this point on, the inner part is called the endoderm, the outer part the ectoderm. The inner cavity of the blastula, the primary body cavity, is narrowed during this process; the "dent" consisting of the endoderm is called the primeval intestine or archenteron .
- Involution, rolling in of the prospective endoderm
- Ingression (immigration), immigration of cells of the prospective endoderm
- Delamination, cells of the blastula pinch off the cells of the prospective endoderm into the blastocoel
- Epibolism, basically an intussusception. In yolk-rich eggs, this occurs when the prospective ectoderm overgrows the prospective endoderm.
Usually, gastrulation and the onset of the following processes such as neurulation overlap .
Description of gastrulation using examples
Amphibians are the classic example of the very original vertebrate evolution. At a certain point in the blastula, the rate of cell division increases locally, so that a cluster of cells develops which, like a dent in an empty soccer ball, invades the primary body cavity ( blastocoel ) of the blastula. The cause is the increased rate of division, but also the amoeboid movements of the cells themselves. A double-walled hollow sphere is created, the inner wall of which, the endoderm, has an opening to the outside where it has turned in, the primal mouth. The outer wall is the ectoderm.
The posterior end of the body is now marked by the original mouth, since the original mouth develops into the anus. While the endoderm is still growing in, cell lobes grow along the center line of the germ from the invasive endoderm on both sides of this center line into the space between the entoderm and ectoderm. These lobes grow from the back of the germ between the primary cotyledons on both sides of the abdomen around the endoderm, the primitive intestine, until they meet on the midline on the abdomen. They make up the mesoderm. In the following, crevices within the mesoderm form and expand along their entire length, forming the secondary body cavity ( coelom ).
At the end of gastrulation, the embryo has a clear orientation through a longitudinal axis with a front and rear end, transversely through the midline between the leaves of the mesoderm, and in cross section through the abdomen and back.
The next step in vertebrate development is the formation of the chorda dorsalis as a dorsal, longitudinally oriented mesoderm cord that forms from the roof of the primitive intestine, where the two mesoderm leaves grew apart. The notochord induces other fundamental processes such as neurulation .
While amphibians depend on reproduction and development in the water, as their water-permeable, sensitive eggs would dry up on land, birds and reptiles have adapted to the conditions on land and made their eggs more resilient through extraembryonic membranes. A significant advance is the amnion that surrounds the embryo, which is why birds and reptiles are referred to together with mammals as amniotes. In contrast to the amphibian egg, the bird egg contains a huge egg cell, which consists almost entirely of yolk (in an unfertilized chicken egg the yolk is actually the egg cell, while the egg white is a nutrient solution for the embryo). Due to the fact that the cells can divide only to a limited extent or not at all in the presence of yolk, no spherical blastula can develop, but only a flat germinal disc created from meroplastic (partial) furrowing, which is open ventrally on the animal pole of the huge yolk sphere and consists of two superimposed cell layers, the epiblast and the hypoblast, which together enclose the blastocoel (thus they represent the actual blastula). A drawn out, long groove, the primitive groove, forms at the rear pole of this germinal disc. It extends approximately from the middle of the hemispherical germinal disc to the end of the germ. Cell material from the epiblast now migrates into this groove from both sides into the primary body cavity: First, cells on the yolk sac form the endoderm and in the process move the cells of the hypoblast downwards, then the cells that sink into the primitive groove turn over and over on both sides form the mesoderm hanging between the primary cotyledons. The remaining cells of the epiblast form the ectoderm. The embryo thus consists entirely of cells from the epiblast. The “primeval intestine” consists initially of only a small space between the endoderm and the yolk and is open towards the yolk; however, the intestinal anlage is still forming, while the embryo is lifted from the yolk mass by folding sideways. This closes the intestine and reduces its connection to the yolk sac to a yolk stalk, which consists primarily of hypoblastic cells and is located in the center of the embryo (similar to the umbilical cord in mammals).
Once all three cotyledons are formed, they begin, first the ectoderm, then the endoderm, then the mesoderm between the two, to grow around the giant yolk , and become extraembryonic membranes. The hypoblastic cells that enclose the yolk after this process is complete form a temporary auxiliary embryonic organ, the yolk sac, which nourishes the embryo. The outer epiblasts form the chorion, which encloses the embryo and the other membranes in the extraembryonic coelom. The allantois, a bulge in the rectum that absorbs waste products of the embryonic metabolism, is formed from other cells. Only the small part on top of the yolk where the cotyledons formed will continue to develop as an embryo. This distinction can also be found in the development of mammals.
While the embryo forms the notochord from the mesoderm and the neural tube from the ectoderm, two major developments take place in the extraembryonic tissue. The yolk has now grown around the three-layered yolk sac from all three cotyledons. On the one hand, the ectoderm and mesoderm fold up over the embryo, meet in the middle, connect, and thus form a protective cavity over the embryo, the amniotic cavity , which is supposed to protect the embryo from drying out and mechanical vibrations. On the other hand, a cavity is created in the mesoderm, so that the mesoderm completely disintegrates into two sheets, between which the secondary body cavity or coelom arises. As soon as the connection between the intra- and extraembryonic coeloma is torn at the border between the intra- and extraembryonic tissue, where the amniotic cavity has unfolded, a shape of the embryo with an outer shell (from the outside inwards ectoderm, mesoderm), a extraembryonic body cavity, the amniotic cavity (mesoderm, ectoderm, amniotic cavity, embryo) and underneath the yolk sac (mesoderm, endoderm, upwardly open connection to the primitive intestine of the embryo), which can be found in the same way in the embryonic development of mammals, but without the direct similarity to the To be able to recognize the basic pattern of the amphibians.
Mammals and humans
Relation to the embryonic development of birds
It is precisely in the early embryonic development that the phylogenetic relationship between mammals and egg-laying birds and reptiles becomes clear. Mammals do not lay eggs (with the exception of the ancient mammals ). Nevertheless, the yolk sac as well as the amnion, chorion and allantois are still integral components of embryonic development: Evolution cannot simply abolish this auxiliary organ again, the yolk sac no longer contains the yolk, but processes such as the first blood formation and the emergence of the primordial germ cells still take place in the yolk sac. The inherited genetic program is thus further developed and partially transformed. Especially in higher mammals such as primates , some processes are modified in such a way that the result is the same, but the way there is significantly shorter.
The blastocyst is - apparently in contrast to the birds, but like the blastula of the amphibians - a hollow sphere. However, it will soon be shown that mammals also develop from a germinal disc . The cells of the blastocyst have differentiated into two lines, the outer trophoblast and the embryoblast (for an explanation see blastocyst ). The embryonic auxiliary organ typical of mammals, the placenta , will develop from the trophoblast, while the embryo itself will develop from the embryoblast, a cluster of cells at one point on the inside of the hollow sphere. One can imagine that the (no longer present) yolk lies in the cavity of the blastocyst and the embryoblast lying on it corresponds to the downwardly open germinal disc of the birds.
Two-leaved germinal disc
In humans, the formation of two cotyledons occurs simply because the cell material of the embryoblast differentiates into an epithelial-like, cylindrical association, the ectoderm, while a layer of smaller, polygonal cells, the endoderm, forms at the border of the germinal disc against the blastocyst cavity , trains.
While in some mammals (dog and cat) the formation of the amniotic cavity occurs exactly as in birds through the formation and closure of amniotic folds, this process is abbreviated and simplified in more highly developed mammals, especially in humans: In the ectoderm, crevices arise that confluence and form the amniotic cavity. It is delimited at the top by a layer of thin cells, which also come from the embryoblast, at the bottom lies the ectoderm and thus the back of the germinal disc (eighth day in humans).
Cells migrate out of the endoderm and line the blastocyst cavity along the inside of the trophoblasts and thus form the primary yolk sac (human: ninth day).
The implantation in the uterine lining and the further development of the trophoblast to cytotrophoblast and syncytiotrophoblast , which will not be discussed in detail in this article, happens parallel and relatively independently of the development of the embryoblast described here, in the first few days it even progresses in terms of size much faster. This means that around the eleventh to twelfth day (humans) the trophoblast grows disproportionately to the embryoblast. It first tears open a gap between the endodermal lining of the primary yolk sac and the trophoblast, which is filled by a net-like, loose tissue that comes from the endoderm (endodermal reticulum).
On the thirteenth day (humans), extraembryonic mesoderm develops from the zone between the two cotyledons . This tissue grows into the gap described above, on the one hand envelops the structure of the amniotic cavity, yolk sac and intervening germinal disc from the outside, and on the other hand forms a layer on the inside of the trophoblast. The cavity formed between these two leaves of the extraembryonic mesoderm corresponds to the extraembryonic coelom and is called the chorionic cavity . The embryo is only suspended from a thin mesodermal sticky stalk. The outer shell of trophoblast and mesoderm from which the placenta will arise is called the chorion .
The shape now reached at the end of the second week (humans) with the germinal disc between the amniotic cavity and the yolk sac, an extraembryonic body cavity and the chorionic membrane shows great similarities with the avian embryo at the end of gastrulation, although the mammalian embryo does not yet have an actual intraembryonic mesoderm . In comparison to the bird, the path to this stage of development is different in many respects in terms of form and timing of developments, but the results are very similar. The connection to the development of primitive vertebrates such as the amphibians can also be recognized through the birds.
In the third week of human embryonic development, the intraembryonic mesoderm, the third germinal disc, forms. From the posterior pole of the germinal disc an elongated, narrow, groove-like thickening develops in the ectoderm, the primitive stripe that grows forward and forms a rounded end at the border between the posterior and middle third of the embryo, the primitive node . Cells migrate inward from the ectoderm to primitive stripes and nodes, descend and form a new cell layer between the ectoderm and endoderm, in which they then migrate again to the side and forward. With the exception of a small round area in the anterior area (the prechordal plate ), the mesoderm expands in the entire germinal disc between the two primary germ layers and also connects laterally to the extraembryonic mesoderm that has existed for some time.
The early development of the human germ in the first two weeks is relatively insensitive to harmful influences. Malformations of the germ and most chromosomal aberrations lead to an unnoticed abortion.
The risk is completely different from the beginning of the third week (two weeks after fertilization, approx. Four weeks after the last rule) as soon as the primitive streak (see above) forms. From then on, every organ has a specific phase in which it is particularly sensitive to teratogens . In gastrulation in particular, there are two clinical pictures worth mentioning: sirenomelia and coccyx teratoma .
If there is a disturbance in the formation of the primitive streak and there is not enough mesoderm in the back half of the embryo as a result, the very rare syndrome of sirenomelia can occur. It includes malformations such as the growing together of the lower limbs, disorders of the lower spine, lack of development of the kidneys and malformations of the genital organs.
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