Gastrulation

The Gastrulation is a stage of early embryonic development. In this phase, the endoderm, mesoderm and ectoderm are the three germ layers of the embryo. Gastrulatory disorders cause severe malformations, which in most cases lead to death.

What is the gastrulation?

During embryogenesis, the human embryo develops its full shape. The fertilization of the egg is followed by the furrowing stage. This is followed by the formation of the blastocyst. The blastocyst is a fluid-filled cavity. This cavity turns inside out during the gastrulation of four-celled organisms. Three cotyledons arise from the blastula. Developmental biologists speak of a triploblastic basis of human embryogenesis because of the tripartite nature.

The cotyledons are the product of an initial differentiation into different cell layers. They are multipotent clusters of different tissues. From these clusters, all structures of the later body are formed during the further course.

The inner cluster of tissues is called the endoderm. The mesoderm lies in the middle. The outer layer is called the ectoderm. Gastrulation is part of early embryogenesis and follows the formation of the primitive streak. The next developmental steps are the development of the notochord and the folding of the neural tube.

Function & task

Gastrulation takes place in all four-celled cells and is similar in the individual species. Bilaterally symmetrical or triploblastic species develop three different cotyledons with the endoderm, the mesoderm and the ectoderm. Cnidarians and comb jellyfish develop two cotyledons and are therefore also referred to as diploblastic.

The starting point for gastrulation in multicellular and lower mammals is the blastula. In higher mammals such as humans, it is the blastocyst. It is a hollow sphere made of a single layer of cells. At the beginning of gastrulation, this blastocyst is transformed into a bilayered beaker germ. This cup germ is the gastrula. This creates the endoderm as the interior of the primary cotyledons and the ectoderm as the exterior of the structures. The endoderm has an opening to the outside called the original mouth. The endoderm is analogously referred to as the primeval bowel. The mesoderm develops at the same time or slightly delayed with the primary cotyledon formation.

The further development of the primal mouth differentiates the bilaterally symmetrical animals into two different groups. The original mouths form the mouth from the original mouth. New mouths, like humans, develop the anus from the original mouth. Your mouth erupts on the opposite side of the blastula after gastrulation.

The gastrulation process can be simplified to several basic movements. The first of these is intussusception. In this phase, the prospective endoderm invades the fluid-filled and inner cavity of the blastula. The cells of a blastula pole deform and thus invade an outer wall part. The inner part is now the endoderm and the outer part is called the ectoderm from this point on. The inner cavity of the blastula is the primary body cavity. The phase of invagination constricts this inner body cavity visibly.

Involution follows these processes. This means a rolling movement of the endoderm. During the subsequent ingression, cells of the endoderm migrate into the structures. This gastrulation step is followed by delamination. The blastula cells cut off endoderm cells and transport them into the blastocoel. With the subsequent epibolism, intussusception occurs again. On yolk-rich eggs, the ectoderm overgrows the endoderm.

The gastrulation phases are the basis of human shape and structure. They mostly overlap with the subsequent processes of embryogenesis, for example with neurulation.

Illnesses & ailments

Disturbances in early embryogenesis result in malformations or even loss of viability for the embryo. Neurulation disorders, for example, cause malformations of the nervous system. Since gastrulation and neurulation often overlap, neurulation disorders are often associated with gastrulation disorders.

This is the case, for example, with the developmental disorder hemimyeloceles. This congenital disease is characterized by swellings and neurological deficits. The early development of the embryo is relatively insensitive to harmful influences during the first two weeks of the germ. However, germ malformations and chromosomal deviations lead in most cases to an abortion of the pregnancy.

From the beginning of gastrulation, after the third week of development, there is a high level of sensitivity to harmful substances. First the primitive streak is formed. From this point on, each organ is exceptionally sensitive to teratogens in a specific phase. Two disorders worth mentioning can occur in gastrulation. These two clinical pictures are known as sirenomelia and coccyx teratoma.

If the formation of the primitive streak is disturbed, there is consequently insufficient mesoderm in the back half of the embryo. This connection is known as sirenomelia and is associated with malformations such as fused extremities, spinal anomalies, missing kidneys or malformed genital organs. The remnants of the primitive nodule often develop into tumors known as sacrococcygeal teratomas and coccyx teratomas, which are among the most common tumors in the newborn.