Gastrulation is a stage of early embryonic development. During this stage, the three germ layers of the embryo, the endoderm, mesoderm, and ectoderm, are formed. Gastrulation disorders cause severe malformations that result in death in most cases.
What is gastrulation?
Gastrulation is a stage of early embryonic development. During embryogenesis, the human embryo forms its full shape. Fertilization of the egg is followed by the stage of furrowing. This is followed by the formation of the blastocyst. The blastocyst represents a fluid-filled cavity. This cavity invaginates during gastrulation of four-celled organisms. Three cotyledons emerge from the blastula during this process. Because of the tripartite nature, developmental biologists speak of a triploblastic basis of human embryogenesis. The cotyledons are the product of an initial differentiation into distinct 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 tissue cluster is called the entoderm. In the middle lies the mesoderm. The outer layer is called the ectoderm. Gastrulation is a part of early embryogenesis and follows the formation of the primitive streak. The next developmental steps are the development of the chorda dorsalis and the folding of the neural tube.
Function and task
Gastrulation occurs in all four-celled organisms and proceeds similarly among species. Bilaterally symmetrical or triploblastic species develop three distinct germ layers, the entoderm, mesoderm, and ectoderm. Cnidarians and ribbed jellyfish develop two germ layers and are therefore also called diploblastic. The initial site of gastrulation in multicellular and lower mammals is the blastula. In higher mammals, such as humans, it is the blastocyst. This is a hollow sphere made of a single layer of cells. This blastocyst is remodeled into a two-layered cup germ at the beginning of gastrulation. This cup germ is the gastrula. Thus, the interior of the primary cotyledons is the endoderm and the exterior of the structures is the ectoderm. The endoderm bears an opening to the outside called the primordial mouth. By analogy, the endoderm is called the primordial orifice. The mesoderm develops at the same time or slightly delayed with the primary cotyledon formation. The further course of development of the primordial mouth differentiates bilaterally symmetrical animals into two distinct groups. Urmouths form the mouth from the primordial mouth. Newmouths, like humans, develop the anus from the primordial mouth. Their mouth breaks through after gastrulation on the opposite blastula side. The gastrulation process can be simplified to several basic movements. The first of these is invagination. In this phase, the prospective endoderm invaginates into the fluid-filled and internal cavity of the blastula. Cells of a blastula pole deform, thus invaginating an outer wall portion. 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 visibly constricts this inner body cavity. These processes are followed by involution. This refers to a curling movement of the entoderm. In the subsequent ingression, cells of the endoderm migrate into the structures. This step of gastrulation is followed by delamination. In this process, the blastula cells lace off endoderm cells and thus transport them into the blastocoel. During the subsequent epiboly, invagination occurs again. On yolk-rich eggs, the ectoderm overgrows the endoderm. The gastrulation phases are the basis of human form and structure. They usually overlap with the subsequent processes of embryogenesis, such as neurulation.
Diseases and disorders
Disorders 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, in the developmental disorder hemimyeloceles. Swelling and neurological deficits characterize this congenital disorder.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 abnormalities lead to pregnancy abortion in most cases. From the beginning of gastrulation, after the third week of development, there is a high 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. During gastrulation, two disorders in particular can occur that are worth mentioning. These two clinical pictures are known as sirenomelia and coccygeal teratoma. Consequently, if the formation of the primitive streak is disturbed, there is insufficient mesoderm in the posterior half of the embryo. This association 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 node often develop into tumors called sacrococcygeal atatomas and coccygeal teratomas, which are among the most common tumors in the newborn.
